What grit to use for lapping the valves

Ken911 · 07-07-2009, 07:09 PM

I'm pulling the motor apart in a couple weeks for new pistons and cylinders and to fix some oil leaks. the heads were done 30k miles ago so I planned on just re lapping the valves. what grit of clover lapping compound should I use. I also planned on lapping the cylinders to the heads. The engine is a 3.3 turbo soon to be a 3.4 turbo.

docrodg · 07-08-2009, 03:58 AM

Grit depend on the condition of the valves and seats. The worse they are, the coarser you go to begin with. Basically, if you need to go with medium or coarse grits, get the seats and valves just to the point of smooth and even then go to the fine lap compound and give it a little work to make it nice.

cgarr · cgarr's Garage

Lapping is really only to check the fit, it can not be used to remove metal or make a bad seat good again.

docrodg · 07-08-2009, 06:08 AM

It cannot make a badly leaking seat good, but minor pitting/wear can be lapped out. The process removes a small, minuscule, microscopic, tiny little bit of metal (it is basically hand-grinding). A quick fine-lap (just a few spins of the valve) is great way to check on machine-shop regrinds.

Just clean the valve and head about 5 times after you do it to make sure no grit is left.

Ken911 · 07-08-2009, 09:54 AM

i know what lapping is i was a machinist in the navy for a very long time. I was just wondering what grit is being used for final lapping on valve seats. These were just done 30k miles ago I was just planning on freshening them up while it was apart.

Ken911 · 07-08-2009, 09:58 AM

Quote:

Originally Posted by cgarr

Lapping is really only to check the fit, it can not be used to remove metal or make a bad seat good again.

Actually a blue check checks the seal, lapping uses fine grinding compound to make the seating surface fit and not leak.

304065 · 07-08-2009, 10:53 AM

Don't lap them, leave them alone.

TimT · 07-08-2009, 11:40 AM

I use the permatex lapping compound then prussian blue to check..

Also found this with a quick search..

Lapmaster

JFairman · 07-08-2009, 01:19 PM

if you want some really fine compund, try some JB bore compound from Brownells.
you might even find some more uses for it.

Ken911 · 01:34 PM

so it looks like something finer than 320 grit. I have everything from 200 grit to 1000 grit in my haz mat building, with expired shelf life. So looks like i'll try 400 grit clover. If they need anything at all after i inspect them. I am also still debating if i should ceramic coat the chambers. I have got the stuff to do the piston domes havent decided on the heads yet.

911pcars · 07-08-2009, 01:57 PM

You can lap the face and seat, but that's when both parts are at room temp. What happens when the combustion chamber is supporting exhaust temps of 800ºF? The valve expands a little in size, so does he valve seat, and the room temp contact surfaces no longer coincide.

In addition, most valve face/valve seat angles are ground to provide a 1º interference sealing surface - it creates a point contact. Grinding an impression of this interference point into the face of the valve doesn't make sense.

I vote for Prussian blue to indicate the location of the interference seal on the valve face.

Sherwood

john walker's workshop · 07-09-2009, 04:28 AM

why bother? was it a poor valve job that has excessive leakdown readings?

Ken911 · 07-09-2009, 03:31 PM

Quote:

Originally Posted by john walker's workshop

why bother? was it a poor valve job that has excessive leakdown readings?

No i do have a lot of blow by pretty sure the rings were toasted. which is why the new pistons and cylinders, I wanted to go to a 3.4 anyway. I just figured if it's apart I might was well freshen it up some if it looks ok. I will blue check them and do a good inspection in any case.