lapping valves to check seating

[Dragsinger]

do you lap the final valve job, or, for a double check of seating is doing so acceptable?

[Steve Salesky]

can do to verify the valve job, just don't go crazy with the lapping compound.

[ProPower engines]

can do to verify the valve job, just don't go crazy with the lapping compound.

And use the absolute finest compound available to just discolour the valve face to confirm seat location

[headshop57]

do you lap the final valve job, or, for a double check of seating is doing so acceptable?

Lapping is a very valid machining process, is use when precision matting surfaces are required, is the only machining process used to calibrate precision granite surface plates to extreme accuracy +/- .00005
On the subjet of finishing valve seats, is a matter of time and cost involved and/or if you have some finish issues with your conventional cutting system, like chatter, out of round, flatness or valves out of round and concentricity, if the later is the case, you must lap every seat, but like any other tool, you have to be careful not to over doit and use the finest grade compound to do the job in a timely manner and clean good afterwards.
I use it on the first seat to check tool setup and on any stubborn seat/valve that don't pass the vacuum test for what ever reason.

[Keith Morganstein]

I don't like using lapping compound to see the seat contact / concentricity. I feel it's misleading. Lapping compound tends to just show the largest area of seat contact all around the valve because you are twisting when you lap. There is usually enough viscosity to the paste to gray up the whole seat unless it's really bad.Lapping may however repair slight irregularities in the seat. You can lap a valve, have everything look nice and gray, and then it still can fail a Prussian blue check or solvent test.

For checking valve to seat contact, the best bet is to use Prussian blue paste. Smear a thin coating on the valve, tap it with your finger into the seat, then push it out (without twisting) and you will see the contact pattern.

[Alaskaracer]

I don't like using lapping compound to see the seat contact / concentricity. I feel it's misleading. Lapping compound tends to just show the largest area of seat contact all around the valve because you are twisting when you lap. There is usually enough viscosity to the paste to gray up the whole seat unless it's really bad.Lapping may however repair slight irregularities in the seat. You can lap a valve, have everything look nice and gray, and then it still can fail a Prussian blue check or solvent test.

For checking valve to seat contact, the best bet is to use Prussian blue paste. Smear a thin coating on the valve, tap it with your finger into the seat, then push it out (without twisting) and you will see the contact pattern.

Great tip, thanks!!!

[PackardV8]

Like the space time continuuum paradox, a valve and it's seat are in a different relationship every cycle. I've done a vacuum test, had one leak a little, hit the stem tip with a brass hammer and it sealed. Take it apart, use Prussian blue, no indication of where the leak might have been. Lap it, just because and everything looked perfect. Where did the leak come from? Where did it go?

Bottom line - lapping or Prussian blue is an indicator of gross mismachining, but it will not really show what is happening in a valve and guide with .0015" clearance and a stiff spring turning 9,500 RPMs.

jack vines

[jsgarage]

Along with using lapping paste AND Dykem-blue, you can get obsessive with this. My old Sioux valve grinder came with a Sioux X825 valve-seat dial indicator that uses a 0.0001" Starrett indicator and a 10X magnifying glass. The rig is more accurate than the valve grinding system.... As someone once said, 'after the first few revs of an engine, the seat clearances have changed anyway'.

[Kevin Johnson]

Like the space time continuuum paradox, a valve and it's seat are in a different relationship every cycle. I've done a vacuum test, had one leak a little, hit the stem tip with a brass hammer and it sealed. Take it apart, use Prussian blue, no indication of where the leak might have been. Lap it, just because and everything looked perfect. Where did the leak come from? Where did it go?

Bottom line - lapping or Prussian blue is an indicator of gross mismachining, but it will not really show what is happening in a valve and guide with .0015" clearance and a stiff spring turning 9,500 RPMs.

jack vines

The valve stem is a spring as well and depending on how the spring, seats and retainers are set up may be under torsion whilst seated and thereby slightly canted and leaking. A blow with a soft hammer could release the temporary "set" and allow the valve to reseat as when it was ground.

Or it could be a local space-time distortion, of course.

[bill jones]

-I'm curious if any of you guys have ever bolted up a set of head plates to your heads and did a valve seal comparison---and/or ever used headplates on your heads while doing the seats----and really paid attention to what happens when you test the valve seal.
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-OR---if you have ever bolted a serious O-ringed plate over each chamber and tested the valve to seat seal with at least 100# of air pressure.
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-OR-if you have ever pressure tested the valve seal on a perfectly good hot engine that has passed all your testing techniques----and sprayed a liquid into the ports to witness any bubbles.

[Larry Widmer]

During my years of doing Nascar heads, we proved conclusively that even light lapping caused premature valve face and seat erosion. It's impossible to get the all of compound out of the pores of the seats and valve faces, regardless of how well you clean them. Today we use a blue or purple Sanford Delux marker to net much better results than Dykem. You should be able to use the marker on the seat (or valve) and spin it with a lapping stick to "read" the surfaces accurately...without getting lapping compound anywhere near the head.

[robert1]

-I'm curious if any of you guys have ever bolted up a set of head plates to your heads and did a valve seal comparison---and/or ever used headplates on your heads while doing the seats----and really paid attention to what happens when you test the valve seal.
--------------------------------------------------------
-OR---if you have ever bolted a serious O-ringed plate over each chamber and tested the valve to seat seal with at least 100# of air pressure.
---------------------------------------------------------
-OR-if you have ever pressure tested the valve seal on a perfectly good hot engine that has passed all your testing techniques----and sprayed a liquid into the ports to witness any bubbles.

Years ago I did aluminum S/S hemies. I had to use a torque plate because of the stud hole right next to the seat. After grinding I had to use shop air and blow around the valves and seats with alcohol in the ports. One gnat fart and they wanted them redone.

[Engguy]

do you lap the final valve job, or, for a double check of seating is doing so acceptable?

Lapping is a very valid machining process, is use when precision matting surfaces are required, is the only machining process used to calibrate precision granite surface plates to extreme accuracy +/- .00005

You are correct, good post.

""": lapping valves to check seating
by Larry Widmer » Sat May 26, 2012 4:52 pm

During my years of doing Nascar heads, we proved conclusively that even light lapping caused premature valve face and seat erosion. It's impossible to get the all of compound out of the pores of the seats and valve faces, regardless of how well you clean them. Today we use a blue or purple Sanford Delux marker to net much better results than Dykem. You should be able to use the marker on the seat (or valve) and spin it with a lapping stick to "read" the surfaces accurately...without getting lapping compound anywhere near the head."""


I have never found any lapping compound on gauge blocks, in the pores or otherwise. And what metal has such porus qualities? Maybe a casting and or sintered metals. So I'm curious about this. What were the valves and seats made of?
So then a diesel engine that was overhauled 30 years ago should not have lasted this long with lapped valves? And yes it is used extensively. One of many gas
and diesel engines. And like Headshop says granite surface plates, ours dosen't show a sign of lapping compound, and it was calibrated 2 months ago via lapping.

[Larry Widmer]

Not here to argue with you.
I just told you our findings, with SS valves / titanium valves, as well as cast iron, copper alloy, and powdered metal seats. We run tight guides and seat/valve run-out tolerances of less than .0001", so we're not dealing with clunkers.

[Engguy]

Not here to argue with you.
I just told you our findings, with SS valves / titanium valves, as well as cast iron, copper alloy, and powdered metal seats. We run tight guides and seat/valve run-out tolerances of less than .0001", so we're not dealing with clunkers.

I suppose with very soft metals there could be retention of lapping compound. As far as erosion on face and seat, I also suppose that can be debatable to wether it was due to the lapping compound or combustion products and heat causing it. Was the erosion pitting or actual wear type recession? At some point if there was abrasives in the material, caused by lapping wouldn't it would have been worn away and blown out the exhaust? How much wear are you talking about?
To actually measure the tolerance you are calling out in real life you would have to have a thermal controlled environment and the part would need to be in it for quite sometime to normalize the temperature. And the most accurate check would be by probe or laser not a dial indicator.
So were tests run of lapped vs unlapped with all conditions equal? How about with hard faces and seats? I'd be interested in the data. Thanks.