Lapping cylinder heads: grit?

[Rohls]

Hi All!

So my r90/6 has had an exhaust leak blowing out from the head gasket for a few hundred miles now. I took the cylinder off and was hoping to lap the head and cylinder's mating surfaces with some sandpaper. I have access to a granite slab to use as a flat surface. I also have new gaskets.

What grit sandpaper should I use?

Thanks
N!

[George Ryals]

Don't go any finer than 320. Any finer won't leave enough "tooth" to grip the gasket.
How coarse you start with depends on how bad the gas leak cut the head and how much time you have to rub the head on wet or dry emery paper.
I would take it to an auto machine shop that rebuilds modern engines (aluminum heads). Take the cylinder too if the liner is cut a little too. Have then measure everything so when they are finished on the surface grinder or milling machine and re measure, you will know how thick of a shim to put under the cylinder so it still matches the other one in compression. I know .5 mm shims are available for '74 cylinders.

[Duane Ausherman]

I usually use 240 or 320 grit wet-n-dry. The intake is in the way of it laying flat, so you must let it hang off the side. It won't take very much to get it flat. I highly doubt that you need to add a shim. Normally, those heads don't warp much. Oh, wait, my experince is with not very old machines, so maybe they do warp a lot after some 38 years.

[Rohls]

Thanks! I'll try the 240/320 sandpaper method as I was hoping to keep from going to a machine shop rather staying on my own garage. Thanks Again!