BigEnds

[Stuart Jenkins]

Hi there,
Are the g80/18m short stroke 2 piece bigend crankpins generally robust (1" diameter)? Is it only the long stroke 1 piece crankpins that have a habit of breaking?
Thanks
Stuart

[dave16mct]

For 1956 the CS had a shorter stroke and was fitted with a larger racing type big end and steel flywheels. It was very robust. AMC always fitted a 2 piece crankpin. Only in the 60's and early 70's was a 1 piece crankpin made by Alpha Bearings. It was just OK in the 350 if fitted properly and not stressed but many broke when the flywheels were'nt chamfered to suit the radius on the 1 piece pin.
Dave

[g80csp11]

I remember the 1 piece Alpha BE , mine broke luckily about 4 miles from home on my 46 G80
In the late 70's thats all that was available even from Russell motors
Bike limped home slowly with the pin retained by the flywheel ( not the best advise to keep riding )
Alpha bearings and other patterns now are 2 piece like the original

[56G80S]

Similar story for me, also limped the 2 miles back. My current pin is a later Alpha "proper job".

Johnny B

[clive]

I managed to get my double adult combo, fully laden with kids and camping gear, about 15 miles with a broken one piece pin. The vibration was "interesting" and at one point the cap on the premonobloc carb fell off quickly soaking my leg in petrol splashing out. We spent about 20 minutes vainly looking for the top beside or on the road. When i got back to the bike i noticed it perched between the swinging arm and mudguard.

[RichardS]

Interesting anecdotes, I had an Alpha bigend break in a Vincent (engine) in my NorVin about 1980 - I came across the souvenir remains quite recently. They swore blind at the time it was the first they'd ever known.... I replaced it and sold the bike not long after.

[g80csp11]

I have to say that Alpha bearings now are about the best you can get , I was talking to them only yesterday .
Its not easy to make a BE assembly correctly . tolerances are critical , machining radius , material etc..
Unfortunately the whole engine is built around the BE and is that last thing you want to fail

I took a old AMC6 BE apart the other day , worn out and sat for a long time rusting in one spot . Id always for Alpha bearings

[56G80S]

I ought to add that my current Alpha BE only replaced it's predecessor because I was having new axles fitted. The Alpha BE that was in had done hard service and was still useable but while the whole was apart, quoting g80csp11, "the who engine is built around the BE and is that last thing you want to fail" so it was changed.

Johnny B