I bought my 1928 JD in April 2012 and the engine was claimed to have had 1500 miles on the rebuild. I adjusted my exhaust valves after about 600 miles of riding and found that the valves had no clearance. The manual says they need to be .08 to .010 so I adjusted them to .010. I discovered that it had the shorter 61ci exhaust pins and then immedeatly got a thudding knock in the engine. We dismantled the engine and found the front cylinder was melted and the top ring was seized to the piston. I think these events are related; or are aluminum cylinders prone to this? Should I switch to forged steel? What do you guys think?