Hitchcocks Forum

Hi Chaps...wonder if anyone can explain this please: On the Enfield big twin engine, the crankshaft has a roller on the timing side and a ball bearing on the drive side, so why on earth is it that way round?
Any other motorbike engine always has the strongest bearing/s on the drive side which is completely understandable as it's this side that has to handle the full power output of the engine which side loads the bearing by the pull on it by the chain transmitting the power tot he clucth. Take for example the BSA twin, all it has is a smallish bronze bush on the timing side and the Norton Domi Twins has a ball on the timing side and heavy roller on the drive side, this set up is common on nearly all motorbike engines so why is it the "wrong" way round on the Enfield twin?

As the bearings are the same dimensions I would imagine someone somewhere might even have tried the bearings in the swapped over positions. BTW: the Dynamic :Load rating of the ball bearing (drive side) is 35kn and the roller (timing side) nearly double at 64kn.

So if any knows I'd love to hear why...much obliged.

Les

Hi Les, Unless you can find the engine designer you may have trouble with this one.

Don't shoot me down for this (I could be wrong), around this time manufacturers of twin engines were having problems with roller bearings supporting heavy crankshafts due to the shaft flexing and destroying the bearings, this resulted in the "superblend" roller bearing which as I am sure you know had rollers which tapered towards the ends to allow for this flex, could that be the reason? and if so why not both ends of the shaft?, could it be that the lightly loaded end was expected to fare better?, it certainly helps with parting the case halves.

Hi Dennis ...thanks for replying...yes pity there doesn't seem to be a similar written article like the interview with Anthony Wilson Jones when he discussed all the design features of the Bullet engine...well there might be one but obviously I've never seen it.

I have read an article that describes the early meteor engine where it says the roller bearing is on the drive side. I've gone through every aspect of the twin design but just cannot understand this arrangement...yes it does allow the crankcases to be parted but this would be the same if it was the other way round too....in fact some engines have ball races on both sides and it isn't that hard to separate them as usually the timing side shaft is made a touch smaller and is allowed to slide through the centre of the bearing.... the timing side pinion nut, when tightened, holds it rigidly when assembled.

Even the Mk11 Interceptor has the same bearings and the same arrangement and as this is a redesign, they could have changed the system but didn't. Triumph twins also used a roller on the drive side....Nope, I just can't understand why it was done that way round, perhaps someone will know. All the best Dennis.

Les

BTW I know this message is going to one long paragraph even though I've written it with spaces and 3x paragraphs...could someone let me know how to format these messages please?...Thanks.

Les,



type this at the end of every paragraph:







A.

The superblend bearings were only fitted to Commandos as far as I know, following the introduction of the infamous Combat engine that lasted about 2,000 miles before the mains went. This was some years after RE ceased production as it was around 1971 or 1972. I don't know why the mains on RE twins appear to be on the wrong side. Maybe it was something to do with the lack of rigidity caused by the twins in effect being two singles mounted on a common crankcase. Who knows?

I think the bearing design of old British twins is because of vertical splitting of the crankcases. That means it is next to impossible to make the bearing housings be accurately lined up so that means a design of bearing must be used to allow for that......................................A horizontally split crankcase means all the bearing housings can be bored in perfect alignment.

Hi Papasmurf, whilst I agree in principle with what you say, all the old Brit manufacturers made the crankcases as a pair and then line bored them as an assembled pair to gain alignment of the respective bearing mountings.

As Rattlebattle correctly says the superblend bearing were developed for the Norton, but could the 700 twin designer have saved them a lot of problems?, was this something he thought about in advance?. We will probably never know but anyone who has stripped on of these motors will know that he left nothing to chance and designed it to be a bloody durable engine, the size of the crank journals and bearings have to be seen to be believed.

Bored as an assembled pair with a vertical split, is never going be as accurate when split and reassembled as a horizontal split crankcase is going to be. Even 1/2 a thou misalignment is going to make a difference to longevity.
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I have back in the dim and distant past been surprised when reassembling engines just how far out mass production engine components can be.
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My brother and I got an extra 10 miles an hour out of a Morris 1000 just by making sure everything lined up, making sure everything weighed what it should, every thing was balanced and polished inside the engine before we reassembled it.

I fully agree that all engineering has come on a long way since the 1950s, half a thou over the length of a 700 twin crankshaft that would be brilliant, even the current honsuziwaki bikes would love to be that close.

Only 10 MPH are you sure you were trying , to be a bit more serious but not too serious as far too many threads recently have gone that way, the way the case halves are located together make for a good accurate fit each time unless they have been abused in the past.

As for design of bearing?, well that is why Les started this thread, some British bikes use two ball races one each end, some use two rollers some use one of each, some use a plain bush at on end and a roller or ball at the other, some have an extra bearing in the middle, BSA to name but one have one bearing at the timing side and two together at the drive side, aint life a mystery?.

Thanks Adrian I'll try that now....



Hopefully this is a new paragraph



Referring to your last point on your last message Dennis, yes there are a lot of variations but ONLY the Enfield twin uses a much stronger bearing on the timing side than the more highly stressed drive side if you know of an engine that similarly is set up this way let me know please...So perhaps someone got it mixed up at the factory and they just carried on that way!...but seriously though, the only thing I can think of is that if a roller bearing was used on the drive side the very slight flex of the crankshaft under full load causes the edges of the rollers to bite into and damage the outer bearing track...funny how this doesn't happen on many other big twins apart from the Commando requiring the superblend bearings. If this is correct and the ball bearing is long lasting the well over spec'd timing side roller bearing should and probably lasts for ever.



Les