Scooterotica

Just reassembling my engine and when I took it apart I noticed the gap between the conrod and piston where the small end bearing goes was quite large. There where no shims as it was an origninal series 1 crank with the bearing built in. I've fitted a new crank and have noticed the gap with a new small end bearing is too big and rather than using shims I have bought one of the wider small end bearings from MB. I've just offered it up to the inside of the piston and have noticed on the edges either side where the gudgeon pine goes through and the bearing would sit inside the piston are a couple of "bobbles" in the casting, one on one side and two on the other. Because of these it is very awkward to get the new bearing in between. Is it ok just to file these off so the bearing fits the gap ok. They are only very small??

The GPM piston was designed with the bobbles as you call them so it could run without shim washers fitted where shims were fitted it was usual to file them off....So get the file out....

I personally wouldn't run the wider bearing. Small ends are self centering in use. Just get a decent small end bearing and it'll be fine.

Special X wrote:I personally wouldn't run the wider bearing. Small ends are self centering in use. Just get a decent small end bearing and it'll be fine.

I've fitted a wider one on my Mugello 186, a DT175 from MB I seem to recall. Is this a bad thing then? Is any damage likely to result?

sideout wrote:

Special X wrote:I personally wouldn't run the wider bearing. Small ends are self centering in use. Just get a decent small end bearing and it'll be fine.

I've fitted a wider one on my Mugello 186, a DT175 from MB I seem to recall. Is this a bad thing then? Is any damage likely to result?

Personally I think that hoping that both the conrod and small end bearing will self-centre is pushing it - if they don't, a disproportionate loading is going on the end of the needle rollers - I fitted the Yamaha small end to a 186 too for exactly the same reason. That way you have the confidence that the small end is always in full contact with a bearing irrespective of end-float without the need for shims.

Special X wrote:I personally wouldn't run the wider bearing. Small ends are self centering in use. Just get a decent small end bearing and it'll be fine.

When I put mu mugello together back in 2006 I also noticed that the small end bearing 'floated' a fair bit. I contacted Camlam who sold me the muggy kit and they told me that it didn't matter as the bearing would just float back and forth. They said they only used the narrow bearings and hadn't had any failures.

A good few thousand miles on and I haven't had any problems with it (well not the bearing anyway )

It may not be best engineering practice but in reality it doesn't seem to present a problem.

Avantone wrote:

sideout wrote:

Special X wrote:I personally wouldn't run the wider bearing. Small ends are self centering in use. Just get a decent small end bearing and it'll be fine.

I've fitted a wider one on my Mugello 186, a DT175 from MB I seem to recall. Is this a bad thing then? Is any damage likely to result?

Personally I think that hoping that both the conrod and small end bearing will self-centre is pushing it - if they don't, a disproportionate loading is going on the end of the needle rollers - I fitted the Yamaha small end to a 186 too for exactly the same reason. That way you have the confidence that the small end is always in full contact with a bearing irrespective of end-float without the need for shims.

Totally agree here I ve done the same with a RB 250 as the conrod could ride over the bearing face to put a disproportionate loading on the end of the needle rollers. Just precautionary really

Remember though it is the big end float that can cause major problem if not correct.
ie a masschelli crank has no big end shim so with certain pistons you do need to shim small end.

So no problems with a shimmed crank and wider small end bearing???

eadmr04 wrote:So no problems with a shimmed crank and wider small end bearing???

IMO none whats so ever m8
Belt and braces

The wider bearing won't go in a mugello mark 1 piston without filing or grinding the piston boss - not a great idea.

Unless of course the MB item is in between the normal and wide ones.