Boxerworks.com

Jean wrote:Diaphragms: Should not make any difference at idle. The BING book says to change them every 2 years...

I've never known ANYONE who does that. I wonder if there's anyone out there who has?

Your 5 years is a good number. Anywhere between 4 and 8 is probably good, but lots of people change them when they fail: every 10 to 30 years.

Yeah.
THAT's how I arrived at the 5-year number...I found the hard way the 15-year change interval wasn't good enough.
(Which relates to the 2-lane-pass story.)