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Location: Cheam, Surrey

Sun Jun 05, 2011 3:18 pm

Been in a couple of years, thought I had holed the piston on the A3M last Monday as bike locked

on initial investigation magnet had come loose on the flywheel with what looked like a twisted crank

further delving showed the below which probably caused the magnet to come loose

350 rod on sil webs, not welded, any ideas


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Sun Jun 05, 2011 3:58 pm

pin too tight when pressed back together with the yam rod and has cracked across the web ?

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Sun Jun 05, 2011 4:35 pm

I like the Yam rod conversion and have had a few done. I never thought to ask about what pressure it took to press the pin in and whether or not it took more pressure than was expected on putting the pin in but it certainly looks like the tolerences didn't match too well on this crank. Probably just a one off :(

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Sun Jun 05, 2011 4:40 pm

yes but its been in for 2 years or so apparently- would have expected problems before now.

Scootering since 1968.

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Sun Jun 05, 2011 5:33 pm

Cool, you don't see many like that, one for the mantle piece.

My money would be on a metalurgy defect. If the pin were too tight it would have split around the thinest point (between the pin and the outside edge of the web.

This crack has been there for a long time and propogated with stress, time and use unlit it has let go. Most webs are forged but it might be possible that they are a cast SG material (like the gearboxes). Inclusions are far more likely in casting but a poor quality piece of metal could have been used in the forging process too.

When you take the webs apart look for inclusion marks on the machined surface in the pin hole, they look a bit like small black, corrosion pit marks.

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Mon Jun 06, 2011 5:48 pm

split the web to inspect the fracture faces

i would bet that they would show a fatigue crack with possible "beachmarks" showing the crack propogating to a final brittle fracture. should also indicate the exact initiation site which may be a minute defect.

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