Author Topic: Valve grinding and lapping  (Read 1926 times)

Online Vixen

  • Full Member
  • ****
  • Posts: 709
  • Hampshire UK
Valve grinding and lapping
« on: April 01, 2018, 09:28:34 PM »
This question has probably been asked many times before.

I am about to lap the valve seats on the Bristol Jupiter. Each valve head is 1/2" diameter and there are 36 valves to be lapped. So I have a few questions.

What is the current wisdom regarding the best medium to be used ?

What grade or grit size will give quick and consistent results ?

What is the best method for rotating the valve, I am sure they do not made dop sticks that small?

Thanks for your advise

Mike :noidea:
It is the journey that matters, not the destination

Offline Admiral_dk

  • Full Member
  • ****
  • Posts: 838
  • Søften - Denmark
Re: Valve grinding and lapping
« Reply #1 on: April 01, 2018, 10:37:39 PM »
I haven't done any our model size, but I have done quite a few full size without the "dop stick", using a piece of plastic tube. This is a very annoying way of doing it, but it can be done, just remember to pull on the hose while doing it .... Hmm - that didn't sound polite  :facepalm2:

.... and come to think about it - I'm sure it would have been quite a lot easier if I had just used a short piece of hose to "connect" the valve with a round stick ...  :noidea:

Looking forward to see your solution ... and a future run of the engine  :Love:

Offline petertha

  • Full Member
  • ****
  • Posts: 219
Re: Valve grinding and lapping
« Reply #2 on: April 01, 2018, 10:50:17 PM »
Hi Mike. The best reference I can think of is Terry's 18-cylinder radial ~ post #185
http://www.homemodelenginemachinist.com/showthread.php?p=251041&highlight=valve#post251041
I've saved some other links related to cutting/prepping the valve seats too, could be part of this build project or one of his priors.

I built a prototype head for the radial I'm working on & replicated his valve seal mini vacuum test apparatus. I'm no expert but I think it has to be a worthwhile exercise. Very subtle lapping changes can make or break resultant seal, so personally I would to improve my odds on the workbench. 

Offline petertha

  • Full Member
  • ****
  • Posts: 219
Re: Valve grinding and lapping
« Reply #3 on: April 01, 2018, 11:00:42 PM »
Not sure if this is applicable to your particular heads if for example ring type valve seats are already in the head. On my radial, the valve cage is kind of an integrated seat/flow chamber/stem bushing part. It gets glued in the head with HT Loktite then the port passages are drilled into the upper cup. Anyway, some pics of my lapping pot & vacuum valve tester. The vac test uses a 'standard' cup so the bleed down comparison to valves is apples & apples. is. any lapping mods are to the valves themselves. Ask me a year from now if it actually works & if the engine runs LOL!

Offline petertha

  • Full Member
  • ****
  • Posts: 219
Re: Valve grinding and lapping
« Reply #4 on: April 01, 2018, 11:12:44 PM »
I bought this progressive grit selection of oil based diamond lapping compound from AliExpress at very reasonable cost. I've also messed around with a variety of different compounds, mostly typical machinist brand name aluminum oxide. But even the smallest pastes are rather biggish quantities for the small amounts I use, it becomes a $$ outlay just to have a few grits to try. I saw a tube of valve lapping compound at an auto supply store, cant recall details but remember it was quite coarse for our purposes.

From what little lapping I've tried with the diamond, it cuts excellent using a aluminum lap. There are plenty of lapping experts on this forum that will chime in, so I'm watching too. One thing I am wondering about is if some diamond might impale itself in the valve seat itself (because it is sharper & doesn't break down?), or if it can be suitably removed. It appeared to wash away with solvent on the aluminum lap bushing.

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 3899
  • Barrie, Ontario Canada
Re: Valve grinding and lapping
« Reply #5 on: April 01, 2018, 11:32:33 PM »
I use 600 moco lapping compound which is aluminum oxide dust in a slurry of glycerin. I always leave the large end of my valves about 2" long to act as a handle. I cut the seats to a maximum of about 0.020" with a 45 degree tool designed a few years ago by George Britnel, which includes a guided end that is a sliding fit into the hole in the valve guide that holds the stem of the valve. The tool is turned by hand with a light pressure. Do NOT cut very much. I cut the valves to have an  angle of 46 degrees (included angle of 92 degrees). I coat the face area of the valve with compound, slide it home into place in the valve seat, then twist the "handle" back and forth with my fingers while maintaining a bit of pressure. I turn the tool clockwise, then counterclockwise about 10 times, lift it off the face a bit, rotate it 45 degrees and repeat, until I've been all the way around 360 degrees. Then wipe up the mess, put the valve "handle" into the lathe chuck and part off the valve. Once a valve has been ground for a specific valve seat, I don't mix the valves up. ---Brian

Offline yogi

  • Full Member
  • ****
  • Posts: 330
  • Duncannon, PA USA
    • Yogi's Workshop
Re: Valve grinding and lapping
« Reply #6 on: April 02, 2018, 12:23:25 AM »
Mike, I grab the valve stem with a collet in the lathe, and hold the head in my hand while I run the lathe slowly. The valve is too small to have good control over how much pressure you put on it while lapping. This has worked very well for me.
The lapping compound I use is Clover 600 grit.

Yogi


Offline Jasonb

  • Full Member
  • *****
  • Posts: 5324
  • Surrey, UK
Re: Valve grinding and lapping
« Reply #7 on: April 02, 2018, 07:38:37 AM »
I use 600g silicon carbide powder mixed with slideway oil. Usually lap before I part the valve off from the parent bar but have used the tube on the stem method and also hot glued a bit of tube to the head both of won't pull the valve over to one side and allow you to lift the valve much like when using a stick. Last resort is a screwdriver in a slot.

Offline Roger B

  • Full Member
  • *****
  • Posts: 3268
Re: Valve grinding and lapping
« Reply #8 on: April 02, 2018, 08:00:36 AM »
I use a 3mm drill chuck with a MT1 arbour to hold the valve stem. The MT1 is small enough to 'twiddle'. Like Peter I have a selection of Chinese diamond pastes and have successfully used the second coarsest. I have been advised that if you use diamond lapping compounds you need to use an ultrasonic bath for cleaning afterwards and have always done this.

The picture is of the chuck being used to back spotface some M2 fixing holes. The con rod is 8mm diameter.
« Last Edit: April 02, 2018, 08:37:04 AM by Roger B »
Best regards

Roger

Online Vixen

  • Full Member
  • ****
  • Posts: 709
  • Hampshire UK
Re: Valve grinding and lapping
« Reply #9 on: April 02, 2018, 07:18:15 PM »
Thanks everybody, I knew I could rely on the MEM members to provide extremely useful advice and their hard won experience.

There appear to be several ways to twiddle the valve from the valve stem side, a small drill chuck, a small collet or even a tight fitting plastic/ rubber tube.

I rather like Jason's idea of attaching a traditional wooden 'dop stick' to the valve side using hot melt glue, that would allow the traditional way of lapping the valves.

Opinion varies as to the lapping media. Some advocate perfectly machined faces which bearly need lapping with nothing more than metal polish. The majority suggest 600 grit silicon carbide or aluminium oxide powders. Others suggest using diamond lapping paste

I must admit to be sceptical about the diamond lapping paste. To work, the diamond must become embedded in either the valve or the seat to abrade away the other part. The question is, how well can one remove the diamond residue once the valve has been successfully lapped to the valve seat. The softer abrasives will progressively break down into smaller particles but diamonds are forever. A cylinder lap is different, the diamond becomes embedded in the soft aluminium or copper, lap which is removed completely when the cylinder is too size.

I found Terry Mayhugh's write-up (post #185 http://www.homemodelenginemachinist.com/showthread.php?p=251041&highlight=valve#post251041) to be most educational. I particularly liked his leak testing method using a small hand vacuum pump. If you can test it and measure it, then you are in control of the situation, no guesswork required. Hand vacuum pumps intended for auto brake bleeding etc. are inexpensive and readily available from Evil-bay. Every engine builder should have one.

Mike

 
It is the journey that matters, not the destination

Offline petertha

  • Full Member
  • ****
  • Posts: 219
Re: Valve grinding and lapping
« Reply #10 on: April 02, 2018, 08:10:24 PM »
I'm no expert but I think diamond between a valve & seat in-situ is not a good idea for the reasons we both stated - any amount diamond grit that 'sticks around' to either part, its probably bad news for running conditions. I suspect that's where the classic (aluminum oxide or maybe silicon carbide) lapping compounds can be used, because of the presumption that they will wash out cleanly.
http://www.americanlap.com/Lapping%20Compound.htm

What I was referring to in my post was a dedicated 45-deg lapping pot for the valve face (shown in the picture). The pot takes care of the embedded diamond issue. But that's different way of going about what I suspect for you is a 99.9% finished valve/seat combination that you are attempting to mate together as a fitted pair by lapping. I hear there are pros & cons, that's why I wanted you to see Terry's method & underlying logic.

ps - for an interesting demo of aluminum as the lapping substrate, watch Tom Lipton's (oxtool) recent Youtube video where he is using aluminum in the form of household kitchen foil. Its called Monday Night Meatloaf 116 Part 1. he has a few lapping demo's in his series, some using copper or similar 'embedding' mediums depending on the application. Then he shows testing equipment that validates the surface.

Offline Tennessee Whiskey

  • Full Member
  • *****
  • Posts: 3307
  • Springfield, Tennessee. USA
Re: Valve grinding and lapping
« Reply #11 on: April 03, 2018, 12:23:03 AM »
The Timesavers lapping compound is really nice. It does not embedd and the “sample kit” is rather inexpensive and would last us modellers a very long time.

Cletus

https://www.ws2coating.com/timesaverlappingcompounds/

Online crueby

  • Full Member
  • *****
  • Posts: 5733
  • Rochester NY
Re: Valve grinding and lapping
« Reply #12 on: April 03, 2018, 12:59:50 AM »
The Timesavers lapping compound is really nice. It does not embedd and the “sample kit” is rather inexpensive and would last us modellers a very long time.

Cletus

https://www.ws2coating.com/timesaverlappingcompounds/
I've used that on bearings and slide valves, works quite well and cleans away when done (it degrades by itself as well). The sample kit is a lifetime supply for me, includes the variety of grits.

Offline Mcgyver

  • Full Member
  • ****
  • Posts: 91
  • Toronto
Re: Valve grinding and lapping
« Reply #13 on: April 17, 2018, 03:20:46 PM »
I'm no expert but I think diamond between a valve & seat in-situ is not a good idea for the reasons we both stated - any amount diamond grit that 'sticks around' to either part, its probably bad news for running conditions. I suspect that's where the classic (aluminum oxide or maybe silicon carbide) lapping compounds can be used, because of the presumption that they will wash out cleanly.
http://www.americanlap.com/Lapping%20Compound.htm

What I was referring to in my post was a dedicated 45-deg lapping pot for the valve face (shown in the picture). The pot takes care of the embedded diamond issue. But that's different way of going about what I suspect for you is a 99.9% finished valve/seat combination that you are attempting to mate together as a fitted pair by lapping. I hear there are pros & cons, that's why I wanted you to see Terry's method & underlying logic.

I think you are right with most of that and agree with the other guys, the short answer is use Timesavers.  I've always been slightly suspect of their lofty claims, I mean who really knows unless you pull out the electron microscope, however I believe its made of garnet which as quite friable and would break down.  Of comfort is the fact that the valves aren't really rotating or sliding agaist the seat, so as long as one kept the abrasive to the lapped area, a tiny bit of embedding shouldn't matter. 

To go a little deeper and comment on some thoughts in this thread......

Valves seem somewhat unique in that in almost every other instance I can think off, lapping means your'e using a lap which is cutting tool.  Otherwise, the notion of applying abrasive between to pieces of work vs a lap and work usually should trigger the hack warning alarm.  Aside from not being able to control the geometry that results, the main reason is that the abrasive (be it diamond, carborandum, aluminum oxide, silocon carbide etc) used loose between two parts will embed in the softer of the two, or both if they're the same material.  (unless, according to the brochure, its Timesaver)

The other thing that is interesting to note is the difference between lapping with a charged lap and lapping with loose abrasive.  With a charged lap, the lap is a cutting tool and base material is never touched.  You machine/grind/scrape into the lap the geometry you want and then charge it.   For the unfamiliar, charging literally pressing the abrasive particles (embedding) into the surface of the lap (cast iron, coppy, AL etc) the result being the charged lap becomes a cutting tool.  Embedding is as simple as rolling the abrasive into the lap with something harder than the lap material.

Lose abrasive lapping is mostly (imo) for cylindrical work; it results in both the work and lap being affected, which is ok in cylindrical work as, so long as they are axially moved relative to each other (ideally such that all of the work sees all of the lap), which makes both lap and part very round.   The point of all this?  loose abrasive is usually a poor choice for a taper as the work and lap cannot move back and forth and you get irregularities, differing amounts of taper, rings etc.  A cross section view would no longer have the taper as a straight line.  The solution is to use a charged lap if you want to accurately lap a taper.

However.....in the unique case of valve lapping, we don't really care that a perfect taper is formed but we do care that the two parts mater perfectly.  So, imo, its best to use loose rolling abrasive between the parts, and to minimize the chance of embedding....which gets us back to the short answer, use Timesavers :)
« Last Edit: April 17, 2018, 04:01:19 PM by Mcgyver »

Offline gbritnell

  • Full Member
  • ****
  • Posts: 1657
Re: Valve grinding and lapping
« Reply #14 on: April 17, 2018, 06:11:40 PM »
I'm kind of late to this thread but here's the way I go about sealing valves to seats.

 First of all I adopted my procedure for sealing valves from full sized engine practice. On engines with inserted valve seats when new seats were needed they were shot with a CO2 to shrink them. They were then inserted into the head and lightly tapped to make sure they were well seated. Once the seats were installed the head was set up an a valve cutting machine and the cutter was set for depth using a dial indicator for repeatability. After cutting the valves were vacuum checked for sealing. In 99% of the time the valves were completely sealed without the need for lapping.
 For motorcycle heads when the existing seats needed to be cleaned up we had a tool called a Neway valve cutter set. This consisted of three cutter heads with carbide inserts. The three heads would be used for what is known as a 3 angle valve seat cut. First the top and bottom angles, 30 and 60 degrees would be used then the seat cutter would be used to create the seat area, usually from .06 to .09 wide depending on the engine or application. The cutters were guided by a hardened post that was inserted into the valve guide. This made sure that the seat was concentric with the guide.
 For lapping aluminum oxide compounds were used to make sure that a perfect seal was made between valve and seat.
 For model engines some builders prefer the valve cage/seat configuration. The theory is that the whole insert can be cut at one time, guide, pocket and seat so that everything is concentric. There are two catches to this method, one being that a center drill is used to start the hole followed by an undersized drill then a reamer. First of all a reamer only follows the drilled hole so if the drill doesn't drill a perfectly straight, concentric hole then the reamer just follows what is already there. Now the seat is cut with lathe tooling. In most cases the seat is perfectly concentric with the axis of the lathe but the guide and seat might not be concentric to each other. The second problem is that when the valve cage/seat is pressed into the head it can distort and further amplify any misalignment with the seat and guide. I know builders who have had good luck using this method I'm just saying that I prefer the following.
 Most of my engines use iron heads. I do have aluminum heads on my V-twin. The V-twin has valve seats pressed into the head. It also has bronze valve guides pressed in. For the iron heads I machine
the valve pocket and guide into the head, leaving the valve seat for another operation. I then make up a valve seat cutting tool, a drawing for which I had posted some time back. For small valves and guides I make the cutter from W-1 drill rod as one piece. For larger valves I make the cutter and guide post separately. The guide being pressed into the cutter after it is hardened. The small cutters consist of the guide post, a short length of diameter .005 smaller than the port diameter then the seat cutting area. The short length of enlarged diameter is to add as much rigidity to the tool as possible. Let's say you make a tool with the cutter and pilot as one. No matter what size the pilot is when the cutter is hardened the pilot will grow. For a pilot of .078 or large this can be anywhere from .0005 to .0015. I make the pilot the required diameter then polish it down once the tool is hardened. You want almost a size for size pilot with the guide.
Once the tool is made I mount it in as small  of a chuck as possible. The larger the chuck the heavier it is and the less feel you have when cutting the seat. Now I insert the cutter into the guide using a little oil so that the pilot doesn't score the guide. Using a minimal amount of pressure I turn the chuck by hand and just let the weight of the chuck cut the seat.

As a side note when making the cutter it always has to have a minimum of 3 flutes but I prefer 4 or more. I cut the flutes using my dividing head and always make one flute at say 5 degrees out of square with the rest of the flutes. The reason for this is because I have found that depending on your feel when cutting the seats sometimes you can create a slight chatter mark. With evenly spaced flutes the they are going to fall into the same chatter pattern but with one flute off it helps to prevent this condition.

 Now once the seats have been cut I lap the valves to the seats. Where possible I insert the valve through the guide and using a small pin vise, hand chuck or I have a 0 Jacobs chuck on a mandrel I clamp the end of the valve stem. I apply a tiny bit of very fine valve grinding compound on the seat and using a back and forth twisting and up and down movement I lap the valve to the seat. After a little bit of lapping I pull the valve out, clean it and the seat of grinding compound and do a vacuum check. If you can't get a good seal within two applications of grinding then you have a problem with the guide and seat concentricity. Yes you can keep going until it seats but if everything is done right the lapping should only take a minute or two.

 On some engines, like a flat head type, where the valve stem can't be held then I use two other methods. On my flathead engine when I made the valves I left a short spigot on the valve side of the valve, just long enough to secure into my chuck. The valves were then lapped using the above procedure. With larger valves a radial slot can be cut into the face of the valve using a slitting saw. A small screwdriver can then be used to rotate the valve to lap the seat.
gbritnell
Talent unshared is talent wasted.

Offline petertha

  • Full Member
  • ****
  • Posts: 219
Re: Valve grinding and lapping
« Reply #15 on: April 17, 2018, 08:02:12 PM »
For model engines some builders prefer the valve cage/seat configuration. The theory is that the whole insert can be cut at one time, guide, pocket and seat so that everything is concentric. There are two catches to this method, one being that a center drill is used to start the hole followed by an undersized drill then a reamer. First of all a reamer only follows the drilled hole so if the drill doesn't drill a perfectly straight, concentric hole then the reamer just follows what is already there. Now the seat is cut with lathe tooling. In most cases the seat is perfectly concentric with the axis of the lathe but the guide and seat might not be concentric to each other. The second problem is that when the valve cage/seat is pressed into the head it can distort and further amplify any misalignment with the seat and guide. I know builders who have had good luck using this method I'm just saying that I prefer the following.

Really good points George. The exact same thoughts crossed my mind. I guess I'll know how this radial engine pans out because integral (bronze) valve guide/port/seat were called for on the plans. The only comfort to me is some YouTubes of them actually running. LOL!

CATCH-1: When I made my prototype valve cup insert for the vacuum testing, I first drilled/bored the larger diameter port ID section of maybe 0.5" depth. As opposed to drilling/reaming through the entire ~1" length of stock corresponding to the much smaller valve stem diameter & then opening up the port diameter as a second operation. The (shorter length) valve stem drilling was still preceded by center drilling inside the port hole with 120-deg spot center drill. But my thinking was minimize the amount of material for the drill to wander.

Catch-1A: Although the valve stem hole is nicely reamed, the valve stems themselves are lapped after turning. They slide pretty nice but I'm guessing they must be 0.0005" undersize (maybe more) just based on my own lapping variation. On one hand this isn't so great, but OTOH might help a bit with valves self-centering & seating where it counts on the valve face itself. Or is that wishful thinking? That's the challenging thing about these little engines isn't it - the same sliding fit annular tolerance on a 1:1 engine valve stem doesn't scale to a 1:4 engine.

CATCH-2: This one worries me a bit. Rather than pressing a delicate valve cup into the head & risking valve seat distortion because the port area wall thickness is quite thin, I am entirely relying on the specs of high temp Loctite. Depending on the grade, an annular clearance gap of .001" is perfectly acceptable. So they slide in on a liquid film & (fingers crossed) stay put once fully cured. Now whether this withstands the head temperature in running conditions is another matter. Again, I am basing this on the designers specs. I made a few slug tests using the same materials though. In one I bored the ID to this .001" clearance slip fit. In another I made with zero clearance but warmed the aluminum so it shrank down on the bronze, again with HT Loctite. Once cured I could not beat them out with moderate hammering. I put them both in my mini oven at 400F & repeated, same result. Actually the shrink fit let go but very unscientific 'beating'. So I'm hoping this will hold true for the actual engine.

My only other data point is Loctite 'gluing' parts to a mandrel as a means of turning on a lathe. It usually takes a butane torch flame plus a whack to release them. I'll keep you all posted.

« Last Edit: April 17, 2018, 08:05:39 PM by petertha »

Offline gbritnell

  • Full Member
  • ****
  • Posts: 1657
Re: Valve grinding and lapping
« Reply #16 on: April 17, 2018, 08:52:19 PM »
All of the processes we use in building engines will work. It just depends on how meticulous we are when machining. The issue with  these miniatures is that .001 in a full size engine with a valve diameter of say 1.50 and .001 with a valve head of .25-.375 makes a world of difference.

I have never personally used Loctite for an end-all assembly although I don't see why it wouldn't work for the valve inserts given the amount of heat needed for disassembly. On a miniature engine I don't think they would get hot enough to release the bond.

One other thing to consider is that when using Loctite with aluminum and accelerator is needed. When using it with another metal, bronze, iron or steel it should work.
gbritnell
Talent unshared is talent wasted.

Offline petertha

  • Full Member
  • ****
  • Posts: 219
Re: Valve grinding and lapping
« Reply #17 on: April 18, 2018, 07:47:45 PM »
Here is a recent integral valve cage example, Terry's Knucklehead (post #2). I recall he employed similar method on his radials. Also some useful trials & tribulations regarding specific bronze alloys. http://www.homemodelenginemachinist.com/showthread.php?t=27584&highlight=loctite

I reluctantly ordered the C544 from MSC, and what arrived appeared to be identical to the smaller diameter material I was used to working with. The cages machined beautifully with no internal chatter, and the test seats cut buttery smooth. The cages were sized for a light press fit in the aluminum heads, and high temperature Loctite retaining compound was used to insure they stay in place. A 9/16" standard end mill was used to plunge cut the major bore in the head for the cage, and its actual measured i.d. was used to set the target o.d. for the turned cages.

Offline steamer

  • Global Moderator
  • Full Member
  • *****
  • Posts: 8964
  • Central Massachusetts, USA
Re: Valve grinding and lapping
« Reply #18 on: April 30, 2018, 05:38:50 PM »
Actually this has been one of the most informative and useful posts I've seen in a while!    Well done everyone!

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Online Vixen

  • Full Member
  • ****
  • Posts: 709
  • Hampshire UK
Re: Valve grinding and lapping
« Reply #19 on: May 02, 2018, 06:58:01 PM »
Here are some of my findings which relate directly to steel valve seats. Some of this information may be relevant to combined valve seats and valve guides made from bronze. However, there is a greater risk of abrasive becoming embedded in the softer bronze valve seat, so beware. Parts of this post are copied directly from my earlier By Jupiter post.

Any valve seat cutter, no matter how well made, will always leave minute radial ridges and grooves on the surface, you may need to use a X5 eyeglass to see them, but they are there. It takes a lot of extra time to lap away these marks. I followed Terry Mayhughs method of using two cutters one after the other to produce a finer surface to the valve seat. The idea being the second cutter should flatten any ridges left by the first. Only the lightest of pressure is used with the second cutter.




The easiest way I found was to grip the valve stems with a small drill chuck. There is a short piece of soft aluminium tube between the chuck jaws and the valve stem to prevent the valve being marked.





In order to test the quality of the valve seat lapping it purchased this little hand pump vacuum tester. It can get to about  400 mm Hg (-15 inch Hg) that's about  -8 psi without too much problem from internal leaks. I cap off the top of the valve guide with a short length of silicon tube, and insert a rubber cork into the inlet/ exhaust port. I can create the vacuum by squeezing the trigger a couple of times. The internal volume of the volume under test is approx 3.9 cc. I can make the volume bigger (20cc) by connecting a longer pipe, but the short pipe is the most sensitive. I have set an arbitrary leak rate pass criteria of 20 to 30 seconds for the -8 psi to leak to zero. Consider the engine running at 2000 rpm, the combustion chamber pressure should be at over 200 psi during the compression stroke and much higher still during the firing cycle. At 2000 RPM that's a duration of somewhere between 20 to 30 milliseconds. So my leak test criteria (8 psi over 20 to 30 seconds) has an enormous margin compared to the running environment.




I experimented with several types and grades of lapping compound. I tried various grades of metal polish (as suggested by Terry Mayhugh) as well as different grades of fine silicon carbide powder made up as a lapping slurry  It quickly became obvious that it would become a trade off between time and quality of the gas seal.

Solvol Autosol uses pumice as the abrasive, Given time (hours) it will lap the valve and seats to perfect finish which can achieve my 20 to 30 second test criteria. This shows that polished valve seats will provide the best sealing possible, but it takes forever. However some valve seats, for whatever reason, just would not ever achieve a perfect seal, no matter how long I lapped with the metal polish, On these problem valves I tried ultra fine 600 grit silicon carbide. The seats lapped in quickly, but this lapping compound leaves a dull surface rather than a polished surface, sufficiently that a vacuum leak rate much over 10 seconds could not be achieved, seems you need polished surfaces for a perfect gas seal. That was rather unexpected, as I have always used grinding paste on full size engines in the past to get an even all round matt finish, which always appeared to be adequate, I never had a vacuum tester. Our model engines have much small capacity, so will be more susceptible to any leaks. Especially an engine with four valves per cylinder.

I have just obtained some 1200 grip abrasive powder and will experiment further (there are plenty of valves in the Jupiter)  to find a workable compromise between the time require for a highly polished valves seal using metal polish against the quicker, but less perfect seal that come with abrasive lapping media.




Some suggest that fine diamond lapping compounds are the answer, but I worry about the diamond becoming embedded in the valves or seat faces and the long term effects on the seats and the rest of the engine.

Mike
« Last Edit: May 02, 2018, 09:44:42 PM by Vixen »
It is the journey that matters, not the destination

Offline petertha

  • Full Member
  • ****
  • Posts: 219
Re: Valve grinding and lapping
« Reply #20 on: May 04, 2018, 07:38:46 PM »
Beautiful!
(Sorry I replied to your Jupiter post, let me know if you want to confine my other question to this post.)

What do you figure the width of the actual valve seat is?

Another tidbit I can pass on from my limited testing: the presence of any liquid, even petroleum based cleaning spirits, can give you false optimistic pressure drawdown readings over dry. Even WD40 with even slight viscosity can influence the numbers. Oil film itself will make you look like a hero. So I washed the abrasive grit with whatever works best & then a final dunk in alcohol or acetone, blow dry & then test. One may argue the running engine is oily so base the seal results on that. I'm just saying for apples to apples comparison, might help to keep everything as consistent as possible.

I also found surfacing blue pretty much useless. The good seals looked about the same 'shiny ring' as so-so seals. There just isn't very much surface area to look at



Online Vixen

  • Full Member
  • ****
  • Posts: 709
  • Hampshire UK
Re: Valve grinding and lapping
« Reply #21 on: May 04, 2018, 08:52:42 PM »
Hi Petertha,

It may be more appropriate to keep all the valve lapping discussion to this topic rather than the Jupiter build.

I looked at the Brownells' gun muzzle cutters, but the cost, shipping from the US and then the UK import taxes etc. can double the price, which makes it unattractive to me. So I machined my two sets of cutters from drill rod and hardened them with a water quench. I did not bother to temper them. The cutting edges were lightly stoned, by hand. The raw machined valve face was originally 1.5 mm wide, I reduced them to approx 0.75 mm by opening the throat with the 60 degree cutter. Measuring the face width is tricky, I found visual comparison with the end of a steel wire of known diameter (0.75mm) was the most reliable. As you found, only the lightest pressure is required, The objective is to dress the previously machined surface rather than cut a new surface. The second cutter serves to refine the surface finish.

If I ever make another set of seat cutters, I will try to slightly stagger the cutting faces, each by a few degrees. This may reduce any tendance towards harmonic chatter and a wavy seat face.

Agree with you about not using oil, it does give an over optimistic reading. I do all my testing dry to be consistent. If the engine later wets the seals with oil, so much the better. Also, marker pen, engineers blue, or visual inspections of the seat faces don't tell you much. The vacuum test is the only practical way to measure the seal quality.

These are my findings which relate directly to steel valve seats. Some of this information may be relevant to combined valve seats and valve guides made from bronze. However, there is a greater risk of abrasive becoming embedded in the softer bronze valve seat, so beware.

However, we should remember that most car (auto) DIY enthusiasts and most model engineers appear to get acceptable results when they simply lap their valves with an abrasive, cross their fingers and hope. Valve seat polishing and vacuum testing may not be essential, but it is the only way I know of, to measure the quality of the seal and so eliminate one possible cause of of an engine's failure to start or run consistently.

Mike
It is the journey that matters, not the destination

Offline petertha

  • Full Member
  • ****
  • Posts: 219
Re: Valve grinding and lapping
« Reply #22 on: May 05, 2018, 02:52:46 AM »
Great info Mike. Maybe this is a good point for a dumb question I've been harboring. What is it about bronze that makes a good valve seat in our application? Attaching my informal cheat sheet of specs pulled from various sources. My limited understanding is it makes for good bearing material against rotating shafts & such. The valve stems see a bit of that but mostly its face to face slapping relationship that defines the seal. The hardness varies by alloy, but in all cases the copper content is quite high. Unfortunately the same element that makes for good laps because abrasive grit likes to embed in there & not let go easily. Full confession, the testing mucking around I was doing thus far was on 12L14 cages, prototyping the heads. Now with some bronze stock in hand I'm concerned I'll have this grit embedding problem when the time comes. I opened up an RC engine OS 4-stroke & sure enough, copper-y looking valve seat in there. So I would have to think there is a good reason even if its not obvious to me.

Offline zeeprogrammer

  • Full Member
  • *****
  • Posts: 5425
  • West Chester, PA, USA
Re: Valve grinding and lapping
« Reply #23 on: May 05, 2018, 02:54:41 AM »
Just letting you know I've been watching...but woohee...way beyond me.
I'm sure some of it will stick.
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
"To work. To work."
Zee-Another Thread Trasher.

Online Vixen

  • Full Member
  • ****
  • Posts: 709
  • Hampshire UK
Re: Valve grinding and lapping
« Reply #24 on: May 05, 2018, 09:28:10 AM »
Maybe this is a good point for a dumb question I've been harboring. What is it about bronze that makes a good valve seat in our application?

Hi Petertha, That's a very good question, Why do we use bronze?

I guess for the majority of model engine makers use bronze because that is what our drawings tell us to use, rarely have I seen a specific grade of bronze specified.

In our miniature engines, a combined valve cage incorporating the valve seat and valve guide as a single item, is a very practical approach. It is so much easier to achieve the all important concentricity of the valve seat to the valve guide bore, in a single bronze valve cage compared with juggling three separate parts (the head, the seat and the valve guide). The combined valve cage is also easier to install and retain in the cylinder head, no shrink fit valve seats to come loose and wreck the engine. Also the high copper content of the bronze efficiently conducts heat away from the valve components and into the cylinder head.

What grade of bronze should we use? A much more difficult question. The valve guide requires excellent bearing properties, it needs to work at high temperature without additional lubrication. It needs to endure these poor conditions for it's life time. This suggests the use of a bronze with excellent bearing properties. Machinability is another criteria, the valve guide needs to be reamed and finished to fine tolerances. I, for one, have had some scary experiences with some grades of phosphor bronze pinching and seizing on a drill or reamer. so I select a bearing bronze with known machinability.

The valves on my Mercedes Benz W165 engines are only 7.5mm in diameter. There are four valves per cylinder and eight cylinders per engine. So I needed a minimum of 64 valves for the two engines. I designed a combined valve cage in bronze for all the reasons identified above. I selected C932 (SAE 660) bearing bronze because of it's bearing and machinability properties. However, I feel that a "harder" grade would be better for the valve seating. Too late now, I have made them and have no intention of repeating the exercise.


This is what the screw in Mercedes bronze valve cages look like


This is what 6 foot (2 metres) of bronze bar becomes


This is the four valve cages filling the combustion chamber. There is just enough room in the center for the spark plug.

You will notice that I have not yet cut the 45 degree valve seats. I plan to cut the narrowest possible seat with a piloted seat cutter. That will wait until I have made the 64 valves to fill those holes.

The bronze is softer than the steel valve, therefore the lapping compound will want to embed itself into the softer material. It cannot be removed or washed of with a solvent. Therefore we need a lapping paste which breaks down quickly, we want a lapping compound which effectively self-destructs. I plan to use 'Autosol' metal polish, which contains a 'soft' abrasive, it's probably pumice or something similar. The abrasive quickly breaks down into smaller and smaller particles and stops working. It should be possible to polish the bronze valve seat for a good gas seal without long lasting side effects. This soft abrasive break down is one reason why metal polish takes so long to polish hard steel valve seats, bronze seats should be quicker.

Hi Zee, How's the retirement going? You could now  find the time to change your signature from "to work, to work" into "to play, to play". Just thinkin.

Mike

 
It is the journey that matters, not the destination