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Crank pin "fretting"?
- Thread starter Terry Keeley
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I've actually upped the oil from 12 - 15% (10% synthetic/5% castor) after break-in and didn't have any problems at 12%.
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is that “pitting”? Or is it caused by impact?
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67 CMB Blue head? I don't think that is fretting as much as edge loading. Looks like it is also happening at the 3 O'clock position closest to the crank web too.
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Needle bearing rod ?
Needles worn or too much clearance?
Looks to me that the needles may be stacking up at an angle and skiding across the pin at those two points..
Needles or bushing,,
Either way, it's starving for oil...
I'd look at improving oil flow into and out of the big end of the rod..
Grooves, extra oil hole, rotor clearance to rod face, not too tight blocking flow to the big end of the rod.. things like that...or it could be the oil is leaving faster than it can be replenished...
Don't forget that centrifugal force plays a big part here,,, an oil wedge must be maintained for lubrication to be effective 360 degrees around the pin..if oil is forced to one side of the pin and slung out, it will in effect siphon all of the oil wedge with it, and oil starvation occurs...
Needles worn or too much clearance?
Looks to me that the needles may be stacking up at an angle and skiding across the pin at those two points..
Needles or bushing,,
Either way, it's starving for oil...
I'd look at improving oil flow into and out of the big end of the rod..
Grooves, extra oil hole, rotor clearance to rod face, not too tight blocking flow to the big end of the rod.. things like that...or it could be the oil is leaving faster than it can be replenished...
Don't forget that centrifugal force plays a big part here,,, an oil wedge must be maintained for lubrication to be effective 360 degrees around the pin..if oil is forced to one side of the pin and slung out, it will in effect siphon all of the oil wedge with it, and oil starvation occurs...
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It's a 67 Blue head CMB, it's happing with the new style cranks as well as some old ones that have made many runs without problems. I'm running the same fuel and oil as always (10% Klotz synthetic + 5% Blendzall castor). Strange too that it's on the bottom of the rod or opposite the compression side.
Found some info from NSK but from the pictures it could be called a couple different things:
https://www.nskamericas-literature.com/en/bearing-doctor/
Found some info from NSK but from the pictures it could be called a couple different things:
https://www.nskamericas-literature.com/en/bearing-doctor/
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Harmonics between loaded and unloaded pins ?
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Don't forget that as the rod is accelerating downward, it reaches the bottom of the stroke and is forced to change direction.
The big end bore will egg shape and can pinch the rollers against the sides of the pin stalling the rolling movement of the bearings.
On the up stroke, compression and the rod shape supports the top of the big end bore better, so there is not as much distortion.. may be why you are seeing this happening on the opposite side.
The big end bore will egg shape and can pinch the rollers against the sides of the pin stalling the rolling movement of the bearings.
On the up stroke, compression and the rod shape supports the top of the big end bore better, so there is not as much distortion.. may be why you are seeing this happening on the opposite side.
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Terry it also looks like you might have a decent amount of side loading on the big end of the crank from the material transfer I see.
I assume you are running a collet drive instead of a square so the crank is seeing the thrust loads. I know of anyone you are probably religious about changing your bearings at the slightest sign of wear, but do check the end play.
What do the needles and the big end of the rod look like?
I assume you are running a collet drive instead of a square so the crank is seeing the thrust loads. I know of anyone you are probably religious about changing your bearings at the slightest sign of wear, but do check the end play.
What do the needles and the big end of the rod look like?
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Weird eh? Needles and rod bore are good, this is the third one in the past few months, different motors.
I've seen this start to happen on the compression side of the rod very rarely but never on the back side.
Bearings and the rest of the motor is in good shape.
I've seen this start to happen on the compression side of the rod very rarely but never on the back side.
Bearings and the rest of the motor is in good shape.
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Has this(these) engine(s) had previously been run on your dyno ?
You are using a one way bearing between engine and load ?
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How do the rod bores look? Same needle brand you've always run?
Brian
Brian
Tim Kish
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Was thinking the same, check the rod I.d. for ovality
LohringMiller
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We had a lot of flex on our CMB 35 crankshaft. It caused high rpm flywheel strikes on the magneto. We added another front bearing to stop this. We never had a rod big end failure though. Our experience with nitro engines on our dyno was that they were loaded more than by a boat propeller. We burned holes in several pistons at the same needle setting that was used for racing. Are you seeing this only on your dyno?
Lohring Miller
Lohring Miller
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Thanks for all the input guys, making me think a bit.
I was using my dyno/run in stand to break in some piston/sleeves and I think (maybe) I buggered up the cranks there by running them with the same rod that has a ton of time on it.
When put together there was very little slop in the rod/wrist pin so I didn't think to check the bottom bore, I do bang out the top bushing at times but just replace it. When I measured a new rod it came out to about a 0.4419" bore, very round and straight. When I checked the old rod it was still round but had worn about a thou. near the edge closest to the crank but it bell mouthed out to over 0.0025" at the rotor side!
I think I've found the problem...
I was using my dyno/run in stand to break in some piston/sleeves and I think (maybe) I buggered up the cranks there by running them with the same rod that has a ton of time on it.
When put together there was very little slop in the rod/wrist pin so I didn't think to check the bottom bore, I do bang out the top bushing at times but just replace it. When I measured a new rod it came out to about a 0.4419" bore, very round and straight. When I checked the old rod it was still round but had worn about a thou. near the edge closest to the crank but it bell mouthed out to over 0.0025" at the rotor side!
I think I've found the problem...
ianfolkson
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In Europe we are now running high castor based oil in cmb motors.
70% Castor 30% synthetic and oil at 16 to 20% no loss of power and better oiling. The CMB factory are running pure castor oil but nitro is not so high so it mixes ok.
70% Castor 30% synthetic and oil at 16 to 20% no loss of power and better oiling. The CMB factory are running pure castor oil but nitro is not so high so it mixes ok.
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