AJS & Matchless Owners Club

It seems to me that most lightweight engines I dismantle suffer from the same rod migration such as this, anyone know why, not a bent rod or worn bearings.

I cannot help... But I am curious... What is rod migration? Can you explain further what the picture shows?

I too am very curious ?? I have never heard of Rod migration. I would be interested to know what that means. I can't see anything amiss in that picture.

Looks like the flywheel is rubbing on the conrod.

Well there seems to be wear on the side of the rod suggesting it has migrated across where it should not.

Big End wear most likely would allow the rod to move more in the flywheels , especially if you have seen it on multiple engines it unlikely to be mis-assembly , plus perhaps small end wear. i Assume you don't just reassembly the engine when you see it

It looks to be around 5 (10?) thou worn off the side which could be due to rock in a worn big end. That would produce a lot of steel dust going round the engine.
Is the conrod worn on both sides?
Does it extend right down to the big end eye or is it only as far as the perimeter of the flywheel at TDC?

The big end has done about 4k miles and has no discernible wear and yes, it is worn right down to the B/E eye just on the drive side, I am going to fit a new assembly as this is obviously compromised. It is odd as I have seen this same phenomenon on several engines, always to the drive side. Bush is fine and so are the bearings on the drive side, the rod when centralised in the crank is bang in the middle of the barrel aperture. The magnetic plug has done a good job here along with the felt oil filter thankfully.

A conrod can tend to 'walk' to one side if the big end outer bearing race has been poorly lapped after fitting so that it has a very slight taper. Perhaps if it has happened to several bikes it was down to poor workmanship or a worn out lapping tool.

Steve

Ohhhh... The two shiny strips on the side of the con rod are wear from rubbing on the flywheel. When I saw the picture I imagined the rod had been machined on the sides.
I didn't realise the little end got that close to the flywheel... Eeeek.