lapping valves and seats necessary?

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Re: lapping valves and seats necessary?

Post by SpeierRacingHeads »

I just hit everything with a stone. A shiny seat is proper.
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Re: lapping valves and seats necessary?

Post by Schurkey »

David Redszus wrote: Mon Dec 05, 2022 2:26 pm But ultimately, I always rely on a vacuum seal test.
I have a valve-seat vacuum tester. No operator's manual, and whoever made it has disappeared. It draws from the port, so it tends to pull the valve tightly closed. Kinda wondered if it should pull from the chamber side, with the valve springs attached.

The machine shop I got it from said anything over 15 inches/50% was fine. I suspect it is...for grandma or a car that's going to be sold soon.

What are your "specs" for vacuum-testing valve seats? Do you install a valve-guide mounted valve stem seal to reduce leakage via the valve guide?

The only time I've used the thing was on Lumina DOHC 4-valve heads; I was getting ~90% sealing which is way better than 50%; but that was with the valve stem seals installed.
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Re: lapping valves and seats necessary?

Post by PackardV8 »

Schurkey wrote: Mon Dec 05, 2022 6:01 pm
David Redszus wrote: Mon Dec 05, 2022 2:26 pm But ultimately, I always rely on a vacuum seal test.
I have a valve-seat vacuum tester. No operator's manual, and whoever made it has disappeared. It draws from the port, so it tends to pull the valve tightly closed. Kinda wondered if it should pull from the chamber side, with the valve springs attached.

The machine shop I got it from said anything over 15 inches/50% was fine. I suspect it is...for grandma or a car that's going to be sold soon.

What are your "specs" for vacuum-testing valve seats? Do you install a valve-guide mounted valve stem seal to reduce leakage via the valve guide?

The only time I've used the thing was on Lumina DOHC 4-valve heads; I was getting ~90% sealing which is way better than 50%; but that was with the valve stem seals installed.
My machinist puts the head on a rubber mat, chamber down and pulls 28" of vacuum out the spark plug hole. If it won't hold 28", he smacks the valve stem with a rubber mallet. Sometimes, that's all it takes, as we know the valve never sits in the same place twice. If it still doesn't hold, he touches the seat with a stone.
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Re: lapping valves and seats necessary?

Post by modok »

I have never seen a vac tester calibrated measure to any actual unit.
But that COULD be done same as a leakdown tester, with two gauges and a calibrated orifice. Then we could measure actual leakage rates.
Downside is the attachment to the head will have to be leak free or at least a constant.
I will admit tho, practically speaking, it's not really necessary that it is calibrated and doesn't matter if it's slightly leaky, just needs to be repeatable so you can use it to tell you want you need to know. Mainly repeatable.

actually.....we could test jets of a known size with our leakdown testers to calibrate it, like we do flowbenches.



I would vac test diesels from either side, it didn't make any big difference either way, and that's often LOW seat pressure.
Sometimes it would leak slightly from the guide if the guide clearance was very loose.
It is possible to get near zero leakage with no cheating, but it isn't necessary.

For instance I could only get zero leakage when the seat angle and valve face angle matched within a fraction of a degree, but we know that in the running engine a half degree or one degree difference doesn't make any difference. But if you are racing vac testers that might be a key to winning.
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Re: lapping valves and seats necessary?

Post by Belgian1979 »

David Redszus wrote: Mon Dec 05, 2022 2:26 pm Did you grind the seats or cut them?

Are the valves and seats concentric?

A very slight lapping will reveal seat contact width and concentricity.

But ultimately, I always rely on a vacuum seal test.
I didn't grind them, the shop did. With a CNC seat cutter I was told.
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Re: lapping valves and seats necessary?

Post by bentvalves »

https://www.youtube.com/watch?v=ZE74dtq96o4

I know many will disagree, but I don't think any high performance valve/seat work is complete until the valve has been properly lapped into the seat.

I am not talking lapping an old valve into an old seat and calling it a valve-job because you used a drill.

The valve will find the seat in short order weather it has been lapped or not.

I just prefer to jump-start the process when the head is on my bench and usually only in a high performance deal when the valves and seats have been machined and the customer isn't beating me up on price as I almost consider this step a courtesy.
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Re: lapping valves and seats necessary?

Post by juuhanaa »

bentvalves wrote: Sun Dec 11, 2022 10:12 am https://www.youtube.com/watch?v=ZE74dtq96o4

I know many will disagree, but I don't think any high performance valve/seat work is complete until the valve has been properly lapped into the seat.

I am not talking lapping an old valve into an old seat and calling it a valve-job because you used a drill.

The valve will find the seat in short order weather it has been lapped or not.

I just prefer to jump-start the process when the head is on my bench and usually only in a high performance deal when the valves and seats have been machined and the customer isn't beating me up on price as I almost consider this step a courtesy.
Hi and thanks for video,

Cool leak test, but i noticed you didnt measure the concentricity of the valve to the seat before and after lapping. Precision machining of steel is often done with stone.



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Re: lapping valves and seats necessary?

Post by modok »

bentvalves wrote: Sun Dec 11, 2022 10:12 am https://www.youtube.com/watch?v=ZE74dtq96o4

I know many will disagree, but I don't think any high performance valve/seat work is complete until the valve has been properly lapped into the seat.
You just said that lapping is done AFTER the valve job,
So why didn't you do a valve job before lapping?

The head you used for you long winded video
Looks like the exhaust seats worn too wide and the intake valves too small for the throat size.
To position the seat correctly on the valve, is the definition of what a valve job is IMO.

That said what you learned about lapping is very interesting anyway, and I'm sorry to be rude, I did not watch every second perhaps I missed something.
Not saying did anything wrong but your testing of that head does no prove your statement above.
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Re: lapping valves and seats necessary?

Post by bentvalves »

modok wrote: Sun Dec 11, 2022 5:11 pm
bentvalves wrote: Sun Dec 11, 2022 10:12 am https://www.youtube.com/watch?v=ZE74dtq96o4

I know many will disagree, but I don't think any high performance valve/seat work is complete until the valve has been properly lapped into the seat.
You just said that lapping is done AFTER the valve job,
So why didn't you do a valve job before lapping?

The head you used for you long winded video
Looks like the exhaust seats worn too wide and the intake valves too small for the throat size.
To position the seat correctly on the valve, is the definition of what a valve job is IMO.

That said what you learned about lapping is very interesting anyway, and I'm sorry to be rude, I did not watch every second perhaps I missed something.
Not saying did anything wrong but your testing of that head does no prove your statement above.


It's not my video. the head was valvejobbed however, and he was working with fresh new valves.

And to be 100% honest, you do need to watch every second of the video to truly appreciate what this fellow was able to conclude and what he is showing us with calibrated equipment.

that was an OEM type valve job (seats cut/ground) that he was working with. you could see the seats were shiny and freshly "cut" even if the 45 was on the wide side.
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Re: lapping valves and seats necessary?

Post by racear2865 »

Ive been lucky or maybe unlucky to work in the industrial world and the auto world for more years than some of you are old. During the industrial years, I had the opportunity to visit many high precision, air tight and or vacuum controlled environment. For lack of a better term "class acts". Ive seen entire plants make parts in such high precision that your body temperature would screw up the measurements, yet many and I do mean many of these operations lapped and still do lap their parts so that there was no leakage that could slip by. There are others reasons to lap but no need to discuss that now. And yet here we discuss lapping as a no no and several of you even believe it is a part of the dark ages. I could care less what any of you have as a personal opinion, as that is what it is, a personal opinion that you want to convince us what we should or should not be doing. And yes Ive seen some of the best shops. The volume we do, in some cases doesn't touch what some plants do in a week. And they have engineering staffs that would make all of us smile and they have testing equipment available to them that we can only dream of. And they have opportunity to see and review any and all failures and to hash and rehash why and what failed. There are several good and bad ways to do any job. Valve lapping is exactly that. And the material we lap with does not have to be the favorite "Clover" compound. There are several lapping compounds that completely get used up in the process and easily washes off and actually can improve the finish. If we question lapping, then we should question cross hatch honing of valve guides versus reaming or or???
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Re: lapping valves and seats necessary?

Post by swampbuggy »

When you get to where you believe that the Valve and its seat are sealed well enough, install the Spring / Springs then position port gasket surface upward and pour 4 ( + or - ) ounces of mineral spirits in . This will tell you something . Mark H .
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Re: lapping valves and seats necessary?

Post by Bob Hollinshead »

I've used a few automotive paint final step polishing compounds for lapping. They are slow and take hardly any material off but the contact point shines up nice and seals well.
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Re: lapping valves and seats necessary?

Post by In-Tech »

racear2865 wrote: Sun Dec 11, 2022 9:42 pm Ive been lucky or maybe unlucky to work in the industrial world and the auto world for more years than some of you are old. During the industrial years, I had the opportunity to visit many high precision, air tight and or vacuum controlled environment. For lack of a better term "class acts". Ive seen entire plants make parts in such high precision that your body temperature would screw up the measurements, yet many and I do mean many of these operations lapped and still do lap their parts so that there was no leakage that could slip by. There are others reasons to lap but no need to discuss that now. And yet here we discuss lapping as a no no and several of you even believe it is a part of the dark ages. I could care less what any of you have as a personal opinion, as that is what it is, a personal opinion that you want to convince us what we should or should not be doing. And yes Ive seen some of the best shops. The volume we do, in some cases doesn't touch what some plants do in a week. And they have engineering staffs that would make all of us smile and they have testing equipment available to them that we can only dream of. And they have opportunity to see and review any and all failures and to hash and rehash why and what failed. There are several good and bad ways to do any job. Valve lapping is exactly that. And the material we lap with does not have to be the favorite "Clover" compound. There are several lapping compounds that completely get used up in the process and easily washes off and actually can improve the finish. If we question lapping, then we should question cross hatch honing of valve guides versus reaming or or???
Hiya racear,
I have always appreciated your posts.
Precision machining is precision machining. It all depends on the environment of the end user. You mentioned some good points with cross hatch...this is all relevant to duty cycle of the particular mechanism. Reaming a hole within .0001's and straight, then supplying an accurately machined interaction device is awesome. As you know it cannot operate at the same cycle at a different depression/speed. You can of course attempt to induce pressure fed oiling. I'm not telling you anything you don't already know. For the others, we invent depressions(cross hatch or whatever) so there will be oil "embedded".

Now, we are talking about lapping valves...If the machining is done correctly, in our performance racing environment, a vacuum test is sufficient. To check mine when they come back from the machine shop, I simply use a sharpie marker at 8 points on the valve and seat, then use a "lapper" stick and rotate. It will show any kind of problems if there are any.

People often worry about leakdown too often as well. There isn't enough time for a leak to matter at 6000rpm and especially above, unless there is a hole in the piston :lol:
Heat is energy, energy is horsepower...but you gotta control the heat.
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Re: lapping valves and seats necessary?

Post by bentvalves »

In-Tech wrote: Sun Dec 11, 2022 10:57 pm
racear2865 wrote: Sun Dec 11, 2022 9:42 pm People often worry about leakdown too often as well. There isn't enough time for a leak to matter at 1500rpm and especially above, unless there is a hole in the piston :lol:
fixed it for ya mate. : ).
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Re: lapping valves and seats necessary?

Post by David Redszus »

Here is the process that I like best. Doesn't mean it is the absolute best, however.

Cut or grind the seat as accurately as possible to deliver the correct dimensions and angle.

Very lightly lap the valve and seat to produce a light matte contact finish.

Scrub the surfaces carefully.

Suck it with a vacuum.

A running valve will deflect upon impact and create its own contact sealing surface.
While the seat width may not change much, deflection will change the contact angle.
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