Mismatched Bearing Clearances In Transmission

[bultacobrian]

Hi! I'm in the middle of my first rebuild, a real basketcase, and I have a question about transmission bearing clearances being mismatched. I ordered CN clearance bearings but one of them arrived as C3, the clutch side 6303 bearing if my memory serves me right. I looked up the clearance ranges and they overlap by a 1/3 but at the extremes there's a 6-fold difference. Is it ok to use this C3 bearing opposite a CN, or will this result in increased uneven wear and poor operation over time? For ~$10 I'm inclined to keep things consistent, but it's another delay in the project and we're talking <30µm here which sounds small at this operating temperature. I would appreciate some insight here if you've got it. Thanks!

 

-bb

[johnjsy]

Hi Brian,let's see if I can help. I have assembled quite a number of Bultos from 125 up over the years,I may be right,I may be wrong,but this is my way these days ! The 4 gearbox bearings,use ordinary standard ones. On the crankshaft I use C3 clearance because 2 stroke lower ends can run really hot. On the timing side I use a NJ 205 ECP/ C3 roller bearing--yes they are rather pricey but make closing the cases so much easier. The later motors towards the end of production used these as well. I also do a dummy run of putting the crankshaft in the cases with no bearings,new gasket & a couple or nuts tightened up to measure the cases to crankshaft gap with feeler gauges. On assembly I try to get the gap equal on each side. A bit of extra care now & you will get a nice sweet running motor. Oil? Now there's a minefield ! That's down to you,but these 50 year old motors don't really like fully synthetic. Most of the time I use Millers CM2T at 33 or 40 to one. No lubrication issues to date, but that decision is down to you. Over to you.......

[markg]

Johnjsy - what do you use to centralize the crank in the cases? Shims outside the bearings?

[bultacobrian]

Hi John! Thanks for the reply. 

 

I've already got C3 bearings lined up for the crank, and I've heard about using a roller bearing on one side, but thanks for the part number! I'll see if I can scare one up for less money than all of the other bearings combined. I'm trying to keep variables to a minimum since this is my first rebuild, so I may just skip the cylindrical roller bearing since I already have a suitable-enough ball bearing for the crank. What's the rational for putting it on the timing side? It's seems counter-intuitive to a beginner like me. Is it a leverage thing, where if the drive side tips a little under load, the timing side sees more actual movement? 

 

I guess it's prolly prudent to just get the right clearance bearing for the gearbox and forget about it. Cheap insurance/assurance.

 

What's the purpose of test fitting and measuring the case-to-crank gap beforehand? I'm not sure how you would use that measurement for anything. Wouldn't you still just be shooting for an even gap when assembled anyways?

 

-bb

[johnjsy]

Hi Brian,back again.First off,before I forget,check out----Bultaco build notes by snarlyjohn. oldjohnno.id.au>b lots of very sound info there. The roller bearing. There is a lot of "fling" from such a heavy rotating mass & a roller race will handle this loading much better than a ball race,especially in a two stroke not having the benefit of a forced oil supply. The fact that it makes closing the two crankcases together on assembly so easy is an extra bonus. Getting the crankshaft central. The piston is descending on a power stroke,right ? The new charge is being compressed to escape up the transfers,so let's imagine the crankshaft is very close at 10 thou on one side but 30 thou on the other to the cases. The quantity of mixture is now having an easier path to move on one side than the other. There is also wind age to consider,drag caused my a moving object very close to a stationery object,the crankcase. Really,apart from the satisfaction of having a nice job with it fitting centrally,is minimalising those losses. To get a reading I hold the crankshaft tight to the left with one hand,then try a selection of feeler gauges on the right to get a reasonably tight fitting. Let's say it is 90 thou,half that will be 45 each side. I have a good selection of shims,some stainless,some brass,some old hacksaw blades which I use with a digital caliper to get 45 thou. With the new bearings on the crankshaft,heated R/H case on two wooden blocks,the shims sitting on the case with enough sticking out to recover with a pair of pliers( keep your fingers away from that hot case) !! --- drop the crankshaft in. Copper hammer ready,you may have to tap a little to get things just right as the heat drops away. I hope this explains it nicely. You may get into a muddle at first-- nerves-- ! But it is easier with practice. Do you have a workshop manual ? Just to mention,the two crankshaft nuts & the clutch centre plus sprocket one need 75 FT/LBS of torque. Now that IS tight! These are fine engines & will reward you with good service if done properly. Nearly bed time,regards,John.

[feetupfun]

Another benefit for using a roller bearing on one end of the crank is that it allows for zero-load differential expansion of the crank and the cases, which can otherwise lead to a loose fit for one or both of the crank bearings with the standard deep groove ball bearing arrangement (even with C3 bearings), from the bearings moving on their seats

[bultacobrian]

Hi John, I actually accidentally found that site recently. Haven't sifted through it yet though. Too busy getting ready for vacation!

 

I don't quite follow you in regards to "fling" but I can appreciate anything that makes closing up cases easier.

 

The crank/case gap theory makes a lot of sense though. I dig it. Even gap, even seating, even flow, even load, even wear, etc. That's a neat technique!

 

I have a shop manual I'm working from, and after having worked as a cook for more than a decade, I've little fear of burns. 

 

 

@feetupfun - Unpack that lingo a bit for me? Do you mean, as things heat up, the inner bearing race will move away from "center" and shift towards a looser fit on the crank? And cylindrical roller bearings have enough slack in that direction while still being within operating limits to soak that expansion up? Please correct me as I'm just making this up, trying to visualize what that could mean.

 

Thanks guys!

 

-bb

[feetupfun]

Yes bb that is kind-of what I meant

The reason why we use C3 bearings for the crankshaft main bearings is because there is differential expansion, meaning that the amount that the crankshaft gets longer and shorter as the motor warms up is different to the amount that the crankcases get wider and narrower as they warm up.

If there is not enough axial movement provided by the two C3 bearings for the amount of differential expansion generated, then axial force will build up in the crankshaft and in the cases until one of the bearings moves sideways on its seat, which will relieve the force, until the motor cools down again, when the force will build up again until it is relieved by one of the bearings moving sideways on its seat. The sideways moving of bearing on shaft or bearing in casing will be repeated each time the motor heats up and cools down, gradually wearing away wherever the movement happens. I've seen it on the crankshaft where a bearing inner sits, and I've seen it in the engine cases, where a bearing outer sits.

By using a roller bearing as one of the main bearings, the force never gets to build up because the roller bearing allows for load-free axial differential expansion, so the bearings don't move on their seats, so there is no bearing seat wear on the crankshaft or in the cases

[bultacobrian]

Got it! I get the principle of differential expansion and whatnot, I just don't know anything about the design of cylindrical roller bearings so while it's an easy idea to accept it's benefits at face value, I don't actually understand the mechanics as it relates to that type of bearing. BUT, your explanation helps a great deal in understanding the shortcomings of deep groove roller bearings in the application, and anything that makes reassembling cases easier is always a plus. Thank you!! 

 

-bb