Originally Posted by
22Racer
I usually measure crank pins in inches but here is my take, I rebuild hundreds of engines a year.
Most crank pins are .196-.1965" new (4.98-4.99mm)
I consider only running as back up engine at .194-.193" (4.90+mm)
I also consider rod slop, if the rod is worn this often doubles the slop.
Rex
Thanks Rex,
I was under the impression from GO that a new 21-0600 con rod big end bushing was 5.000mm and a new crank pin was 4.997mm - giving a brand new factory tolerance of .003
How correct that is I don't know, but it came from the horses mouth at the time, so pretty reliable info.
So on a set of basic verniers you would likely see a reading of 5mm for a new crank pin - not 4.95mm
Like you I've measured a fair few new and used crank pins, and to the best of my knowledge all GO cranks from the GXR Series going forward have a 4.997mm pin to match the 5.0mm bushing on the 21-0600 Pro Series con rod used in all recent models (GXR Series, GXII-5RHO, UDP, Champion and GXII Plus).
I like to err on the side of caution with the forces involved inside these little motors spinning at up to 40,000 rpm.
If there is slop at those revs on a single cylinder motor, something is going to give sooner rather than later
