Z1OwnersClub GB

H

Tightening the head cannot affect the cam chain at all or even the crank.

Without the cams in, the cam chain can be dropped out of the way down the cam tunnel where it will sit on the bottom roller well clear of the head, any idler sprockets and the tensioner and is easily fished back out when needed with a wire hook or one of those telescopic magnet thingies you get in the quid shop or cheapo market tool stall.

Without the cams in the only way the chain can stop the crank turning is if it is bunched up in the centre cap.

It's a stupid question but you do have the chain running between the idlers and not around them or on the wrong side of the tensioner wheel ???

Just a thought
Is the motor running hi-comp pistons or had the barrel skimmed ??
Your piston crowns might be just clipping the head or head gasket at TDC with the head tightened and backing off the head nuts is giving just enough clearance for the piston to go over.