drive-side bearings

Tom Gaynor

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At about 50,000 miles, 6000 of them mine, I sensed a vibration above 4000 rpm that hadn't been there before. I decided it was time for a look-see. Making a short story shorter, the inner main outer (sic) had gone walk-about, and was machining its way through the crankpin nut. I jaloused (look it up, but it's Scots for "half-suspected") that the vibration was initiated by crank deflection. Trevor bored and sleeved the cases for me. I bought a new crank from Simon Linford who makes his flywheels from EN 36.
Some of the problem may be that ally creeps. My muffs had slackened around my liners by about the same mileage - and the same time, 54 years. I won't pose as a "Vincent expert", but my advice to anyone who needs a rebore after 50,000 miles and 50 years, and maybe the latter is more important than the former, is, instead, buy oversize liners, have the muffs bored to restore the interference, and fit standard pistons.) And while you're at it, check that the inner main outer is still tucked up in bed where Vincent put it.
But I might argue that the fundamental cause of the problems described by Timetraveller is that Vincent cranks were NOT made from EN36, but from something closer to EN3, aka mild steel. So when mainshafts and crankpins are pressed in, the material yields, reducing the interference. Vigorous use merely accelerates what the passage of time will achieve eventually anyway. This will strike a chord for many of us. I say no more.
In answer to any protests I tell a Rudge story: George Hack was shown a beautifully restored Rudge Ulster, early 30's, cosmetically perfect, largely original parts, that went as well as it did when it left the factory 50 years before. Asked what his feelings were, he replied "if we'd known when we built them that they would last that long, we'd have used cheaper materials"
 
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timetraveller

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With regards to materials to be used for modern flywheels, mains shafts etc: I was aware of some of the development and problems which were experienced when John Renwick and Roy Robertson were developing the type of flywheel described by John Renwick in the May MPH. Originally these flywheels were not hardened and what was discovered was that one fitting of the pressed in crankpin was enough to distort the unhardened flywheel material so that an attempt to refit the crank pin again meant that the interference fit had been lost. The same was probably happening with the main shaft holes as well. The solution was to case harden the EN36 and tests showed that this was enough to prevent the material stretching. It also meant that it was no longer necessary to use hardened washers each side of the crank pin to take the wear caused by the big end eyes. EN36 is a case hardening material and was used extensively on Vincents as case hardening was what was done in the 1950s and earlier. I am no metallurgist and I do not have access to a list of the material properties of a wide range of modern materials but I do know that these days for camshaft, and many car crank shafts, one would use EN40B. This is a nitriding steel and that process results in little or no distortion of the original material. It used to be “well known” that nitriding only gave a thin hard skin. This is not correct and I have spoken to several hardening companies who can provide depths of several tens of thous. It all depends upon how long the material is left in the process. Note that there are several ways of nitriding, e.g. flame or plasma or others. I have been told, but have not verified, that the core strength of EN40B is higher than that of EN36. Possibly some of the real engineers in the Club could clarify the facts here. All this means that that these days it is possible to design and build a better crankshaft than the original. Just to put into context the size of some of the Vincent components; I used to know the head of development at Royal Enfield in the days when they were developing their big twins and building the bike used by Bob Macintyre. (The Royal Enield chap was a great fan of Vincents but used to keep quiet about it because of his job.) If I remember correctly they ended up using two inch diameter drive side main shafts, in plain, pressure lubricated bearings.
 

roy the mechanic

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Certainly EN40B makes for a very nice crank for cars. Farndon Engineering have made several for me, for Bentley, Jaguar+very expensive(and succesful) Ford twin cams. Roy.
 
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