Author Topic: Need some good advice on valves  (Read 1153 times)

Offline michelko

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Need some good advice on valves
« on: February 26, 2020, 04:23:06 PM »
Hi
I am still fighting with my bugatti typ50 motor. I noticed some serious valve leaks and dismateled the head.
I lapped that valves severaltimes but cant get them airtight. With the chamber pressurized with air i can still hear the air escape out of the intake or exhaust. :help: :help:
How much clearance are you guys aiming at the stem?
How do you manufacture the valveseats? Guides? Any secret wisdom?
How long do you lap the valves?
At the moment i am realy sure my spark plug problems where in reality these valve problems.
Thanks in advance.
Michael

Online Vixen

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Re: Need some good advice on valves
« Reply #1 on: February 26, 2020, 04:55:01 PM »
Hello Michael,

This is what I did to lap the valves (36 of them) on my Bristol Jupiter. You can decide if it is good advice

Go to www.modelenginemaker.com/index.php/topic,7559.100.html    The story starts at reply #104

You will also find further discussion on valve lapping if you go to  http://www.modelenginemaker.com/index.php/topic,8006.msg173066/topicseen.html#new

I found ultra fine 1200 grit abrasive produced the best results and the lowest leak rates. The vacuum leak tests are very important.

Hope this helps

Mike

« Last Edit: February 26, 2020, 05:05:37 PM by Vixen »
It is the journey that matters, not the destination

Offline Roger B

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Re: Need some good advice on valves
« Reply #2 on: February 26, 2020, 07:32:26 PM »
Some quick thoughts as I am travelling at the moment;

I make my valves from stainless steel screws. I believe (probably incorrectly) that as these are forged the material grain lines are good.

I have made seat cutters at the same time as I have made the valves but leave the hexagon so I can rotate it. I have also made them to fit into the ports.

The valves are lapped with a fine diamond paste and then the valves and head go in an ultrasonic bath.

I have had some valves distort in initial running, probably due to residual stresses.

Valve cages seem more difficult to seal, again possibly due to distortion.

I have read that the guides should not be to close fiiting to allow the valve to find it's natural seat.
Best regards

Roger

Offline gbritnell

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Re: Need some good advice on valves
« Reply #3 on: February 26, 2020, 09:56:13 PM »
Some of my valve information.
First off valves and seats should be 45 degrees (+-) I have seen fellows talk about cutting the seat and valve with 1 degree difference for whatever reason. I'm assuming they feel that the valve will eventually pound itself into the seat making a better seal. The actual valve seat area on full sized engines is critical to proper wear and sealing. You can have what's called a 3 angle valve job on the seat. This cuts a 30 degree chamfer on the combustion chamber side and a 60 degee chamfer in the port side. Doing this can move the seat position higher or lower and these cuts can reduce the seat area to the industry standard.
I have made most of my engines with the seat machined directly into the head. I only have one engine with inserted valve guides and seats. The reason for this is because the head is aluminum and I feel that over the long run aluminum isn't a good material for the seat area.
Now for those of you that machine the seat right into the aluminum and don't have a problem, so be it. I am expressing my opinions.
As to valve/guide inserts. I only tried this procedure once and it took more time dealing with alignment issues than was necessary.
One of the biggest issues concerning valve seating is because the valve guide and seat aren't concentric. For seats machined into a head I have come up with my own procedure. I first drill and ream the valve guide. I then use an undersized drill to cut the port pocket. This is followed with a ball mill of the appropriate diameter to cut a nice clean hole for the port pocket. If your port pocket is a bastard size then just grind a new radius onto an existing ball end mill. The port pocket can be a little out of concentric with the guide, it won't hurt because the valve seat will be cut relative to the valve guide.
I make up special cutter for cutting the valve seats. I have posted a drawing of it many times. It's generally a 4 fluted cutter made from drill rod. For very small valve guide diameters I machine the pilot right on the tool. For larger seats I drill and ream the cutter to accept a  pilot.
The cutter is used by hand. It's mounted in a chuck and set against the sharp edge of the port pocket then lightly rotated by hand until a nice seat is produced. This is generally between .025-.035 wide.
With the seats cut this way it generally takes very little lapping to get the valve to seat.
As far as valves go I have made them from solid drill rod, solid 303 stainless steel and I have fabricated them from standard diameter rod with silver soldered heads.
I have a home made set-true ER-11 collet holder for my 6" Atlas/Craftsman lathe. Once adjusted to a given diameter it is repeatable to couple of tenths. I put the finished valves into the collet and cut the seats on the fabricated valves. When making one piece valves I cut the seat while turning the valve.
In reading what people have to say about poor compression in their engines the biggest issue is valve seating. It's not to hard to seal the piston to cylinder whether using rings or O rings unless the engine just has very bad bores. The bigger problem is the valves. Ideally you want a perfect seal between the valve and the seat but just for the sake of argument lets say you have a full sized engine with a 2.00 diameter valve. Although a .001 gap between the valve and seat wouldn't be great just think in proportional terms how bad the condition would be when the valve only has a diameter of .250.
If you machine the seat and valve as closely as possible it should take very little lapping to get both to match. If you spend more than 5 minutes trying to lap the two together you have bigger issues.
Valve guide to valve stem diameter clearance. TINY! If you have a .093 diameter valve stem then generally a hole reamed with a .093 reamer will give the needed results. In other words for a .093 diameter you would want no more than a couple of tenths clearance. If the clearance is too large it will cause other problems. When the valve opens and closes it will round out the seat area because it will hit on the side of the seat and slide into position. Do this enough times and the seat becomes rounded. The next issue with to much clearance it that when the intake opens you will draw oil (if the top end is lubricated) or air. If you draw oil it's like having bad valve seals in a full sized engine so you get smoke. If you draw air then you are leaning out the fuel/air mixture. To check my valve seating I use a vacuum pump. I make a hollow tube with a very slight taper on it and wring it into the port. If the port has bolt holes around it then you can make a tube with a flange and bolt it to the head. When you go to check the seal always put grease on the valve guide before inserting it into the head and make sure there is a small fillet of grease around the valve stem/ guide area.
Valve springs. This will depend on the size of the valve but generally for a small valve (.187 dia. head) I would use nothing heavier than .020-.025 wire. This will also depend on the diameter of the spring. The larger the diameter the softer the spring will be with a given wire size. If you make the springs too light the worst thing that will happen is the valves will float at higher rpm's. Here again if you don't rev your engine very high it's a moot point.
Building engines is an experiment in progress. There are no hard and fast guidelines for most things, not that I have ever seen. I make a part and if it doesn't work I make it again.
Hopefully this essay will give you some insight into my knowledge and what I have learned over the years.
gbritnell
Talent unshared is talent wasted.

Online Jasonb

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Re: Need some good advice on valves
« Reply #4 on: February 27, 2020, 07:16:14 AM »
I much prefer to machine the head in the lathe and turn the taper rather than use any form of seat cutter all done at the same setting as the hole and guide hole to keep things concentric, only had to do it  once in the mill with a seat cutter because of the design and had problems sealing.

Valve guide hole will be drilled and reamed with H7 reamer first and then the valve turned to be a suitable fit rather than actually measured so can't really tell you what the gap is.

Valves only get a quick 1min or so grinding in with 1000grit powder

Valves are cut from 303 stainless steel rod though large ones may have a separate head finish turned once assembled. 45degree angles on valve and seat with seat usually being very narrow.

Offline michelko

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Re: Need some good advice on valves
« Reply #5 on: February 27, 2020, 04:27:08 PM »
Hi guys many thanks for the replies.
just ordered a vacuumpump for testing. I will make a piloted 45 deg. cutter to do the seats again.
Once i bought a cutter with a Pilot hole but didnt find it anymore, i am alos not sure how accurate it was.
What i understand from the reading there should be no hearable or feelable leaking when testing the valves.
More work to do.

Michael

Offline fumopuc

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Re: Need some good advice on valves
« Reply #6 on: February 27, 2020, 05:41:09 PM »
Hi Michael,
here a sample of a good sealed valve
Kind Regards
Achim

Offline michelko

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Re: Need some good advice on valves
« Reply #7 on: March 01, 2020, 01:01:39 PM »
Hi Gents,
again thanks for the usefull Input. I made a cutter and that worked out my Valve issues.
The cutter has pilothole reamed to 3mm and is made aout of drillrod.
After milling the flutes the cutter was hardened an annealed.

After cutting the new seats nearly no lapping was needed to seal the valves.

One Thing i noticed is that some leakage at the stem can be noticed. For testig i put some heavy oil on the stem an then i saw relay slow dropping of the Manometer needle.


Offline michelko

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Re: Need some good advice on valves
« Reply #8 on: March 01, 2020, 01:04:22 PM »
This is the cutter i initially used. I think the Pilot hole was not exact enough.

Online Vixen

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Re: Need some good advice on valves
« Reply #9 on: March 01, 2020, 01:29:54 PM »
That cutter looks like a countersink bit for wood screws. The seat cutter you made is much more accurate, as you found out.

Well done, soon we will hear the Bugatti running sweetly

Mike
It is the journey that matters, not the destination

Offline michelko

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Re: Need some good advice on valves
« Reply #10 on: March 01, 2020, 03:53:58 PM »
Mike you are right. I was lazy  :facepalm: and thought i could go the easy way and buy a cheap tool.
Now i know that one i made cost nearly nothing and took only minutes to make but i had the full controll over quality
Learned something again.

Michael
« Last Edit: March 01, 2020, 08:44:23 PM by michelko »

Offline petertha

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Re: Need some good advice on valves
« Reply #11 on: March 01, 2020, 11:10:48 PM »
Looks good. Let us know how it all turns out.

When I was doing valve testing I found that presence of fluid can give you false good readings of good seal. Particularly any kind of oil because of the high viscosity and particular along the valve stem area because of the small annular gap for slip fit & relatively large area. What got me onto this was just cleaning parts before testing. Alcohol was a bit faster pressure draw down & dry was faster yet. I guess in reality there will oil in the running engine but for testing I figured I would go this approach. I'm not sure if this is right or wrong but I made a standard testing valve to first confirm the valve seat in isolation. The valve stem has a slight flat ground so there can be no false seal.

When I bonded the cages in the head, plugged off the port & drew vacuum using the real valves I noticed even the slightest amount of lint or whatever could be enough to increase the draw down from say 30 secs to 15 secs so I clean with a Q-Tip & alcohol & re-test. Interesting, with partial vacuum if I gave the top of the valve a spritz of water or Windex, the needle would stop dead & stay that way for minutes. I don't think the real engine will be that neat & tidy. Makes you wonder how they stand the abuse & function.

Having said this, my engine is still being built. So its all just speculation at this point!

Offline petertha

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Re: Need some good advice on valves
« Reply #12 on: March 01, 2020, 11:18:51 PM »
I really like the cutters you guys made. My first attempt didn't work out too well. I think it was milled accurately enough, but I suspect it underwent some distortion in my Neanderthal heat treatment. I want to pursue this until I become proficient. But in the mean time I was able to get the 45-deg (gun) tool from Brownell shipped to me in Canada. It is very sharp. I had to re-make the centering pin, my first attempt was not accurate enough. Eventually I settled on a hardened/ground M3 pin and machined a bushing which fits the tool ID.

Offline petertha

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Re: Need some good advice on valves
« Reply #13 on: March 01, 2020, 11:29:56 PM »
I used 935 bronze for my cages & another thing I learned is that bronze is a grabby material. It can drift drill bits enough over these lengths to cause issues that cascade to how the valve squares up to the cage lip. So I changed my procedure & much more consistent results.
- machine the entire cage blank in one setting
- flip the part in the (ideally really accurate) collet chuck
- counterbore the cage ID, spot drill the bottom of hole & drill/ream the (now much shorter) valve stem hole. I found a short length carbide drill recommended for alloys was better than a regular twist drill

That just leaves blueing the lip with a felt pen & gently rotating the 45-deg cutter until a consistent chamfer is made. Mine is about 0.010-0.015" at the most. If everything is done right & the planets align, it passe vacuum test first time and no lapping is required.

Now... to keep them clean & un-scratched until final assembly!

Offline Brian Rupnow

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Re: Need some good advice on valves
« Reply #14 on: March 01, 2020, 11:39:25 PM »
Please tell me more about the tool from Brunell. I would like to see a catalogue showing it and order one.---Brian