Why would I lose ONE big end

new 1980 bike; bought with noisy engine.
I stripped it to replace the front cam chain guide, after finding the top strainer full of bits. The sump filter was breached.

On getting down to the crank, I found the right hand (as in clutch side) big end bearing shot. I mean really shot; like 3/4mm up and down play.The left hand side is smooth and well within spec. The little end isn't worn either.

Funny thing is that the engine has original std yam pistons, with rings that seem unworn along with an unmarked bore. All the other bits seem in really good condition.
Where I have had old brit bikes with knackered roller big end bearings, then the main bearings are rusty from lack of use and moisture. None of that here either

Obv, I have no idea on the history of the bike but the last few MOT's show no little to no mileage and it's likely spent it's life on the sidestand being run up.

Question is .. what else should I look at .. the oilways seem clear and everything .. I am just surprised it's knocked out a roller on one side.
Is there an oil supply issue I should be aware of ..

I think I will replace the one knackered side, but take a look at the good side at the same time.

I stripped the engine completely.
Every bolt/nut cam off without stripping or problems
Every oil seal was soft and pliable
The bores were on standard Yam pistons unworn and the rings likewise
The valves hardly needed a regrind
The clutch plates were virtually unworn too.

The front camchain guide was, however, buggered. The remains of the rubber were flopping about in the engine and the chain had worn two grooves in it.
Obviously the one crank bearing was shagged.

I replaced everything obviously screwed, and some parts because I was there, and re-assembled

My question is .. given the state of the bore and everything else .. then what is the reasonable life expectancy of roller big ends and this front cam chain guide - are they really prone to wearing so much faster ?

The bike had no history, although recent MOT certs show almost no mileage in past 4 years. 34 year old bike though

I guess there's two ways to approach this:

Analytically - Perform a proper post-mortem failure analysis on the failed components, including hardness testing. Would also include researching past owners and lubrication history. From your pics, I haven't a clue.

Statistically - Failure of this type is very rare, but someone's going to draw the short straw. You're the lucky one who took one for the team.

I understand your reservations. The collection in your stable shows a good respect for machinery. Most of us are understandably jittery about the various clinking sounds, but these machines continue to plod along.

The front cam chain guide is a different story. This forum is full of failure stories of that part. Using the more expensive factory official part may reduce that failure risk...

The big end is likely an anomaly, esp with no history on the bike.
Like I said originally, I did wonder about the starting/revving/idling on the side stand for years and years. It looks like the oil would pool over the other side of the engine leaving only spray lubrication..

The failure of the cam guide is the one that bothers me.
The cam chain that I took out wasn't that much longer than the one I put in.
I can see that there are folks on here changing those after 15,000 miles ....

Like 2many sez front guide failure is more or less normal. It's an age thing. The RH big end failure does seem to be an anomaly. I suppose a "chunk O something" could have snuck into the race and started a downward spiral. Or a manufacturing defect in the bearing. Holed pistons are normally on the RH side but that's usually lean jetting from an aftermarket pipe thing. When checked the RH side is often found to be running hotter than the left.....



There is some disscusion of this temp difference here go a good bit down the page, discussion starts after you see the pic above.

I lost a right side crank bearing on my original 1981 motor. my guess is that my bike was parked on its sidestand for many years outside(exactly how I found it), with the right side bearing above the oil level. condensation inside the case caused little rust flecks, which turned into pits later. once I started putting miles on it, the pits caused the bearing to wear faster and then finally fail.

Now that's some thinking there james...hmmm very possible

In an article in one of our old Club newsletter there was a case where one of the main bearings failed after a few klm's, (cant remember how many but more than one would have thought), After stiping everything down it was found there was no hole bored, (from the factory), in the case for the oil to squirt onto the bearing. The other side was drilled and ok.

Did you check that both oil passages were clear? Squirting water through them when the cases are disassembled is a good way to make sure.

Skull,
I have just such a motor. The right side oiling hole is not drilled out. Bike had 45,000 km on it when I bought it.

No damage found when stripped for and upgrade to a 270 crank build.

It wasn't until it was back together that I came to realize I should have drilled it out.

Donor engine that I had gotten for the crank build had the same no hole on right side situation.

Crank on my 78 engine was in great condition before I upgraded to new rods and 270 pin.

Previous owner was a true biker and enjoyed years of trouble free service and was what I would call, easy on his equipment and maintained it every year of use.

There didn't appear to be any blockages, I ran an air line through it all.
The bearings seemed fine - on both sides - good enough to go back in certainly.

I am very familiar with the state of old british bikes, left standing for years and the condensation eating into the races etc.... none of that here.

I am just scratching my head as to why just that bearing was shagged; hopefully it's just an anomaly.
I just worry that I missed something, on the rebuild, that killed the bearing and is going to cause me to strip it again shortly ..... while I can do that (probably faster this time) I really don't want to.

If the cam guide is an age thing .. then I'll be dead in 35 years, when the guide is next due for replacement *-)