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lapping valves and seats necessary?

General engine tech -- Drag Racing to Circle Track

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fabr
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Re: lapping valves and seats necessary?

Post by fabr »

I assume if using coated valves we should measure run out to verify within very tight specs and use uncoated sacrificial valves to do the seat lapping? That would assure as concentric seat to guide as could be had yet it would not assure matching valve face/seat angles. Probably as good as can be had though?
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Re: lapping valves and seats necessary?

Post by bentvalves »

as good as can be had requires optical comparators because you never know if your valve grinder is grinding a true 45, and you also don't know where your knife angles are at in relation to the valve grinder angles, and where the tool holder is holding the knife, or what angle your diamond nib is laying down on your stone dresser. it all adds up.
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Re: lapping valves and seats necessary?

Post by fabr »

IMO,the best a valve job can be is 1-seat to guide concentric within .001 or less (depending on stem diameter ). 2 seat width and location as desired for the application. 3 valve margins are as desired and adequate for the application. 4 of course the stems and guides need to be straight/true/correct clearances depending on application. I keep saying correct for the application because it must be correct for the application,duh!

So far as the seat and valve face needing to be absolutely the exact same angles well,no they do not need to be perfect. A slight interference angle +/- will seal as well or even better than a perfect match. A very small interference angle of less than a 1/2 degree will quickly be a perfect match once the engine is started and that is a good thing since it is almost impossible to get perfectly matching angles without lapping.

If your valve work is done as above there is no need for any lapping. At that point though if using uncoated valves and you wish to lap I say go for it,clean well and be happy you have "perfection" . If using coated valves,that you will NOT be doing any lapping on, be happy when the vac test passes.
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Re: lapping valves and seats necessary?

Post by vortecpro »

I want to see exactly how my contact patch looks. Also it gives me a guide on my back cut. And this is what I want to see.
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fabr
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Re: lapping valves and seats necessary?

Post by fabr »

vortecpro wrote: Tue Jan 24, 2023 10:11 pm I want to see exactly how my contact patch looks. Also it gives me a guide on my back cut. And this is what I want to see.
Personally,if back cutting I'd do a quick lap with 600/800 for that. All you need is a witness band. If not back cutting I'd use dry erase. I've done it that way for 30+ years anyway.
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Re: lapping valves and seats necessary?

Post by sbcharlie »

My thoughts lapping a valve seat is foolish use Prussian blue to check contact
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Re: lapping valves and seats necessary?

Post by BCjohnny »

All this 'holier than though' crap about lapping is just that ....... if you have a process that works for you then by all means use it

Wished I had a better pic, roughed out seat, bit more needs to come off the top chamber angle :

20230119_153717.jpg

[Dark band at the lower insert in the throating cut, artefact of a hand ground cutter despite being a pretty clean cut, guess there's a dink in the carbide]


Seat is obviously the dull grey line in the middle of the five angles

Home spun 1200 superfine SiC polishing paste ....... no 'embedded' particles and certainly no grooves, but then how would you get the latter if the average mesh size was 0.0005" ?

If the seats are being bumped, what grit are the finishing stones people are using ?
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Re: lapping valves and seats necessary?

Post by SpeierRacingHeads »

Llyods stone holder was the shit! Easily make any seat .001 better. RIP Lloyd..
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Re: lapping valves and seats necessary?

Post by David Redszus »

I wonder what a strip of Plastigage, laid perpendicular cross the seat with the valve pressed in place,
would look like. It might provide a visual of seat width and seat angles.
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Re: lapping valves and seats necessary?

Post by skinny z »

vortecpro wrote: Tue Jan 24, 2023 10:11 pm I want to see exactly how my contact patch looks. Also it gives me a guide on my back cut. And this is what I want to see.
What was your method for creating an observable contact patch?
Lapping compound? Layout dye? Marker?
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Re: lapping valves and seats necessary?

Post by bentvalves »

SpeierRacingHeads wrote: Wed Jan 25, 2023 11:04 am Llyods stone holder was the shit! Easily make any seat .001 better. RIP Lloyd..

know where any .375 top stone drivers may be kicking about?
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Re: lapping valves and seats necessary?

Post by Dave Koehler »

Lapping necessity. Perfection meets reality = depends. 8)
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Re: lapping valves and seats necessary?

Post by 1980RS »

vortecpro wrote: Tue Jan 24, 2023 10:11 pm I want to see exactly how my contact patch looks. Also it gives me a guide on my back cut. And this is what I want to see.
Nice valve unshrouding on that closed chamber head there Mark.
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