Nylon Bearing Sleeve

Potentially there is quite a number of engines that could need converting, since the original nylon component can no longer be sourced anywhere. I have tried quite a number of dealers in various European countries that used to handle ROTAX 100cc engines, but without any success. To be honest, nobody seemed to be particularly interested in helping out until you came along. Many thanks once more.
 
Hi Pete! I thought i´d lift this old thread as these engines are getting more and more popular again.
Just wanted to say thanks for all your help on this matter. I machined some rings using more or less
your measurements. Worked like a charm!

Dimensions of the ring i machined (free state) is:
OD - 56,06mm
ID - 51,99mm
Width - 15,00mm

The real tricky bit is to get them in the bore before they expand to much from the heat
generated by the heated housing.
(They expand about 0,3mm in 1-2 seconds, no kidding)
This became my solution, fasten the ring to a wrench socket using some tape and pack
the socket full of snow. Put it in the freezer for about 15 minutes. The socket then helps
cool the ring during the mounting process.
However I plan to machine a better tool for this.
 

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Did the interference on the bearing turn out close to what you were looking for?

PM
Yes, it came out pretty much perfect i think. I got a final bearing interference of about 0.04-0.05mm (0.0020")
which is what i measured on other engines without sleeves. So all-in-all i´m real happy with it.

Here are all the dimensions for future reference

Bore in case - 55.99mm - 2.2043"
Ring OD - 56.06mm - 2.2070"
Ring ID - 51.99mm - 2.0468"
Bore to ring interference - 0.07mm - 0.0027"
Bearing OD - 51.99mm - 2.0468"
Ring to bearing interference (free state) - 0.00mm - 0.000"
Ring to bearing interference (after mount) - 0.04-0.05mm - 0.0016-0.0020"

Edit: Interference measurement
 
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The plastic sleeves were an attempt at vibration damping. If this was an engeneering bullseye they would be
on every engine since then. Rotax stopped putting them on their engines after the VMC/DSC and very few
other manufacturers used them after that. This tells me it was not as good as they initially thought.
Besides, i believe the use of plastic steals some power as it flexes.

Something that happens on several of these engines is that the big end pin sheers right of. It´s my belief this
is caused by the plastic rings, creating flex between the bearings and putting a tremendous strain on the pin
causing it to snap. This happened on one of my engines back in the 90s and i´ve seen a couple of others with
the same problem. When this happens at 20.000rpm the crank halves are forced outwards, pressing the
bearing out of the case (usually on the ignition side, where there is less material) causing a very expensive
wreck.

So my conclusion is: If you use plastic rings on these engines (Rotax VMB, DSB, VMC, DSC and some Italsistem
engines) swap the big end pin, bearing and con rod more frequently then on other engines to prevent
problems.
 
I am not a fan of the nylon type sleeves but I have I think 6 of them made for a rotax got them from europe years ago, I would love to sell them. contact me at 309 786 3204 or magmo44@att.net Thanks Mike
 
Ring to bearing interference (after mount) - 0.05-0.06mm - 0.0020-0.0024"

Excellent documentation, thanks.

That is quite "heavy" on bearing interference. I would recommend making an adjustment so the final bearing interference at 20C is between 0.032mm and 0.038mm for best results.

PM
 
Excellent documentation, thanks.

That is quite "heavy" on bearing interference. I would recommend making an adjustment so the final bearing interference at 20C is between 0.032mm and 0.038mm for best results.

PM
That is a small error on my part. When i measure the bearing it is actually 51.99mm so that interference would be 0.04-0.05mm. That is still more then what you recomend and i´ve had other people tell me the same thing. However i measured a Italsistem which have the same bearings without rings which is the reason i chose to go with that interference.
Also i´m always using C4 bearings which have a little more play then C3.
Any thoughts on that Pete?
 
In my opinion, bearing interference should typically be based on a number of considerations: design of the case (mass, casting type, how it cools, etc), type of bearings used, whether the engine runs on gasoline or methanol, state of tune (direct drive, clutch, restricted or not), and possibly even slight adjustments for different customers.

The range of bearing interference of all the different engines I've re-machined over the years (mainly 52mm bearings, but also 47mm, and CR125 engines which are just slightly larger than 52mm) is 0.022mm to 0.0.040mm. The vast majority were machined to .030/.035mm.

As far as using C4 bearings, the specs for internal clearance are quite "wide" -- a C3 bearing on the loose end of the C3 spec actually will have more internal clearance than a C4 bearing on the tighter end of its spec. The bearings can be measured for internal clearance, which gives a bit better picture.

PM
 
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