What grit sandpaper to use on block and head?

MikeKanterakis

MikeKanterakis

Registered User
I did a search for this, but came up empty handed.

1990 Automatic, original head gaskets, 236,000 miles.

I'm replacing the head gaskets and am using 400 grit paper to clean / flatten the heads and the block (where the heads sit). Is 400 grit paper too rough or too fine?

I've heard in another post that you want to make a mirror finish. I've also read that you don't want a mirror finish...

So, i've sanded down the heads and the top of the block to the point where you can just see some of the "imprint" of the head gasket. Same on the block.

My question is, is this good enough, or do I want to rough up the surface a bit so that the gasket will "catch" better? or should I go for an even smoother finish?

BTW, my arms are ready to fall off. :eek:

Also, I originally notices some major streaks on the top of the block where it was initially machined by the factory. Now some of those streaks/creases led directly to the two chambers where the head gasket was starting to fail (splitting apart on the metal ring part). Now that I have sanded the block, those creases are barely there, and I have a nice smooth surface just around the cylinder where the metal ring on the head gasket will sit.

I've also noticed that the sand paper was getting dirty and fouled very quickly at first, but now that the block is 98% clean, the paper seems to just loose it's grit and not make any real difference to the "cleanness" of the block.
 
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I think most have machine shops surface the heads .005-.010 inch to insure flatness. The surface finish depends on what headgaskets you are using.
 
i think you're going to put yourself in more trouble if you are sanding by hand. This allows for a lot of surface imperfections. You can never sand something flat by hand. I'd say you might cause some sealing issues later on, regardless of what gaskets you are using.
 
so, then is it safe for me to assume that you are on the "mirror finish" side of the fence, or do you still think that there should be some "roughness" to the block and heads so that the gaskets will "catch"?

If I may ask what did you do last head gasket change?
 
I took my heads to get milled they shaved them down .005 each side. Then on the block I used a razor scraper to get the old gasket crap off and left it smooth as I could get it. I might have used some sand paper on it, but only to chew up the gasket material, not flatten the deck.
 
Yeah, like i said in my first post, the 400 grit was really good at getting the left over gasket material off. It would get dirty and useless really quick, but then after a while, I would rub the same point on the block with the paper, make about zero change, and the grid on the paper would just get ruined fast. So, I'm pretty sure that 400 grit is relatively safe to use on the block, but I was interested in what other people thought worked well for them.

btw, i used a flat razor initially as well.

thanks for the feedback. :)
 
Get the block decked and trued to crankshaft centerline and ask the shop for a finish fora set of composite gaskets, they'll know what you mean since most rebuilds require this on later model motors, there is a specific micron finish but I've always asked for a finish for composite gaskets.
 
That sounds like a great idea. I never thought of having the block ballanced in regard to where the heads sit on either side. How much does something like that cost?
 
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