main shaft bearing

What type main bearings do yammis have, C3 or C4 clearance?

This is not specified on the OEM bearing.

Single one costs as much as all the bearings in the engine combined including 2 main ones, if bought in specialized shop, all in C3 clearance.

Not sure about main bearings though, C4 are a bit hard to come by, even in standard sizes.

Also what makes bearing outer ring and housing surface go like this?




Prolongued axial fatigue??

Bike was running completely normal before disassembled.

Had other issues.

Hello 5valve,

Most crank bearings are C3 bearings. These having a greater clearance than a standard bearing.

The marks on your cases and bearing show that the bearings have been turning in the housing during use.

When you reassemble it I would clean the cases and bearings and apply a liquid locking agent ie Loctite make products for this purpose. Like Super Bearing mount etc. Read up on these products to see whats best.

hope this helps


Bearings on crank are certainly C4 which have even more clearance than C3.

(just found out)

It is almost no problem to get C4 6306 bearing, that is 30x72x19 sized bearing, original part on yamaha KOYO 83C072C (SH), used in all 400, 426 and 450 at least till '05.

"Problem" is SH added to bearing type, because this means termicly processed ball running surface and thus longer life, up to 10 times.

So as for now, original or OEM only is advised to use.

Still looking options.

Thanks on turning bearing info, will use green locktite.

Why does bearing turn in housing and does this to some extent prevent complete locking of bearing and cracking everything up by blocking.

On the suzuki RM500's they had a problem where the cases would stretch/fret and increase the bearing bore in use. So a fix was to fit a metal sleeve to cure it.

In this case I guess the bearings will turn because of a few different reasons. Say the bike sits for a while unused. You start it and there will be an increase in friction on the main bearings because the oil has drained away.

Also the cases being alloy and the bearings being steel. Different expansion rates.

I wasn't aware that they used C4 bearings so thanks for that.

I recently overhauled my 01 YZF 426 engine. Fitted a Rod kit , new cylinder , piston kit and replaced the valves and timing chain.The bearings where fine and I didn't replace them. The previous owner kept the aircleaner spotless but didn't oil it. Go figure.

When you reassemble the engine you will notice the crankshaft is a tight fit into the left case bearing. I assembled the crank into this bearing first. I heated the bearing center with a hot air gun , not too hot though. (Also I heated the cases and freeze my main bearings before assembly). You know the expansion thing.That way the crank just falls in. No need to press. I then put loctite on the right side bearing upon assembly.

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