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Author Topic: ClarificationMore on Royal Enfield twin engine numbers  (Read 2431 times)


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ClarificationMore on Royal Enfield twin engine numbers
« on: June 24, 2012, 04:16:52 am »
All of Royal Enfield's twin cylinder engines were assembled at their Greenland Mills factory in Wiltshire, where they also had the Albion gearbox attached. These units were then sent to Redditch where they were fitted to the cycle parts to build a complete motorcycle.

The crankcase halves are matched after machining by having identical numbers stamped on top of the flat "deck" behind the cylinders. If these numbers are not the same, your case halves are not an original pair.

Up until sometime in 1956, a "true" engine number was stamped across the front of the crankcases. These numbers have a W or W5 prefix for 500cc and (usually) a W7 or SM prefix for 700cc. In the few photos I have of these stampings, the prefix is on the right hand half and the number is on the left. Please note that this is right and left as seated on the bike, not as viewed from the front. The published lists referred to in my other posting indicate that there are some T7 and TR prefixed numbers in 1954, but I have yet to see an example of this.

All of these engines with a "true" number on the front were paired to frames at Redditch by having the frame number stamped at the top of the left hand crankcase just below the cylinder joint. The 500 twins have a T prefix and the 700 twins have T7 or 7T. It is this number on the side of the engine that is recorded on the vehicle registration document here in the UK and not the one on the front. I have no idea what happens in other countries.

From late 1956 onwards, Greenland Mills stamp the W5 (500) or SM (700) engine number at the top of the left hand crankcase. The engines only have the one number from now on, and are no longer paired to frames at Redditch. Although records from Greenlands give the starting point for the numbers on the side only, I am not going to quote them as the Redditch ledgers seem to contradict this for the 700cc models.

The prefix letters also change over subsequent years, becoming SM with furher letters for both 500 and 700cc machines from 1957 on, eventually changing to other prefix letters altogether from 1959. I am not going to make any attempt to explain the significance of these changes.

Please note that these prefix letters are never recorded in the Redditch despatch ledgers. When they were matching engines to frames, they did not bother to record the engine number at all as it was (supposedly) the same. This custom and practice was obviously entrenched, as it has become apparent that not all of the non-matching engine numbers from 1956 onwards have been recorded either.

Thanks to the internet and helpful owners, I have managed to harvest a few of these missing engine numbers. Some of these have been engine only, but where a complete bike, I do have to assume original pairing to frame as the ledgers fail to verify this due to rather lax record keeping.

The gearboxes arrived at Greenland Mills with Albion's own alpha-numeric number stamped on top, and this number was stamped onto the front of the engine crankcase on assembly to pair them off. This applies throughout production, from the first 500 Twin to the last Series 2 Interceptor.

The photos I have of early engines show the alpha prefix on the right hand half and the number on the left, in similar manner to the "true" engine number. Later engines have all of the gearbox number on the right hand half only, and I suspect this coincides with the engine number being transferred to the side. This is a personal opinion for now. as I do not have enough photographic evidence to sure of this claim.

If the number on your gearbox does not match that on the front of the engine, then either the gearbox or right hand crankcase half have been changed. In the latter case, the matching half numbers wouldn't of course!!


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Re: ClarificationMore on Royal Enfield twin engine numbers
« Reply #1 on: June 24, 2012, 10:57:16 am »
Thanks Graham
That answers a few questions I've had for many years. There is very little information out there about how & where RE did some of their work for these machines. Of course, a lot changed after Major Walker Smith passed in 1964 if I remember correctly.