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OS FS .70 Crankshaft bearing fit

Old 10-20-2021, 11:19 PM
  #26  
MrRover75
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Originally Posted by 1967brutus View Post
Maybe a silly question, but... have you checked whether the bearing was a C3 clearnce class bearing?
Typically, standard (C2, usually not even marked as such) bearings have a slighly looser fit on the outer race, and a tighter fit on the inner race. These bearings will usually come out with the shaft.
C3 bearings usually fit a bit tighter in their seat (outer race) and a bit looser on the inner race, and will stay put while the shaft is pulled out.
Not a rule set in stone, and I have seen wallowed out bearing seats, but most engines that I fix for people, if the bearing comes out with the shaft, it is because it is not a C3 bearing, and fitting one usually solves the whole issue.

Brgds, Bert
Hi Bert,

The bearings installed was exact the same type as installed from the factory. A Standard (C2) bearing was fitted, so the same was used on replacement. I have noted that a standard (C2) bearing is also installed in my OS 48Surpass. On the other hand I see C3 bearings used the most on 2 cycle engines. Might have something to do with RPM and heat build up.
I was always of the impression that the C2/C3/C4 designation had to do with the internal clearances in the bearing and not the fit on the outer/inner ring, but I might be wrong for all I know.
Old 10-21-2021, 08:32 AM
  #27  
1967brutus
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Originally Posted by MrRover75 View Post
Hi Bert,

The bearings installed was exact the same type as installed from the factory. A Standard (C2) bearing was fitted, so the same was used on replacement. I have noted that a standard (C2) bearing is also installed in my OS 48Surpass. On the other hand I see C3 bearings used the most on 2 cycle engines. Might have something to do with RPM and heat build up.
I was always of the impression that the C2/C3/C4 designation had to do with the internal clearances in the bearing and not the fit on the outer/inner ring, but I might be wrong for all I know.
I was taught the same about clearances, but invariably, if I replace a C2 bearing with a C3, then those differences in fit is what I see. How that works, no idea, but if a C2 is tight around the shaft,a C3 is a bit looser.
So far, I have only found C2 bearings in Saito's (actually, some Saito's use bearingsizes where C3 is not even availlable, as I have noticed).
No expert on that matter, just my observations.
Old 10-22-2021, 09:27 AM
  #28  
jaka54
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Hi!
C3 or C2 has nothing to do with how they fit in the crank case,or how they fit against the crank, it is the ball play inside the bearings!
Old 10-22-2021, 09:39 AM
  #29  
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Originally Posted by jaka54 View Post
Hi!
C3 or C2 has nothing to do with how they fit in the crank case,or how they fit against the crank, it is the ball play inside the bearings!
I will give you this challenge: Take the same engine, fit a C2 and a C3 bearing as rear bearing, fit the shaft properly in place (all the way home, I mean), then tap the shaft out again and watch the difference between the two bearings. Use heat if you like, or not, does not matter much, as long as you play fair and heat both to the same temp.
I can 100% guarantee that the C2 will come out with the shaft and the C3 will stay in the casing.

Last edited by 1967brutus; 10-22-2021 at 10:37 AM.
Old 10-22-2021, 12:24 PM
  #30  
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I have done so with the WIB bearings I use in my pylon racing engines and there is no difference! Why should it be...The crank should sit firmly in the bearing whether you use a C3, C4 or heaven forbid, a C2 bearing. Its the play between the balls and the inner and outer surfaces that dictates if it is a bearing with more play (C3 and C4) compared to a "standard" bearing.

Last edited by jaka54; 10-22-2021 at 12:28 PM.
Old 10-22-2021, 01:48 PM
  #31  
1967brutus
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So... you're saying you put bearings with at random clearance class in your racing engines? Not very credible if you ask me...

Other than that, what can I say? I do on average 25 3rd party glow engines per year for fellow clubmembers and via local RC forums. My experience clearly is different from yours. Not talking about one or two experiences or one or two engines either. Literally dozens of all miakes and models...

I am not joking, not repeating old wives tales, and working as a marine engineer I consider myself a fairly experienced mechanic... Not talking about specifically toy engines either, I saw the same with the gearbox bearings of my Ducati, and with basically every electric motor I overhaul on the job.

EDIT: I'd like to expand on this, because reading back it comes a bit across as a "he said/she said" thing, and I do not intend this to be some sort of argument or whatever.

Full story on the Ducati example, and I think any decent Ducati mechanic can confirm this as being true (and if not, no doubt he can simply test it).
About 23 years ago (when I was not half as exprienced as I am now, and still was under the impression that clearance classes only referred to the clearance between rings and rolling elements, this part is crucial), the plastic chain guide of my old '86 Ducati Paso 750 broke off, and jammed between the chain and the forward sprocket, destroying the bearing of the output shaft.
I dismantled the engine, and when splitting the crankcase, this is normally done by lifting the left half of the assembly off by hand, no need for any special tools. The outer bearing of the output shaft remains in the left side casing half, while the output shaft itself remains in the right side half, together with crankshaft, and the entire gearbox assembly. In other words; that bearing has a slide fit on the output shaft. No need for heating, no need for pullers, just a few taps with a plastic hammer to break the split-gasker (nowadays a sealant, back then a paper gasket), and lift.
My local bike shop (not a Ducati dealer) only had that bearing in "normal" clearance, not in the C3 class that came out, and which was also listed in the workshop manual.
Me, being young, inexperienced, and totally convinced of my own infallibility, AND being convinced that that C3 code was only about the internal clearance but nothing to do with the fit in seats or on shafts, and being pressed for time, I accepted that bearing.
I should not have: on assembling the crankcase halves, the bearing had to be forced over the shaft with a piece of pipe and a soft hammer. that in itself would not have been an issue, were it not that the bearing is first locked in its seat in the crankcase half with a locking device, making it impossible to ever remove that crankcase half again, because there are no options to place a puller.
With a C3 bearing, it would have slid over the shaft and assembly would have been no more difficult than closing the lid of the garbage can a Paso crankcase actually is. And I know that for a fact, because I had the crankcase open and closed 2 or 3 years earlier to replace 2nd gear, a weak point of the early Paso engines. At that point the bearing in question was still AOK, and it slid off and back on the output shaft without issues. So I most definitely know, the lose fit was not due to the bearing damage.

That's how I found out, that C3 actually ALSO has a different fit on the shaft compared to a normal bearing.
If you read up on it, it is not said in so many words, but the internal clearance of a bearing is specced "as installed", NOT "as measured straight from the package" and in part effected by either compression of the outer ring, or stretching of the inner ring, and for that to happen, you need a different fit.
I hope this clears things up a bit, I really do not want to come across as argumentative.

Last edited by 1967brutus; 10-23-2021 at 01:31 AM.
Old 10-23-2021, 01:41 AM
  #32  
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Hi!
But why should the inner diameter fit (on the crank shaft ) differ whether it is a C2.C3 or C4 bearing?
The crank shaft should not spin on the inner race of the ball bearing! Have you used WIB bearings?
I know the dimensions vary depending on who the manufacturer is but whether it is a C2 ,C3 or C4 ...no! Not with WIB bearings, Which is one of the best bearing manufacturers.
Old 10-23-2021, 02:12 AM
  #33  
1967brutus
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Originally Posted by jaka54 View Post
Hi!
But why should the inner diameter fit (on the crank shaft ) differ whether it is a C2.C3 or C4 bearing?
The crank shaft should not spin on the inner race of the ball bearing! Have you used WIB bearings?
I know the dimensions vary depending on who the manufacturer is but whether it is a C2 ,C3 or C4 ...no! Not with WIB bearings, Which is the best bearing manufacturer.
I am unfamiliar with WIB, never even heard of the name, which is a bit weird since I am in engineering for about 30 years.
I cannot tell you about the "why", I can only tell you what my experience has invariably been with the known brands like SKF, NSK, Nachhi, FAG, and all those. They all do as I said.
I never said anything about cranks spinning in the inner ring, and to my knowledge, I have never seen signs of that happening in model engines.
All I can tell you, is that when pulling a regular model engine apart, if the rear bearing taps out with the shaft, it was either not a C3, or the seat in the casing has been mistreated/damaged or really extreme cases corrosion or castor residue made it stick to the shaft.
This even is valid for the few Saitos that OEM use "normal" rear bearings: the bearing comes out with the shaft. I did a Saito 80 this summer, and was amazed that that particular bearing size is not even availlable in C3 (I never buy "original OEM" in the engine branded packaging).

But I am pretty sure, that if anybody would send me a regular and a C3 bearing of the WIB brand and I would compare them side by side on the same engine and crankshaft, I will see the same results as I see with all other bearings.
Mind you, with the stuff I have at my disposal for measuring (Inner diameters to roughly 0,01 mm accuracy) I cannot tell a normal bearing from a C3 with any degree of certainty. But when mounting them on a shaft, I actually CAN tell them apart. C3 invariably is the one that fits the shaft easiest, and as said, at times being the difference between a slide fit and a press fit. That's how marginal these differences are.

Last edited by 1967brutus; 10-23-2021 at 07:31 AM.
Old 10-23-2021, 05:42 PM
  #34  
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Originally Posted by jaka54 View Post
I have done so with the WIB bearings I use in my pylon racing engines and there is no difference! Why should it be...The crank should sit firmly in the bearing whether you use a C3, C4 or heaven forbid, a C2 bearing. Its the play between the balls and the inner and outer surfaces that dictates if it is a bearing with more play (C3 and C4) compared to a "standard" bearing.
Quite so, the C3 ratings etc are the internal clearance ratings, as installed.
As implied, two bearings with identical radial clearances un-installed, a bearing with a tighter shaft fit would have less internal radial clearance.

"WIB" (?)

Yessir, know them well. When setting up production test equipment in Canada 40 years ago they were my first choice and readily sourced from dealers in Ontario. "Suisse made, very high quality. Walzlager Industrial iirc



Last edited by Jesse Open; 10-23-2021 at 05:49 PM.

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