250 Crank Pin Bearing

[LaceyDucati]

Hello Matt

You are correct about sleeving rods, there are issues and it will normally weaken the rod. The picture eldert posted was actually mine and I won't go into that particular issue again

That said sleeving can work if there is sufficient parent metal and sometimes it's needs must, rather than a first choice. I recently built a 175 crank with a sleeved 250 rod (which is thicker) and a 30/38/18 bearing. The thickness of the parent metal of finished rod was actually thicker than the original 175 rod, so hopefully strength will be fine.

That's interesting the Mach 1S has a bronze cage, is it original? Seems an unusual choice and I've not seen it's use as a rod bearing cage before. Is it machined and wasted away to a thin section rather than the thick alloy cage section.

Nigel

[double diamond]

Hi Nigel,

I’ll have to have another look at the M1S crankshaft, haven’t had it out for a while. I do recall a bronze bearing cage. I was going to post pictures of it just for posterity. It has a number of curious design features.

Matt

[dsmess]

I located a KZK 32/40/18 bearing. It is used on the Yamaha YZ250F. The 39mm big end could be bored and honed to 40mm. I know this would weaken the rod somewhat. Has anyone tried this?

What bearing clearance do you recommend Nigel? I was reading an article about Godden Engineering. He claims a wider clearance than INA spec of .0015" to 002" or .0025" will increase the big end life. He was talking about Ducati twins, but it should apply to the singles.

Dave

[Eldert]

Dave

that is not a good idea . you go thru the hardened track and the bearing wil run on the soft material .
wont last long .

Eldert

[dsmess]

I think you are right.
I doubt the existing rod's hardening is deep enough after boring out 1mm. It would have to be heat treated again..maybe Gas nitrided.
The INA sheet for their bearing specs 0.5mm deep. They also spec a minimum hardness. This could be pricey. I have no idea. Rather than pour money into a 50 year old rod, it probably makes more sense to use an Arrow or Carrillo rod if you have the $. This is a reminder that proper surface hardening is another critical factor affecting big end life.
Dave