Scooterotica

I have a black meceur crank in and one kicking about. When I measure across the top the conrod the measurement is 17. odds mm and the other is 21.odds. The reason I ask other folks opinion is there is alot of side play in the area where the gudgeon pin goes through to the extent im thinking the small end bearing will be able to move to one side throwing the piston out of line possibly causing piston slap or worse.




is there a reason these two conrod sizes differ?

Small end eyes vary for a number of reasons. Originally, with standard cranks, the differences related to how the rod was aligned; at big end or later by little end and shims, however, MEC race cranks should all be aligned by shims at the big end, meaning that the small end no longer aligns the rod, As such additional space here (small end eye) normally equates to better lubrication of the bearing.

BUT you are right in being concerned about the standard bearing being able to run off centre to the rod and wear badly, which is why many people run wider Jap small end bearings that are kept aligned by the piston bosses, with the rod remaining central because of the big end shims. The fact that the rollers of the small end might be wider than the small end eye doesn't matter, the rod always runds true to the rollers, unlike if a narrow bearing moved sideways and allowed the eye to run on the edge of the narrow rollers.

Luckily enough for you, lots of UK dealers will sell you a wider Jap small end bearing for this very purpose. MB certainly sell them but I'm sure many others will too. One thing to watch out for, sometimes the bearings are a little tight on the piston bosses so, when I've come across this, I've worked the edge of the cage on a flat bit of wet and dry just to ensure that it doesn't expand with heat and put too much side pressure on the piston bosses (I have known pistons to crack!).

Adam

thanks for your reply. It is the wide bearing thats in there. My paranoia will no doubt lead me to change it over.

Wow, then that piston is pretty wide between the bosses!

Adam its a TS piston. Im led to believe this mec crank has a forged rod. i still dont understand why the conrod eye width is 4mm less that than the one which will replace it. Im gonna have to bash on with work and try and get the crank swapped over before thursday night. friday am departure for scabby.