I don't have the exact measurements, but it was definitely rubber shrinkage for cold blowby. IIRC, warm rubber plunger in room temperature cylinder - tight. Room temperature plunger in room temperature cylinder, average. Refrigerated rubber plunger is basically unusable in a room temperature or slightly warm cylinder - and borderline with a cold cylinder.
The copolyester seems to have a different coefficient of expansion than the rubber plunger (I know this may be a "duh" moment for some), but my brother has an old polycarbonate AP that is still going strong and it's a couple years older and doesn't seem to be affected by variations in temperature as much.
I looked up thermal expansion coefficients: polycarbonate and copolyester are similar, with polycarbonate actually a little higher. Not too surprisingly the rubber plunger (thermoplastic elastomer) is about twice the value.
It's hard to say why your brother's AP is still doing well as there could be a number of possible reasons.
Instead of heavy Pyrex, how about an AP based on a stainless steel vacuum bottle? Make it taller and add a valve closure at the bottom and... it would be too expensive!
By the way, I had originally estimated my AP use based on time (16 months X 2 cups/day), but when I looked at the number of filter papers I've gone through (350/pkg) it appears the number of cups is closer to 750. I started having problems about 100 cups ago and the crazing appeared evan longer ago. Plastic.
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