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.