ET: Engine (Twin) Primary side main bearings

Albervin

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I am now into the rebuild of the Shadow engine. I have heard there is a mod. for fitting two tapered roller bearings, back to back on the primary side. Has anyone done this? Details please.
 

greg brillus

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I've just done it to my race Comet engine, it is a great mod.......not the easiest thing to carry out as the distance spacers between the bearings are critical. It does rely much more on the clamping affect of the crankshaft nut being done up very tight to keep the bearings in place. I'm not sure for a stock road engine if it would be worth the extra effort over just replacing all the stock bearings with new ones.
 

Robert Watson

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How much crush did you give the bearings?

I have to say I have replaced the bearings whenever I have taken it apart because they are so cheap why wouldn't you? Nice for assembly as well. Crank in the timing side first and then just drop the drive side on and cinch it up, no fussing about trying to get the rollers to go in the outer race.

Mine was done by the previous owner, I think the bearings got changed about 60 or 70K miles a go, winter of '99- 2000.
 

greg brillus

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No crush on the bearings, they have a tiny amount of float, difficult to measure.......John Trease here in australia told me to shim it up then do the crank nut up very tight, then you should be able to wiggle the timing side shaft just the smallest amount, so just a small amount of play in the bearings. They do not need any crush , because they are back to back, there is no expansion between the two, not like a Velocette or a bevel Ducati crank that do expand. I did not modify the bearing races either, I used the center stock circlip which I de -burred first to remove any protruding edges, then I modified an old ET 19 alloy spacer to get the inner bearing located where I wanted to locate the crank as best I could, the outer bearing cup up against the outer side of the circlip. Then made up several spacers to go between the two center races, and fine tune to get the shimming correct........this takes many goes to get right. The tension of the crank nut is especially important, if it comes loose then the bearings separate and the crank would become quite noisy I would imagine. Like I said, it's a great mod but takes some setting up to get correct.........This is the same bearing set up that Neal and Rodney used on the new series"A" twins, I would say from advice given by John Trease who did all the machining on the new twin engines.......sadly no longer with us.
 

Robert Watson

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Mine is a new steel housing that held in place by a flange with several c/sunk screws going into the cases. it has a shoulder near the midpoint and the outer races go tight up against that. The cups then go face to face and are held apart by a hardened spacer. Put it together with a spacer that is close and tighten it all up. Then measure the end float. If you end up at 20 thou, the spacer is ground to reduce the float to .003 less than zero (in the sample stated .023 reduction) and voila. It been that way a loooooong time! It also has a seal housing which screws LH thread into the outer end to seal the whole plot.

Sorry no pictures, as I said I haven't seen it in close to 20 years.
 

Martyn Goodwin

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This drawing was passed on to me a while back. Sorry about the quality as I do not have access to an original, only a photocopy received second hand. It author is Dr Doug from Australia. and here are some of his words.


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Vincent Main Bearings. Almost from their inception the Series B and C machines have had the drive side inner main bearing move in the crankcase and thence the bearing cup move in toward the crankpin.

Some riders seem insensitive to the “Clonk, Clonk” as the very hard bearing “Machines” a groove in the crankpin nut that delivers “Shrapnel” to the sump to be pumped through the entire lubrication system causing inevitable havoc. The original “OEM Mains” were supplied by Ransome and Marles and were of high precision and quality. The R&M bearings were selected from THREE possible “Fits” that were available at the time. The “Fit” was achieved by the internal clearance within the bearing. The bearings selected were of the type that allowed for a Tight fit in the crankcase and a tight fit on the mainshaft. The machines were produced many years before the advent of “Loctite”.

The method of fitment was to heat the crankcase to 200 Deg C preferably in an oven, the coefficient of expansion of the aluminium used (DTD 424) was 0.000022 per Deg C. The correct “Crush” on the bearing O.D is 0.002 Thou in TOTAL.. This was obviously not enough. When the housing was originally machined at the factory the level of precision required would be doubtful with the machinery available at the time on a “production line basis”. (ie) Bearing O.D. – 2.500 Dead, housing 2.498. The actual machining marks (turned” would have allowed “Bed Down” and in due course the bearing Rotates in the housing and results in an “under-sizes fit.

Further, ham fisted subsequent Re-fitting of mains WITHOUT a guide mandrel caused more damage as the bearing is entered into the crankcase. Many owners would not have the facilities to get the cases evenly heated and or hot enough without suitable pyrometers and adequate heating gear. Over the years the original mains have been not only in short supply but “extinct”. Some owners have fitted the wrong type of “Clearance Bearing” resulting in very rapid bearing wear. Many bearing supply houses are not even aware of what the “Right Bearing” is. The fact that the bearing I.D, O.D and width are the same has nothing to do with the INTERNAL clearance of the “Correct FIT”.

What Then is the Answer? Tapered Rollers are the answer.

I first heard of the fitment of same by Gordon Colquhoun of Moto Vincent fame. I wrote to him and he was most generous with his information. The “Moto Vincent” was a formidable machine ridden by Charlie Rouse.

Timkins take BOTH radial and axial Thrust and are fitted in pairs BACK to BACK and therefore CANNOT move inwards as the original ball and roller set up did. They can also handle more RPM than a Vincent could ever rev to. Some precision machining is required but it’s not rocket science.

Drive Side Rollers 006.jpg
 

vibrac

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I must admit I have not heard of this mod it sounds an excellent wheeze my only caveat is that I am supprised that this mod has not been offered by maughns or other engine gurus over the years I wonder why?
 

Bill Thomas

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Is it only me who likes the Lipped outer race for the inner main bearing ?.
Our "A" Man told us about many years ago,
They can't walk out !!.
Like all Mod's it needs some playing around, I helped fit them to Big Bill Telfer's a long time ago and
Have a set for my new Comet.
 

passenger0_0

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Interesting idea Bill to use lipped bearings.

My view is that main bearings walk out due to three causes.
1. Misaligned flywheels.
2. Misaligned main bearing bores.
3. Crank flex (too many revs using standard crank).
My view is that loose or walking main bearings are a symptom not the cause of a problem. A good indicator things are not good.

I can't help but wonder how much as been written on this subject?
 
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