Creeping Mains

[Chessiegolf]

I thought the article '50 Years On' in this month's Jampot was excellent - well written, light hearted and informative.

In the article Bob Lawmon describes the phenomena of Creeping Mains, apparently a common occurrence with AMC twins. During the strip down on my 1960 G12, the repairer said that he had noticed that the drive side bearing had contacted the flywheel and worn a grove in it. The main and centre bearings were being replaced together with the crank being reground and balanced so I didn't give the rubbing too much thought.

Having now learnt that the twins are prone to this, should we be concerned? Bob talks about the crankcase getting hot, does this mean that if a bike isn't thrashed there is less likelihood of the bearing creeping or it perhaps something that is inevitably going to happen after x miles?

Does anyone have additional knowledge about creeping mains and what, if any, are the early signs?

[Groily]

I was also very interested to read about this creeping mains syndrome in that excellent article.
A new one on me, too.
On the rare occasions I've needed to replace mains, I've found the outer races (which should be peened I think) are quite hard to get out, even with plenty of heat and a good slap on the bench.
So I'm not going to worry about it because 45 years with my Mod 20 tell me it's not inevitable, although of course I don't doubt that it could happen. Probably will now!

[John Jarrett]

I have a couple of bikes where previous owners have modified the crankcases to bolt the outer races in place on both sides. Pretty extreme as it requires a drilling through the crankcase to enable it to be locked by a nut on the outside. The outside of the race is machined to stop it turning when the bolt is in place. The bolt itself has a shallow round head to engage with the circumference of the machined area in the outer race. On the drive side it requires the inner chain case (alternator) to be modified on the mounting flange to accommodate the bolt but it is hidden from view. On the timing side it's in the 6 o'clock position and case be seen externally but isn't too obtrusive. Hope that gives enough to visualise what I am trying to describe without pictures.

[John Jarrett]

I thought it would be helpful to post some pictures to explain my previous post... drive side first.

IMG_20201206_141215a.jpg

IMG_20201206_141259a.jpg

[John Jarrett]

... and timing side.

IMG_20201206_141253a.jpg

IMG_20201206_141228a.jpg

you can see that there were a couple of attempts to get the machining correct on the outer race.

[Chessiegolf]

John,
Thanks for the self-explanatory photos, I don't know now if I should be even more worried if that's what a previous owner thought was necessary. Presumably, the races are normally only kept in position by being an interference/pressed fit?

[Stan Palmer]

Hi John. Do you know the history of those cases. I ask because I have seen Triumph engines reworked for production racing and they go to endless trouble to eliminate known trouble spots. There would only have to be a suspicion of a problem for those boys to take pre-emotive action. And it does look a nice job.
Stan