Lapping in valves - how to tell when the surfaces are mated

**RobST162** · 21-12-2008, 01:58 PM

guys I'm just lapping in the valves on my 3sgte head here

and yes, done a bunch of reading and searching

but haven't found the best way to tell when the surfaces are nicely mated together yet

I tried marker, but then, it was tough to turn the valve because of the friction I guess and results seemed inconclusive.. ie did the marker just smear a little etc

so can I make sure the two surfaces are mated super well? any thoughts?

thanks heaps!

**fixeruperer** · 21-12-2008, 03:04 PM

well besides seeing a shiny constant ring on both surfaces you can put the valve in and pour petrol into the port and see if you can see fuel weep out.

**damo101** · 21-12-2008, 03:05 PM

what about "bearing blue" or something like it? like the same stuff diff specialists use when setting up diffs.

**fixeruperer** · 21-12-2008, 03:08 PM

bearing blue is to thick.
you can normaly just see it if its lapped enuff, you can also get it vac tested.

**allencr** · 21-12-2008, 03:37 PM

Originally Posted by RobST162

and yes, done a bunch of reading and searching

?

You don't need to mark anything as you're already doing that, marking, with the grinding compound.
Look at the valve & seat - bent valve will will show only one spot on it and a non-concentric valve/seat will only show one spot on the seat. The seat width should be wide enough to dissipate heat & narrow enough to cut through/crush carbon buildup, exhausts are almost twice the intake width. The seat should be positioned for the greatest diameter without getting to the edge of the valve & seat.
Why lap, cost/availability/dicking around? Lapping is inferior to the mirror finish received from a stone.

**RobST162** · 21-12-2008, 03:47 PM

thanks guys, seriously I have been looking around google/youtube/toymods to find a little more fuel for my brain on this issue and didn't turn up that much handy info

not familiar with stone finish etc

but I just have the head apart and it's a simple thing to do once the head is apart - just run a bit of past around and make sure there's a good seal

**RobST162** · 21-12-2008, 03:48 PM

ps, is it ok to clean the carbon build up on the valve stem etc with a wire brush bit - I don't have a brush on the grinder

**hosking1991** · 21-12-2008, 03:59 PM

Originally Posted by RobST162

ps, is it ok to clean the carbon build up on the valve stem etc with a wire brush bit

that should be fine mate. also just check around the valve face for pit marks if there is pit marks they WILL have the be faced (machined) and just check valve seats for cracks also. and when you are done it would be wise to get it vac tested and surface ground (i can vac test and surface grind it for you mate if your local)

**rms** · 21-12-2008, 07:27 PM

Originally Posted by allencr

Why lap, cost/availability/dicking around? Lapping is inferior to the mirror finish received from a stone.

Try lapping the valve and seat with a fine metal polish. Will show you just how bad your stone cut mirror finish is !

**hosking1991** · 21-12-2008, 07:43 PM

you should see the finish that our new cnc valve seat cutter gives looks awesome

**jeffro ra28** · 21-12-2008, 07:46 PM

Stone cut? Thats old.
Stoned seats do require lapping. So often you will think that your stone cut mirror seat is perfect but lapping will soon tell you otherwise.
Modern day valve machining should not require valve lapping anyhow! After the valve has bashed the seat a few times all your so called surface imperfections are dulled anyhow. Its a dam good way to see if its done properly however. Another common occurance is a seat that was machined with a flogged valve guide, then see how good your seats are!

Some valves such as titanium are specificly not ment to be lapped!

But to answer your question. If you cannot get a perfectly even width, eccentric "frosted look" ring around your valve and seat with FINE paste it should be recut.