Re-shimmed the valves again the other day while the cam belt was off changing the water pump. 2 exhaust valves down to 0.1mm clearance (spec .28-.38) 2 more down to .15, a couple at .25 rest were in spec. Funnilly enough, the most worn valves were on cylinder 2 and 3. 1 & 4 were fine. Inlet valves don't seem to suffer regression. I didn't keep records of milage last time but I suspect probably about 30K miles since last re-shim.
Hopefully I'll have another engine going in within a couple of weeks as this one is still suffering mild overheating even with the water pump now done. Suspect warped head / gasket or hairline crack in block - almost certainly a direct result of water loss from pump. I'll keep records of mileages and initial valve clearances on this one to give a better idea of wear rate.
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If at first you don't suck seed, try drier grain.