Author Topic: Valve grinding and lapping  (Read 3019 times)

Online Vixen

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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
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