Re: Too wide clearance on exhaust valve,won't close
If the tip of the valve stem gets 'mushroomed', it may bind in the guide(will.).
If the valve stem has enough deposits of cooked oil/fuel/carbon, it may bind in the guide.
If the guide has enough of the stuff noted in the above, it may cause the valve stem to stick in the guide.
Lapping the valve seat and valve sealing surface should not have any possibility of causing a valve to 'not close'. As asked above, how did you lap the two surfaces together if the valve is not closing? They have to 'touch' to lap the surfaces...
Is it possible you have a sticky lifter/support(lima) that is jammed in the 'fully extended' position, holding the valve from closing? Before I blamed the valve, I'd do two things:
1) remove the follower and pull the lifter/support. Check its full length, and then try to compress it. It should be compressible about .030-.050(I don't know exact off the top) after applying pressure constantly for a while. If not, pull another and compare(or swap)
2) use a hammer handle(hold onto the hammer head, and push) to compress the valve spring on the valve in question. You should be able to push the top down at least 1/4" with no problem. Not easy to do, but with a little effort, it should move a bit. Once compressed, let the spring expand, and listen for the 'pop' as the valve seats. If it makes no noise, or you cannot move it, eh, you may have problems. See #1 previous.
tom
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