short stroke 750

L.A.B. need your historic knowledge. I took this photo of what the owner said was an original short stroke 750 engine on his '71 production racer. In case you cannot see it the engine number is 235713. I don't see how it is possible this engine could have been original to a '71 proddy racer, but then I'm not that up to speed with factory racer numbering. Still, an 850 type barrell existed in '71??? Thought the short stroke 750 engine was introduced in '73.

illf8ed said:

I don't see how it is possible this engine could have been original to a '71 proddy racer

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Well, I don't honestly see how that engine could be from 1971? I doubt the through-bolt barrels were available before 1973?
The crankcases also appear to be late 1972-on standard 750 long stroke type and how could there have been a 750 short stroke version of the 850 two years before the 850 model actually existed?

illf8ed said:

the engine number is 235713

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[Edit]As we've previously discovered on this forum, a few 1974 850 Mk2A models (and one 1973 standard 750) are known to exist which also have 235*** serial numbers-but I think all are lower than 235713.

I'm sure LAB will have the info you need, however, I'm pretty sure that number indicates its a very late 750 engine from 73ish.

Cash

Definitely no short stroke 750 engines available in PRs in 1971. My first race bike was an original '71 Production Racer, and I notice two other things on this one that don't look original. The intake manifolds look like stock Commando cast items, not the fabricated steel ones that came on my PR. I can't be sure, but from the picture, the exhaust pipes also look like stock pipes, not the original PR pipes, which were cut and re-welded near the head to pull them in a little closer to the frame for ground clearance. I'd suggest making sure that the bike really is an original PR, not a repro. There were quite a few converted by racers from standard Commandos using the Norvil 750 racer conversion kits. There are a lot of small details you can check to verify it's originality.

On the other hand, the Andover shop was known for sometimes using whatever they had on the shelf when assembling PRs, so they didn't all necessarily have the correct parts as advertised at the time. Mine came from the factory with a stock head and standard cam, not the big valve head and 3S cam it was supposed to have, although it did have the high compression Powermax pistons. Brian Slark told me that it was not uncommon to find anomalies like that in PRs.

Does the head have the proper RH7 stamp above the right side exhaust rocker cover? That is the marking for the original short stroke head with re-angled and larger intake valves.

Ken

illf8ed said:

L.A.B. need your historic knowledge. I took this photo of what the owner said was an original short stroke 750 engine on his '71 production racer. In case you cannot see it the engine number is 235713. I don't see how it is possible this engine could have been original to a '71 proddy racer, but then I'm not that up to speed with factory racer numbering. Still, an 850 type barrell existed in '71??? Thought the short stroke 750 engine was introduced in '73.

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Pic of the rest of the bike?

Everyone with a proddy will exclaim it's original and hyper-rare. But like Triumph T120TT, they made a few but there are thousands now.

I'm surprised you didn't call the BS right there.

The owner could be correct in what he stated. The engine could be a genuine short stroke 750. The rest of the bike could be a '71 production racer replica.

Ron L said:

The owner could be correct in what he stated. The engine could be a genuine short stroke 750. The rest of the bike could be a '71 production racer replica.

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Good call. Waiting to see the rest of the bike to know if it's a real 72 proddy.

Production racers never had short-stroke engines. Their short production run had ended before these engines were produced. Add the fact the short stroke engines that came into the market after Wolverhampton had sold surpluss stock to Rhind-Tudd of WASP were un(der)developed lumps with cylinder heads devoid of squish which coked up, overheated and produced virtually no power in unmodified form.
Peter Williams once told me they had one on the test bed during the racing season when it became available, but lost all interest when the best they could get from it was well below the standart long-strokers (standart production bore/stroke) they were using.
The short stroke engines were only produced to satisfy the homologation rules for F750 racing but, to the best of my knowledge, were never sold in motorcycles bar, perhaps, the "Thruxton TX750 racers" of which probably less than a dozen were made and sold around 1973/74.

ZFD said:

Production racers never had short-stroke engines. Their short production run had ended before these engines were produced. Add the fact the short stroke engines that came into the market after Wolverhampton had sold surpluss stock to Rhind-Tudd of WASP were un(der)developed lumps with cylinder heads devoid of squish which coked up, overheated and produced virtually no power in unmodified form.
Peter Williams once told me they had one on the test bed during the racing season when it became available, but lost all interest when the best they could get from it was well below the standart long-strokers (standart production bore/stroke) they were using.
The short stroke engines were only produced to satisfy the homologation rules for F750 racing but, to the best of my knowledge, were never sold in motorcycles bar, perhaps, the "Thruxton TX750 racers" of which probably less than a dozen were made and sold around 1973/74.

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Yes, but if you re-read it like I had to you'll see that it says a Production Racer with a short stroke engine, it doesn't specify the engine came with the bike. :mrgreen: