Why we call it "inspection"...(2018)

'74 Roadster, 43,000 miles (of which, the last 31,000 were well cared, not abused, and there is no reason to think the first 12,000 were different). No unusual noise whatsoever, and I listened.
Timing side main has failed.

It can happen to ANY bike, at ANY time.

Glad you caught it before it became a catastrophe.

I too am glad you caught it. I would like to know what prompted you to go that deep into the engine to find it. Being there was no noise was there another symptom. I never disassembled an engine that far for an inspection. If there was an indication I would like to know what it is so I can watch out for it. In a month or so I plan start on a '74 Roadster project. Disassembling the engine was not in the plans, maybe I should. Thanks

Most likely fretting caused by bearing race rubbing at the crankcase due to cranshaft undergoing cyclic load.
https://en.wikipedia.org/wiki/Fretting

In addition to replacing the bearing, it's worth considering a replacement of crankshaft bolts (or better still, fit larger fit class bolts as I believe Steve Maney does on his crankshafts).
https://www.accessnorton.com/NortonCommando/crankshaft-bolts.12672/

Well caught sir!!

Was this an OEM factory installed bearing or a result of a rebuild?

As far as I can tell (and I do a lot of industrial forensics) it (the lower end) was all original.

Here is why I began exploratory surgery:

I had seen the came lobe worn through the case hardening upon removing the cylinder. I was going to split the cases to replace the cam. Also, check the rod journals to see if (as long as I'm here...) it needed a re-grind.
And, of course, inspect/replace main bearings. I was a bit surprised to find this.
My theory, as yet, is that the (hard) material from cam failure got into the bearing, caused the initial imperfection, which then eats itself up.

On a positive note, the crank looks great!

concours said:

View attachment 3649 '74 Roadster, 43,000 miles (of which, the last 31,000 were well cared, not abused, and there is no reason to think the first 12,000 were different). No unusual noise whatsoever, and I listened.
Timing side main has failed.

Click to expand...

10000 rpm sideways wheelies?

Worth a read.
Broken link removed

MikeG said:

10000 rpm sideways wheelies?

Click to expand...

Nope, never over 7K on my watch...
and even then, only a near there couple times. The torque seemed to give great results lower, and flattened out near 6.

kommando said:

Worth a read.
Broken link removed

Click to expand...

Thank you kommando. We have the same wall chart from SKF & NTN posted in the room where I rebuild machine spindles. Chicken or egg is difficult now, flakes left marks. I welcome any and all theories...

Flaking like that occurs when a roller gets a bit brindled so every dozen or so revolutions it lines back up on same area and same crank angle of piston banging. Must turns a used dry bearing very slowly many times before able to sense slight tick which is missed if just turned by finger drag or spun. Oil can conceal this with just hand pressure loading.

concours thanks for the pictures looks like a magnetic drain plug for me

olbbeezer said:

concours thanks for the pictures looks like a magnetic drain plug for me

Click to expand...

I have 3 on that bike, sump drain, suction strainer plug (added) and oil tank.

Doubt it is oil contaminant related as spalling/fretting seems to be limited to one bearing and in one area. Unless you wet sump heavy and wet sump often, it is really difficult for any significant large contaminants to get back there as these bearings remain happy with only a misting of oil. Furthermore, roller element bearings are tolerant of contamination. Just consider that this spalling managed to pass from within the bearing without doing more damage.

I doubt it is brinelling (maybe false brinelling). It looks like the damage is localized to maybe 10 degrees BTDC.

I had experienced a similar failure (though on the drive side and I do not recall the orientation) back in the day (1970s) while running a Drouin with 10 psi boost. I attributed the failure to my carelessness in overheating the inner race of the new roller mains or something along those lines when rebuilding the motor for the blower.

Upon your dismantle I would take note of the class of fit of the bearing as well as how tight it was on the main shaft as well as the outer fit in the cases - maybe too tight. Also take a look at the main shaft and inside of the race to see if there were any upsets of metal causing a tight spot.

You may never be able to determine exactly why it failed but most importantly, whatever you do, don't do it again!

Open the picture and check out what can be seen from this one view:

Take a look at how the Red and Green areas compare. There's a fairly prominent dark, presumably untouched ring just inside the edge of the race in the Red oval, whereas the Green oval shows heavy wear next to the crank cheek. If these rings maintain their positions around the entire race, then I'd expect to see some taper to the bearing seat on the crank.
However, if those patterns seem to traverse back and forth across the race face as you go to the opposite side (that can't be seen in this view), then my money would be on a bent crank.
If neither of those, then my money's with Dances about some piece of swarf fouling the race-to-crank surface. From this view, it does appear that the wear may begin to taper off as you move away from the damaged area.

Keep us posted!

Nathan

And the oil pressure trend had fallen by 5 lbs. in 31,000. Rod bearings had left some metal behind.

Is it the camera angle or it the oil hole in the rod being partially obstructed by the bushing shell?