Why do these cam bearings wear out?

[lingeringstin...]

I've taken apart a Honda and two Chinese 125's recently and every one of them had this exact same problem:


https://s12.postimg.org/uxw79tq0t/111.jpg



They all had the same bearing in the same place worn in exactly the same manner. It hadn't actually caused any real trouble yet because the cam chain tensioner takes up the slack and if you keep adjusting the valves you can happily run for ages hardly noticing that the cam is eating through the bearing.

My question is why it happens. Is this down to bad oil flow in the head, or simply that these engine have no kind of actual oil filter so unclean oil just gets recirculated, or is the cam chain tensioner to blame because it keeps the chain under constant strain?




Looks to me like I could change the Honda style cam bearing to an actual needle bearing by fitting a couple of these:

https://s14.postimg.org/eos4uhc4h/RNA4902-_XL.jpg

https://www.kramp.com/shop-gb/en/149907/515415/0/Needle+roller+bearings+INA+FAG%2C+series+RNA49




But would they get enough oil? I don't know how much flow there is on this kind of head or if the oil would get into the needle rollers good enough for this modification to work.

I suppose I could always rout some external oil lines to dribble onto the bearings but I'd rather not have to go through all that.

I've had some thoughts about the automatic cam chain tensioner. Seems to me there's no need for the cam chain to be under constant strain if it's adjusted enough to keep the chain on the sprockets.

I remember older bike engines I've had where you had to manually adjust the cam chain tension occasionally and they always seemed to work fine if you kept an eye on things. Those old type of tensioners just took up the slack, usually by a nut and bolt kind of doodad, and didn't actually add constant tension to the chain automatically.

I was considering making some kind of manual cam chain adjuster out of a small sprocket. I think it would be easy enough to do. But am I barking up the wrong tree here? Is the fault with something else like oil flow?

[stevo as b4]

Needle rollers are very tolerant of low oil flow. They need some form of low pressure constant oil feed, but really don't need and don't want pressure fed oiling to work best. If you have a feed that drips a few drops on oil on them every revolution of the crankshaft that is perfect.

Even the oil mist from a heavily breathing crankcase would probably oil them all they need.

I bet it's possible to engineer a needle or ball bearing solution for most bike engines with just two cam bearings, and more if the cam is a split design in the centre to allow centre bearings to be replaced.

[stinkwheel]

It's a chronic problem with Honda twins.

Those bearings were a quick and dirty fix because they original designs had them running directly in the head and they often ovalled within a few thousand miles.

At a guess, it's an inherant lack of oil flow/pressure built into the design, exascerbated by the fact that no one ever cleans the gauze filter screen over the oil pump because you have to remove the right hand panel to get at it.

People used to do a roller bearing conversion on the old ones that ran the cam directly in the heads, it was a fairly common fix. I presume they must have line-bored the head with the cam-caps in place. I'm not sure if they were using double caged rollers like you pictured or just a caged needle roller like you'd use in a 2t small end running on the alloy.

One concern I'd have is for the oil feed. Doesn't the oil come up through the head under those bushes then through a hole in the bushing to lube the cam? So wouldn't you then need a hole in the outer bearing race and a locating pin to keep it in the correct place to avoid blocking the oil feed entirely? Maybe I'm wrong, my memory is a little sketchy on that kind of detail.

Even then, thinking out loud, perhaps all is not lost because a roller bearing would tend to be volume fed rather than pressure fed. If there is a sufficiency of oil in the vicinity of the bearing, all should be good. Be interesting to see where the oil level on top of the head sits with the engine running. If it's higher (or splashing higher) than the bottom of the bearing, I'd expect it to go well. That leaves the option (if my memory is correct and the oil feed is below the bearing) of relieving the oilway sideways so the oil is still flowing out into the rocker box.

Or consider running a caged needle roller direct on the alloy head?

Or just buy a couple of new iron cam bushes and go for another 20k miles?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[weasley]

Just throwing this out there:

In some engines the force-fed plain bearings act as a flow restriction in the oil circuit, which is what generates the oil pressure as the pump pushes against it. Could changing this affect the balance of oil flow and or pressure around the head/engine?
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Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[lingeringstin...]