Lapping valves

[37 chief]

I just had a valve job done on two ford V-8 heads from a machine shop. I am in the process of putting them back together. They came from the machine shop not lapped in. Is it a waste of time to use some fine lapping compound on them? Stan

 

[Greg_Ky]

Lapping the valves will tell you what kind of valve job was done (good/bad) It will not correct a bad valve job.

 

[RobMD]

100% agree.

In other words, lapping will either make your day or BREAK your day...lol

 

[Duner Wi]

I have done it both ways. Lapped and not lapped but I always assemble the had, turn it over and cover valves with petrol. If it does not leak you should be good. I do a lap to see where the seats are ground and thickness.

 

[Janicholson]

Proper surfacing of the seat will result in a single line of contact around the outer edge of the seat. This is the result of the seat being finished at 1 degree difference than the valve face. If yours were done this preferred way, they do not need to be lapped. If not, I would lap. (and ask why not) JimN

 

[msb]

My late FIL was a GM service manager for many, many years. GM recommended that lapping i n valves be discontinued a long time ago. The GM service engineers reason was that the lapping compound is never really all cleaned off the valves off seats and will continue to grind on the seats for the life of the overhaul, no matter how hard the mechanic tries to clean the compound off.

 

[RAB]

Some new inlet valves have a thin anti wear coating. If you grind it off you are back to (probably) worse than old technology.
RAB

 

[Walt Davies]

Really depends on the valve job if it was done right then just put it together if not then take it to another shop and it done right and then put it together.
I had my heads done on my Saab Sonett years ago and had to to do them all over again, the shop never trimmed the grindstones between valve jobs and it was a mess. it doesn't hurt to use a very fine lapping compound and do a quick check to see if the valves are correctly ground. But be sure that you know how to lap them also. Turn them 1/4 turn back and forth a few times then check to see how the seat.Don't spin them all the way around it will give you a false reading.
Walt

 

[Walt Davies]

here a great looking site for grinding and lapping valves in with lots of pics.

Walt
Valve lapping site