Interesting comments on open vs closed. I've been running closed tin primaries on my belts for 35 years, and belt life seems to be normal. I can't remember mileages now, but I'm guessing at least 30-40K miles. Also, until just recently, they always had some exposure to oil leaking past the "slinger' on engine mainshaft. Finally got the new seals on all the machines. What a difference. Just one man's observations of course. I've always been curious where all this damaging heat was coming from. I mean, we've been running closed timing belts on car engines in engine compartments for ages, with no apparent harm. I can't imagine the temps inside my primary cover approaching that under a car hood. But hey, I'm just a dumb ole shade tree mechanic.