Cylinder and cylinder head block prep.

[Guest]

I'll soon be making the attempt to put my cylinder block and head back on my 850.

Here's the question. The guy whose doing the valve grinding and cylinder honing is recommending that I scratch up the surfaces of the cylinder head and block that fit to the head gasket so that there will be a better gasket seal. He recommends 80 grit sandpaper.

I know he's probably right, but the thought of scratching up those nice smooth surfaces is hard to take. 80 grit sandpaper will leave some real scratches!

Is this a common practice? Recommended? Different ideas? Thanks.

[Dan Ruddock]

Waist of time just make sure it is clean and flat and your good. Dan

[Nessism]

Waist of time just make sure it is clean and flat and your good. Dan

Agree.

You want to make sure all the old gasket crud is removed and there is no oil. Scotchbrite works well but 80 grit is way too rough.

[Biker326]

no to the sandpaper

I did that project twice to get it right. The second time I used a scotchbright pad and it worked beautifully; I highly recommend it.

[bwringer]

The mating surface on my cylinder head was horrible. I had a machine shop skim off the teensiest possible sliver of metal to give a fresh surface.

I forget how deep they had to go, but it was only something like .003 inch (furshlugginger machinists still think in inches around here...). Not enough to alter compression, but enough to expose fresh metal and ensure it was flat.

The machined surface did have a little "tooth" or roughness from the machining tool - it wasn't perfectly smooth. And it sealed perfectly.

I wouldn't use sandpaper by hand, but you could very carefully and very lightly use sandpaper as long as you had a way of making sure that things stayed flat.

I use Scotch-Brite pads to clean up gasket surfaces all the time. You just have to make sure you quit before you start removing aluminum.

[49er]

Mark
Check your head and cylinder surfaces with a straight edge. If you can slide a .003 in or greater feeler gauge under a gap, you should have them re-surfaced. Rather than machine the aluminium which leaves the roughness bwringer mentioned, you can tape some sheets of 600 wet and dry sandpaper to a surface table or a sheet of glass and work the surfaces across it in a figure of eight patern. Reverse the direction of the patern every minute or so.
This helps to keep the wear patern even. Recheck with a straight edge regularly.
If you are using a composite gasket such as the Vesrah ones, you can get away with some warpage on your surfaces, probably in the region of .001-.002 ins.
The key is to remove all remnants of old gasket and not have excessive gouging left on the mating surfaces.

[mark]

Thanks for the input guys, I REALLY appreciate it. Block and head surfaces have been measured and checked out at the machine shop and are said to be perfect, so for me its just a matter of surface prep. Think I'll avoid the 80 grit. Thanks again.

[Footy.]

The type of head gasket is important when considering the surface finish. This is a good article that describes the problems.

http://www.babcox.com/editorial/us/us80222.htm

I recall the Vesrah head gasket set i used to rebuild my GS750, had a multi layer "metal" head gasket. The coatings on these need a much finer surface finish.

I would say the finer the surface finish the better with any kind of head gasket. I think the idea of a rough finish helping the gasket to seal, "seems" to make sense, but has adverse effects in practice.

Footy.

[mark]

The type of head gasket is important when considering the surface finish. This is a good article that describes the problems.

http://www.babcox.com/editorial/us/us80222.htm

I recall the Vesrah head gasket set i used to rebuild my GS750, had a multi layer "metal" head gasket. The coatings on these need a much finer surface finish.

I would say the finer the surface finish the better with any kind of head gasket. I think the idea of a rough finish helping the gasket to seal, "seems" to make sense, but has adverse effects in practice.

Footy.

That article was interesting. I'll keep my surfaces as smooth as possible I think.