Hitchcocks Forum

I pulled a motor apart this morning that was running ok on Sat, but it had a strange knocking noise that didn't sound like a failed floating bush but I couldn't believe my eyes when I found the crank pin completely broken in half through the crank pin oil hole. One thing with Enfields when you think you have seen it all, there is another surprise around the corner

Hi Norm, I had a 535 crank to repair after doing that a few years back. I also know of at least one other machine that also broke the crankpin, so it's not unknown. A former machine of mine which spawned the 'Asbo' bikes had a crank mainshaft runout of 1.5mm !!! [yes, 1.5mm really] when I stripped it to sort out a chewed drive side roller main bearing, so presumably a broken crankpin might have been on the cards for that machine if I hadn't stripped it when I did. Regards, Paul.

Norm, which engine was it?

Hi Paul, what threw me was that it was running and revving fine, just had a strange knocking noise that even had me thinking it was something in the gearbox. Pulled the cases apart and a wheel was sitting in each case.
Thack it is a mid seventies 350 Super Star

Paul what I believe was part of the cause is the fact that the oil feed hole to the floating bush came up directly under the thrust washer on the timing side, not in the centre of the bush, as this was external of the conrod it was being subjected to flexing forces causing it to fracture right across the oil hole drilling

Sounds like your timing for disassembly was impeccable!

Hi Simon, I knew it had to come apart, first thing I noticed was the inlet valve had lightly touched the piston, why? then I checked the big end and it felt ok so I then had to split the cases

It is remarkable that the big end lasted any time at all, with the oil hole over to one side like that. I guess the inlet valve touched the piston due to the cam timing being able to wander, with only the recess in the inside of the flywheel to take up the drive to the timing side. Like Simon points out, you got there in the nick of time!
Regards, Paul.