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CharlesJones 05-17-2005 02:39 AM

Lapping in valves

Is it advisable to lap in valves? I have read contradictory opinions about this - some people say it can damage valve seats, and other people say it will help to ensure a good seal. What do you guys think? And how much lapping should I do?

Any advice you can give would be greatly appreciated - thanks.

john walker's workshop 05-17-2005 07:52 AM

lapping old valves is useless. you would lap new valves on a freshly cut seat to see how concentric the seat job is, in other words, does the lapping show better contact on one side of the seat than the other, and it also gives you a good look at where the valve face contacts the seat, which should be just inside the outer edge of the face. water based compound is best. you only need to do each seat for 10 seconds or so to get a gray mark. i suppose that it finely finishes the seat surface, but if you use decent equipment, it really isn't needed, other than to check your work, as stated above. i lap all my valves to their seats for this reason only.

CharlesJones 05-18-2005 02:34 AM

Thanks for the advice John. Unfortunately it is old valves I am working with, so if lapping isn't worthwhile with old valves I won't bother trying. But is there any way I can check that the valves seal okay before I put everything back together?

NevenM 05-18-2005 03:52 AM


Firstly if you've got the engine apart to this level you should have the valves and seats refaced if they are reusable. I use bearing blue to check the valve face, put the head in a vice angled so the valve seat is horizontal, smear the blue on the valve face and then 'pop' it against the seat once, this transfers the blue to the seat, now clean the valve and 'pop' it against the seat again. If you look carefully at the valve face you will see blue transfered where you have a match with the seat. I agree with john you don't need to lap for hours and hours but a minute is usually enough



CharlesJones 05-18-2005 04:06 AM

Thanks Neven - I'll try that, but what's bearing blue? Presumably its some sort of engineering die, but where do you normally buy it from? Is there anything else I could use if I can't get hold of it?

Thanks for your help.

NevenM 05-18-2005 02:07 PM


Bearing blue should be available from any engineering supply store, I do caution though there is no point in doing this unless you reface everything


snowman 05-18-2005 05:07 PM

John is correct.

An old valve that has been reground is now a "new" valve, the same for the seat.

The current theory is that you grind or cut the valve seats at 46 degrees and grind the valves at 45 degrees. This creates an interference fit and an ideal seal. Once the engine warms up the contact point may move but the seal remains. If you lapped a set of valves for a newer Porsche (one with 4 or more valves per cylinder) you will ruin them, so don't. But if the valves are refinished its ok to lap them. The new ones have a special coating that is damaged by lapping.

Lapping as John is doing it, to mark the contact surfaces, is not really lapping as in the sense it once was. If you really lap a valve in you will create matching groves in the seat and valve. Really seals good, until the engine warms up, the seat gets larger in diameter and the valve grows in length, and now the grooves no longer match up and the valve is unseated and leaks. Thats the reason lapping is no longer recommended.

The bluing is called Persian Blue (sold as Markum) and is available in many hardware and auto stores as well as machine shop supply houses. But why bother, all you need is a permanent marker, it will accomplish the exact same thing.

In the "good" old days when no one had a valve grinding machine, and the like, one just put a lot of lapping compound between the worn or new valve and seat and lapped untill the two surfaces matched. Your then grand for another 2000 to 10000 miles. But the seal was not really that good.

Just a note of caution, ALWAYS replace the valve guides BEFORE doing any work on the seats or valves, everything iis referenced to the guide. If the guide is bad the work will be bad, If the work is done with the old guide and then it is replaced nothing will line up anymore.

TimT 05-18-2005 06:25 PM


The bluing is called Persian Blue (sold as Markum)
hmm that would actually be "Prussian Blue"

snowman 05-18-2005 07:47 PM

YEH YEH YEH pick on the poor spellur

Yesterday I wasn't an engineer, today I are one.

NevenM 05-18-2005 07:49 PM


Wasn't the spelin it was the jeograffy


snowman 05-18-2005 07:53 PM

North pole , South pole, same difference.

snowman 05-18-2005 07:58 PM

A tip I learned from an old timer, very echonomy minded old timer. There is no need to buy blue stuff or a marker or even a pencil when grinding valves. You just pick up a little of that black grinding dust with your little finger and use that to coat the valve seat or valve and then you can see exactly where the two mate, fer FREE!!!

NevenM 05-18-2005 08:01 PM


Yes, the advantage of the method I was taught above is that you cant get a false positive by smearing too much on which is a common mistake

afterburn 549 05-18-2005 08:16 PM

please just throw your old valves away and buy new.....I will sleep better and so will you!.....margin,margin want margin,sharp edges burn ez

snowman 05-18-2005 08:18 PM


I have to agree with you. My personal favorite is yours, the blue dye, followed by the marker pen. I also use the light lapping like John uses. I think I switch back and forth because I am not sure which one is really better than the other.

Leave the cheep dirty method to the really old timers.

Mark McClure 05-19-2005 12:20 AM

I have been under the impression that lapping in valves is a way of reseating them in the seat. The more you lap the lower the valve is seated and the more the face is matched.

Snowman - I can see that if you just spin the valve you would get ridges. and as things expand and change it causes an issue .

the way I have been taught is to rotate only a small amount then lift the valve and move it around and repeat. This way each face is guaranteed to mate well without groves

My dad has a hand tool like a hand drill that actually rotates 90degrees both ways. you then lift the valve and move it round.

I have been planning to lapp my vavles on the next rebuild but you have me thinking about it now!

CharlesJones 05-19-2005 02:51 AM

Thanks for your explanation of the problems with lapping Jack, but one thing I'm not quite sure about is how everything is cut or ground referenced to the valve guide. I can see how the seat is cut referencing the guide, since I guess the cutting tool rotates around the guide, but I can't see how the valve grind is made to reference the guide. Is the valve somehow ground to specifically match the guide and seat? I always thought people just kept valves matched to their cylinder heads due to wear patterns and lapping.

NevenM 05-19-2005 02:53 AM


The Valve is ground referenced to the stem, the stem fits the guide voila, This is the reason worn guides must be replaced


CharlesJones 05-19-2005 03:27 AM

Ahh.. I understand now! If the valve guide is worn the valve might not sit straight and so the valve face won't be flush to the seat. But then why is it so critical to keep the valves matched to their cylinder heads? Maybe its not so critical if everything has been refaced?

NevenM 05-19-2005 03:30 AM


Yes its critical to keep them matched only if you are not refacing them, I usually lap, blue test and assemble all at the same time (get a bit bored by the 6th head!)


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