850 base gasket

No need for one unless one was there before, if one has been fitted then if you do not fit one and you do not get rid of the unworn section at the top of the bore then the top ring can be broken when its next run. I would use Loctite 518 as a sealant. 

Wise comment above.

When I rebuilt my 73 850 like yours there was no base gasket. I had a rebore and new pistons and fitted a base gasket just to be sure of no oil leaks.

There are write ups about the change in compression but it's  all down to you.

Good luck.

I've always fitted a base gasket, simply because back in the early days I followed the parts list. The NOC 'Service Notes' (I believe) indicate that the gasket was used instead of jointing compound as too many owners were blocking the oil drain with the stuff.

What worries me about the use of a gasket is that there is no way of re-torquing the through-bolts after the gasket has compressed without removing the head....I do this after a rebuild, but it means throwing away a head gasket with a hundred or so miles on it.

Each time I do it, I find that the through-bolts require extra tightening so they can be giving little support. Maybe the big 850 block hardly needs them and is adequately clamped from the centre fittings ?