Barrels to head

Better that than dished....

Should be flat. As should the head.

Unusual for the cast iron barrel to go out of shape; you might check that your straight edge is truly straight. But have the barrel skimmed flat and level if more than a thou or so out.

For information, the excellent Victor Reinz composite gaskets sold by Andover Norton are 0.8mm or 0.032" thick but they work best when faces are flat.

Surface plate with some wet and dry will soon show up the issue, same with the head. 

All engine barrels (allegedly) distort slightly when bolted to stiff crankcases, don't they?  And again when they warm up.  So how perfect is ... perfect?  What are flatness tolerances?

Gary, A4 sheets are large enough.  Most DIY places sell that size.  I get what I need from Halfords, who keep a decent range of grades as fine as 2500. 

I really don't think trying to flat a Commando barrel with a piece of wet 'n dry on a kitchen worktop is a good idea. I had the impression Ashley was merely suggesting that a quick rub over would give you some idea of what's going on.

But a good straight edge and a feeler gauge will also give an indication of how far out it is.

If you want to do it right, then take the barrel to a good engine reconditioner or machine shop - one where you get to talk to the person doing the job, not just the receptionist. Have them check that the base flange is flat and the cylinders are bored square to it. Machine the top parallel to the base.

If it was ok before you took it apart and you're not that bothered then bolt it back together.

I've just looked at the head gasket picture on the AN website and composite ones appear to be 1mm thick although the one's I had from them last year were 0.8mm. Copper gaskets are typically 1mm thick.

Tighten the cylinder and head bolts as per the workshop manual. And re-tighten after running then repeat at service time. Don't be tempted to over-tighten.