If it is too tight, that might cause the temp related problems. I sized mine down years ago while on the phone to Steve, it has been perfect ever since in all weathers :-) That is in my 25mm 80. Also spot on in my 25mm 77, but that is .22.
Must get that 80 out and have a few shots, thanks for that reminder. Laterz
Target tune seal fitted, makes power very well. Not letting grease past like the old one either