Topic: Early VIN CB500K0 main-shaft needle roller bearing failure

[AshimotoK0]

My 3/71 500K0 bottom end (less than 1000 VIN) was in great shape when I stripped it down but the cage inside the main-shaft needle roller bearing had started to break up. Bearing is P/N 23931-323-004

I managed to get a shot of the surface on my digital microscope to show what I found on the shaft. The cage on the needle roller that I got with a replacement shaft is steel but from memory it was some form of alloy on the original. I will try to find the old one to confirm. I reckon Toyo changed the internal construction of the bearing but Honda still gave the new part the same P/N as it was still being used on the K2 model.

[754]

Is that a sleeve ? A lot of needle bearings, had an optional hard sleeve that you can mount on a softer shaft.
Not sure if that is what it is.

[AshimotoK0]

Is that a sleeve ? A lot of needle bearings, had an optional hard sleeve that you can mount on a softer shaft.
Not sure if that is what it is.

No ..the bottom end had never been apart as far as I can tell. It's some form of micro crazing to the shaft surface...  I assume caused by the needle roller bearing starting to fail. The rest of the bottom end was in really nice shape. I gather this was a known issue on early CB500K0's (mine has squared off top crankcase at the front) See pic

[754]

What I am asking I'd the bearing surface part of shaft or a sleeve ?
 It's a manufacturing method  to sometimes use a sleeve.

[AshimotoK0]

What I am asking I'd the bearing surface part of shaft or a sleeve ?
 It's a manufacturing method  to sometimes use a sleeve.

Sorry its the surface of the shaft ..there is no sleeve

[jaytee-nz]

I'm still a bit confused (not hard for me) about what your actual question is ?
Are you concerned about the bearing material - alloy versus steel, or are you asking if the end of the mainshaft is still usable given the wear marks shown in your photo ?

[AshimotoK0]

I'm still a bit confused (not hard for me) about what your actual question is ?
Are you concerned about the bearing material - alloy versus steel, or are you asking if the end of the mainshaft is still usable given the wear marks shown in your photo ?

I am saying I found it and anyone with an early 500K0 should check out that bearing because it was prone to failure. A UK engine builder told me he's experienced this with almost every very early 500K0 motor he has rebuilt. I have a replacement shaft & bearing which I am fitting thanks.

[jaytee-nz]

Ok, thanks for clarifying this.
Just curious - how many miles had your bike traveled before you stripped it down ?
I just rebuilt my K1 engine and the replacement bearing was definitely steel.

[754]

40 years will do that..
 The reason I was asking was its easier to fix.
 But also if you turn and grind shaft down, if there is enough meat, you can save the shaft