Hi Steve . If your valve springs were trying to pull the cam around it sounds like you may need to turn the flywheel over one more time so the cam lobes are on the opposite side when you set the tappet clearance. the dot on your cam should be up when the lines are lined up with gasket surface. Try this both ways as you put it back together to see how it changes tappet clearances . I just rebuilt mine and dont remember it trying to turn away from TDC.

I agree, I don't remember mine pulling away from TDC either.

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

My xt600 does that a bit... I woldn't worry.
Just verify you're at correct TDC

Apologies in delay posting back here, the last weeks been hectic! The piston ring end gaps were too great so they need replacing, cylinder needed a hone too so that's in the engineering shop, unfortunately they do motorbike stuff on Fridays and I only got into them Friday evening. They said they'd try and fit it in but didn't sound optimistic. Anyway, that should (fingers crossed) sort the smokey exhaust and high oil consumption.
Checked the valves again, this time pouring petrol into the exhaust and intake chambers (as opposed to turning the head upside down and putting petrol into that) and could see a slight leak coming through. Not even a trickle, you'd just see the edges of the valves getting wet after a while. Gave them a light lapping and they seal good now. The valve seat width however was almost 2mm whereas the manual says just over 1 max IIRC. I don't see why it wouldn't work or if it was cuase any problems(possibly the valves are now further into their seats so the stems are closer to the rocker arms?).
When I get the cylinder back I 'll get it back together and test it all out. Anyone know of a good UK online shop for piston rings? Local motorbike shop is good but can be fairly slow for getting parts. Don't have a good source for that kind of stuff in Ireland. The piston has 0.50 on it, is this the first oversize?

Yes, .050 is the 1st oversize. Good that you did solve the slight leak. The seats really need to be 1mm wide and can only be narrowed by a seat cutter. Your engineering shop can usually do this then lap them again. The bike would probably run but with a wider than usual seat, sealing may be compromised as the valve pressure on the seat is spread out over a larger area. The narrower seat also helps to punch thru any residue. Much easier now than pull it down later....

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

Engineering shop rang me this morning, cylinder needs rebore, oversize piston. Gonna drop in the head and get the valves done too. That's me eating tins of beans for the next few weeks! Ah well, I was half-expecting it and at least once it's done it's done properly.

It will be money well spent...good show.

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.