Steel on steel - bearing

[Truckedup]

I have yet to see any OHC engine with an aluminum head and separate cam bearings. They always ride directly on the aluminum. The spring loads are very low so this is not a problem.

Ducati OHC motorcycle cams run on roller bearings...But they also have no valve springs... The newer DOHC 4 valves engines do run with no cam bearings ,still no valve springs

[strokersix]

Hardened steel running in cast iron is a good friction couple. Tappets in cast iron block example for this crowd.

Steel on steel with caution. Probably works on floating pins in steel rods because the surface speed is low and also relative motion is shared with the piston pin bores.

[cjperformance]

I have yet to see any OHC engine with an aluminum head and separate cam bearings. They always ride directly on the aluminum. The spring loads are very low so this is not a problem.

Ducati OHC motorcycle cams run on roller bearings...But they also have no valve springs... The newer DOHC 4 valves engines do run with no cam bearings ,still no valve springs

Z900 Kawasaki run bearing inserts for dohc.
Im pretty sure a mid 70's Mercedes 6 cyl i dad a few years back did aswell

[rfoll]

My Kawasaki DOHC 250 engine runs the cams on the aluminum cylinder head journals. Each one is a matched set, so if they get cooked for lack of oil, you can't swap the parts from a different head. My ancient, (1978), Briggs & Stratton flat twin 16 hp lawn tractor uses aluminum rods running directly on the crank. I did a re-ring and valve job a few years ago and expected to replace the rods, but they were immaculate. This machine gets worked hard, the pto drive rototiller uses all 16 hp at wot, and it gets a little hot, but obviously they did their homework when designing the engine.

[Circlotron]

Okay, so it sounds like steel on steel is not a good idea for just any old application.
Can anyone tell me exactly what happens and why? After all, if the oil film keeps the two surfaces apart then that should be the end of the matter. Or is it when the two surfaces just begin to move, before the hydrodynamic wedge has fully formed, and there is some amount of scraping and neither surface will yield the other? For simple, low speed applications would grease fix the problem?

[Geoff2]

I can think of many steel-on-steel bearings. Roller & ball bearings are steel on steel. Any some run on veeeery little lube. No better example than the humble front wheel brgs in the grocery getter. They do 0000s of miles on a very thin film of grease.

[Circlotron]

^^ I think roller and ball bearings, particularly roller, are a different case because the moving parts actually roll across each other. If, and for as long as the mating surfaces remain perfectly smooth I imagine a roller ought to tolerate a certain amount of load with no lube at all. A plain bearing is where the discussion started. They are purely sliding surfaces of course.

[modok]

Okay, so it sounds like steel on steel is not a good idea for just any old application.
Can anyone tell me exactly what happens and why?

I'm darn tempted to try, a scientific explanation, but it's going to be very long.
Instead how about hands on? Steel rockers. I've sorted many hundreds of steel rockers from detroit and Vw and several other engines. They usually don't just "wear" out, rather they score eachother to death, big patches of scoring develop if something goes wrong.
The specification is that if more than 1/3 of the surface is scored/galled then it's bad. about 30% of them are bad. So, I'm going to say that's about your chances. You CAN run a rocker or a piston pin steel on steel because even if it does gall.....your ok, but a rod bearing? No thanks, too risky.

[machinedave]

I had a 2 cylinder air compressor with aluminum rods that developed a knock. I resized the rods and put it back together and it ran fine for many years.