Main Bearings .
- Thread starter roger v
- Start date
If you are referring to C3 clearance…. here’s an example (assuming I’m correct). Vic will straighten me out if I’m wrong.
The need for C3 clearance is really dependent on the interference fit between the bearing and the crankcase.
Some roller bearings were honed out to 3C by the vendor, so marked C3 on the box, but may not be marked on the bearing.
The need for C3 clearance is really dependent on the interference fit between the bearing and the crankcase.
Some roller bearings were honed out to 3C by the vendor, so marked C3 on the box, but may not be marked on the bearing.
The one that clamps up readily is the small ET 94 on the timing side, these often need honing out, as the inner race ends up too tight within the outer.......at least 50% of the ones i do need this done, even using the supposed C3 bearing........of course the 5/8" width timing main roller is the tricky one........if they are too tight, then the bearing becomes scrap as you cannot remove it without destroying it. Some might disagree about the honing method, if you have a better fix for this, then I'd like to know. I figure a correctly running bearing fit is better than it being too tight, I've seen bearings suffer total failures from running with no clearance...........worst case, it locks up and the crankshaft ends up spinning within the actual center race, thus wearing the actual shaft badly.......hardly ideal that one.
The rollers in my current project (including the small one) were not C3, just standard bearings and then lapped(in place) to size (not by me). I figure that unless you have complete control of the interference fit of the bearings, then honing or lapping is the answer.
It’s amazing how accurate lapping can be when done by someone with the correct tools and skill. Yes some disagree, but I’m with you on this one.
Edit… changed honed to lapped. Sometimes I interchange the two term’s which I shouldn’t do.
It’s amazing how accurate lapping can be when done by someone with the correct tools and skill. Yes some disagree, but I’m with you on this one.
Edit… changed honed to lapped. Sometimes I interchange the two term’s which I shouldn’t do.
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In some handbooks occasionally C 3 bearings are specified but I have never come across one of these or looked for them before scrapping them. I cannot believe you need C 3 in alu engines - provided the shafts are as should be , meaning max. 10 micron oversize, I do 5 micron for light press fit on shafts. So then a new roller bearing with 30mm i.d. inner race typically got internal play 20-45 micron (0.02-0.05mm) in CN normal, a C 3 bearing would have 35-60 micron. So with a correct press fit on shafts this reduces by 5 microns only.
Next look at the shrink fit in the alu case: I like the bearings to free at around 100-120 degrees C , not much higher than operating temps, means the outer race will show little to no reduction by shrink fit when warm. So that is why I never care about C 3 bearings as they will have sufficient play from the start - I mean from factory, and quite a bit in warm engine.
Certainly with a very cold engine and you are busy with assembling the lot the crank may not want to slip through the bearings so just get the heat gun and warm it up , no troubles. Yes, a cold engine will be a little bit tougher to turn over from a bit of preload from the CN type possibly, but I do not care about this, you don´t do a lot of riding with a cold engine, it will heat up soon. Remember, Velocettes or some HDs got taper roller mains, so rollers as well. They have to have some decent preload in cold state for not having sloppy cranks when hot later. Don´t worry, be happy - with a little preload from CN types, C 3 won´t help much . Honing a new race for extra play would not be my idea, will be sloppy later in hot engine. When some bearings fail I do not believe from wrong C specs, rather some serious defects in other places . . . .
Vic
Next look at the shrink fit in the alu case: I like the bearings to free at around 100-120 degrees C , not much higher than operating temps, means the outer race will show little to no reduction by shrink fit when warm. So that is why I never care about C 3 bearings as they will have sufficient play from the start - I mean from factory, and quite a bit in warm engine.
Certainly with a very cold engine and you are busy with assembling the lot the crank may not want to slip through the bearings so just get the heat gun and warm it up , no troubles. Yes, a cold engine will be a little bit tougher to turn over from a bit of preload from the CN type possibly, but I do not care about this, you don´t do a lot of riding with a cold engine, it will heat up soon. Remember, Velocettes or some HDs got taper roller mains, so rollers as well. They have to have some decent preload in cold state for not having sloppy cranks when hot later. Don´t worry, be happy - with a little preload from CN types, C 3 won´t help much . Honing a new race for extra play would not be my idea, will be sloppy later in hot engine. When some bearings fail I do not believe from wrong C specs, rather some serious defects in other places . . . .
Vic
After a good run on a hot summer day here in Aussie, after letting the bike sit for 10 or 15 mins I've checked the crankcase temp with a digital temp gauge between the cylinders and seen temps of 140 deg Celsius.......not much of a shrink fit beyond that.
Vic, I don’t think I would want to run my bearings that tight unless I had your crankshaft to go with them. With clearance like that a person wouldn’t want much runout on the crank.
Expansion on the back to back tapers like a Harley (or in my Comet race engine) does not exist.......more so on bearings like in a Ducati that have some pre load to account for growth in the cases when they get hot. Nil clearance in the bearings.......I would think you are asking for trouble......Its all in the crank......these do not stay in perfect alignment forever.......the loose bearings in cases does not happen by themselves, its the crank that causes them to come loose. I've seen plenty of bearings that are tight to rotate in these engines.......never leave it like that.......the rollers skid and then rapidly overheat......the outer races then develop cracks.......not good at all. Big end rod liners do the same, especially the very thin ones, found many that start to crack, these can break away and some even lock up the crank all together.......But this is mostly due to the big end eye not being round and the liner has tight spots on the sides......
I believe C3 is to do with the internal Clearance of the bearing, ball to race clearance. Nothing to do with internal or external fit. C1 the tightest C5 the slackest.