Yamaha SC500 big end bearing failure ?

[YZB 500]

Hi All,

Has any one out there experienced big end bearing failures in Yamaha SC500s. I have an SC500 in a YZ360 frame running at the pointy end of A grade.

Easter weekend I had a failure, the rod bearing and crank pin were only 6 meetings old.

It was a 32 degree plus day but I failed in the second race of the day. ( AM ) I was running silkolene at 25 to 1 on 98ron, 440 MJ jet. the appears to be jetted correctly.

No signs of heat seizure on the piston. I have since modified the oil gallery in the rod and changed back to R30.

If any one has experienced a similar fault and has some sound advice please post it.

Regards Gibbsy

[Freakshow]

Sounds weird the main bearing went, if it was lean etc i say it would happen up the top first.

Maybe the crank wasnt put  back correctly if it was only 6 rides old.  Maybe the crank was pressed too tight and its been rubbing away till it let loose ?  picture would help
was it a genuine yamaha koyo bearing or from a bearing shop ?

[Rossvickicampbell]

not good news Gibbsy - hopefully not too much on the gas when it happened??

[YZB 500]

Hi gents,

more info, mains did not fail. its the big end bearing.

Crank was correctly set up to O.M.E. spec.

Rossco i siezed it at Narrogin second to last corner, off the throttle.

I have modified the rod, opening up the oil lube gallery round hole, brought a cut off die grinder, using a 2" disc cut down to 1.1/4" for a smaller radius. i cut a slot similiar to more modern rods, radius all the edges.

If it works i should not have another bearing failure, if it dosen't then you will see a posting for wanting SC500 engine cases.

Cheers Gibbsy

[Tossa]

Point one:  only a kiwi would put this in the Foto Forum section!!!


Point two: loose nut on the throttle!!!

Point three:  still a vacancy in team Tossa for you Paul!!haha

Barry

[YZB 500]

Hi ya Tossa,

What do you mean loose nut on the throttle, i have no problem staying on the throttle its the bike i have trouble stay on!

Hence the BSA 50MX Knobbly dents in freds old tank.

Oh would you like me to shear barbara with my next time out, as i have always said, We Kiwi's dont shear our sheep, go find your own!

Regards Gibbsy

[Tossa]

I'd like it noted i made no sheep jokes!!lol