Forum

[John M]

To cut a very long story short, I have managed to knacker the big-end bearing on my 1956 Bullet engine...............I have two questions, firstly I intend replacing the existing bearing with a roller bearing big end, are there any short comings in this conversion, or any compelling reason to use the standard type plain bush?...............
Secondly, is it practical to attempt this repair at home, I am thinking about truing up the flywheels rather than the disassembly of the engine?

[papasmurf]

I have tried to find a video on YouTube that would help you. However all of them are accepting crankshaft run outs I would not accept if I were rebuilding a crankshaft, especially on a single cylinder engine.
To me 4 thou run out is a mile.

[Bullet Whisperer]

OK, Papasmurf, I have bitten - here's one of mine, certainly good enough not to cause any problems. Regards, Paul. ... https://www.youtube.com/watch?v=xPyCZZ66WQE

[papasmurf]

At least the crank "run out" is in the same place both sides in your video;-

[Tim NZ]

What ever was the cause of the floating bush failure will kill a rolling bearing just the same. Find the cause.


Check the OPR inside the timing shaft....


If you dont have the correct gear to strip and ACCURATELY reassemble the crank shaft you WILL KILL it! Not least if your crank shaft is in need of an oversize crankpin?


IF the crank shaft has been flexing excessively and the 'eyes' in the flywheels have any appreciable TAPER(?) an oversize crank pin will be required.


Nearly every worn out crank-shaft that I have encountered has been in need of an Oversize crank pin, and the fitting of such requires specialist machine tools to re-true the flywheel eyes..

[Leon Novello]

As they say on TV, don`t try this at home.

[John M]

Thanks for the replies, decision number 1 has been made and I will get the big-end professionally fitted, it's not worth the risk of making a pigs ear of it myself as the engine is what I believe to be a very rare variant, a 350 with a pre-1956 500 style bottom end............. I'm more than a little annoyed with myself as the cause of the failure was a leak from the oil filter housing and oil starvation, unfortunately I only noticed once it started knocking..........I changed the oil and re-filled with 225cc of oil, but judging by what I drained off it may have got down to 75cc, a very expensive mistake!

[John M]

Just to clarify, when I said that I changed the oil, that was before the big end went, the bike was on the trailer before going to an event and the oil filter housing failed to seal properly. I do know that 225cc of fresh oil doesn't fix big-ends.

[Chris Tindal]

Out of interest, can you explain the rare variant John?

[Mark M]

John, I'm not sure anyone answered the other part of your question, ie, can you replace the bush? I have come across several failed floating bush big-ends that have been successfully repaired with a new bush alone. Obviously it will depend on inspection but this is one of the overlooked advantages of the bush. But I didn't do the work myself!


REgards, Mark