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Hope all is good for both of you at home , Pierre
Another short stroke
- Thread starter Xpongebob
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Hope all is good for both of you at home , Pierre
Hi Steve,I have a pair of 850 barrels that have been sleeved I am not currently using, standard bore and one race meeting use, they just need the sleeves pinning to stop minor rotation! Norman White did the barrels for me, and he has offered to pin them for me, but I haven't been in the UK to take them.
The rod Jim shows is what I have in my short stroke, with his 'standard' lightweight pistons and Fullauto head. Easiest way forward is to get your RH4 to the spec you want and use JSM flat tops....or....get a new Fullauto
Forgot to say I am in Bordeaux!
I have been the Fullauto/Maney Cylinders/crank on my previous build, and would like to go a bit cheaper way for this one if only possible.
. I am interested in those cylinders then!
Xavier
lcrken
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Back in the early '80s, I bought an ex-factory flat track race bike that NVT had given to John Hatley when the company was in the process of shutting down it's racing activities, just to get the short stroke 750 engine to use in my PR. I raced it for several years with no serious engine problems. I eventually replaced it with a 920 engine, and loaned the 750 to my racing buddy for use in his Commando racer. One of those nice looking steel rods (the same ones you have) broke and trashed the bottom parts of the engine at a race in Park City, Utah. I later learned that such failures were not uncommon. The general consensus at the time was that the factory got a little too carried away in making the rods light weight. The rods in my engine had been Magnaflux inspected for cracks and shot peened the last time I had the engine apart, but one of them still broke. To be fair, they did have a lot of harad racing miles on them. I built and raced a couple more short stroke 750 engines after that, but switched to Carrillo steel rods. I never had a problem with them, but did manage to break two crankshafts. If you still want to use the stock steel rods, I'd definitely recommend you have them crack checked, as well as shot peened, and try not to over-rev them too much
. I'd also, as Jim already recommended, cut (and polish of shot peen) the radius into the crankshaft cheek at the base of the mainshaft.FWIW the stock short stroke rods weighed 530 g, and the Carrillo ones weighed 528 g. I don't know what Jim Schmidt's Carrillo short stroke rods weigh, but I'm sure it's significantly less than the ones I had.
Ken
lcrken
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Also, for comparison purposes, here's a couple of pictures of a stock short stroke crankshaft that came with the flat track bike as a spare. It has a little different treatment on the mainshaft radius than recommended above. The crankshaft had cracked after a lot of racing use, so I had a local crankshaft shop turn a deep enough V-groove to get to the base of the crack, fill and build up the area with their crankshaft wire welding machine, and then grind it to a .090" radius at the transition between cheek and shaft. That was a little different from grinding a radius into the cheek, but it worked well. Unfortunately, this is the crank that was in the engine when the steel rod broke, and the crank was damaged beyond repair.
lcrken
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Hi Knut, here are some pictures of RH7 short stroke 750 heads. First one is stock, as sold to customers, except for the KPM valves and stepped studs, which I fitted for use with one of my early alloy cylinders. The second one is from the factory flat track race engine, and has even larger valves. And the third is one modified by Axtell for Ron Wood's flat track race bikes.
Ken
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When I was young, my race bike motor was a 650cc Triumph with a 63mm stroke instead of 82mm - which made it 500cc. It was made to race against the 500cc Manx Nortons. Increasing the rev limit definitely delivers more horsepower - but less torque. My 500cc Triton was extremely fast , but it only had 4 gears close ratio. I could choose where I was going to lose a race. On a very big race circuit, it was better than on a short circuit. With high gearing it would accelerate slower but forever.
With a long stroke Commando engine, gearing is still very important. I never believed in my 850, however I now believe Norton did not get it wrong - it is simply a different way of achieving a result - and I suggest it is better.
With a long stroke Commando engine, gearing is still very important. I never believed in my 850, however I now believe Norton did not get it wrong - it is simply a different way of achieving a result - and I suggest it is better.
lcrken
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For those who haven't already seen it, this is a great thread on a pretty trick short stroke 750 build.
Ken
Ken
Thanks a lot for sharing your story Ken.
I’d never thought one would be able to break one of those sturdy looking factory rods, but I guess if I was to reuse them I wouldn’t dare to rev the engine to that point. It would be more of a tough road use than race.
What pistons would fit on those, besides the heavy and unobtainable Omega ?
I’d never thought one would be able to break one of those sturdy looking factory rods, but I guess if I was to reuse them I wouldn’t dare to rev the engine to that point. It would be more of a tough road use than race.
What pistons would fit on those, besides the heavy and unobtainable Omega ?
lcrken
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Jim Schmidt can supply forged pistons for the short stroke 750 engine. I suggest you contact him directly at http://www.jsmotorsport.com/. He only lists them for his long Carrillo rods on the website, but he can also supply them for the stock Norton steel rods, although they may be special order. Note that the steel short stroke rods are 6.200" center to center, while the stock Commando alloy/steel rods are 5.875". Also, the Omega short stroke pistons take a larger diameter pin than the standard Commando pistons. I'm sure Jim can sort out something that will work for you.
Ken
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With the 850 Commando engine, a lot seems to depend on how it is used. It took me longer than usual to note what it actually does. I think my gearing is insane, however it has made the biggest difference. The Seeley frame allows the Norton motor to be used very effectively. Whenever I ride any motorcycle for the first time, I ride it fairly fast into the middle of a corner, and gas it hard while on a lean. I take note of which direction it turns. If it does not run wide, I can advance the point at which I start to accelerate on my way out of the corner. When that is done, the actual power of the motor can be less relevant. However, the power delivery must be smooth. With some bikes it is possible to accelerate hard from beginning to end, right around a corner, and to the end of the next straight. Close gear ratios are better, but 6 speeds are preferable.For those who haven't already seen it, this is a great thread on a pretty trick short stroke 750 build.
Ken
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I have a nos pair of Omega 77mm std domed Norton pistons with .75” (19 mm?) holes for gudgeon pins and always wondered why they didn’t take standard pins. Now I know they are probably short stroke pistons. I’ve had them for so long that I’ve forgotten where they came from.
Trouble is that they have pockets machined to fit the 8 valve head so wouldn’t be of any use to anyone. Nice novelty items though.
Trouble is that they have pockets machined to fit the 8 valve head so wouldn’t be of any use to anyone. Nice novelty items though.
Well, perhaps making custom bushes for the rods could solve the pin diameter problem ?
What are these Omega Andover are selling ? :
They seem to quite raise the compression.
What are these Omega Andover are selling ? :
Andover Norton - OMEGA FORGED PISTONS PAIR 850cc (77.50mm + 0.020
OMEGA FORGED PISTONS PAIR 850cc (77.50mm + 0.020
andover-norton.co.uk
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SteveA
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They are a modern Omega piston sold 'simply' as a quality high compression piston for standard rods.Well, perhaps making custom bushes for the rods could solve the pin diameter problem ?
What are these Omega Andover are selling ? :
They seem to quite raise the compression.Andover Norton - OMEGA FORGED PISTONS PAIR 850cc (77.50mm + 0.020
OMEGA FORGED PISTONS PAIR 850cc (77.50mm + 0.020andover-norton.co.uk
SteveA
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My pair are a novelty item too! But they are the short stroke Omega piston but machined from the blank with the smaller Commando gudgeon pin size.
Done to build 850 motors with RH7 heads for 501 to 1000 class racing which was popular in the UK at the time.
Dave Croxford has said it was the best configuration of works Norton he rode.
Having run one of these engines for 4 seasons, I know exactly what he meant. Mid-range by the bucket load, and better top end than anything based on a long stroke head!
acadian
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For those who haven't already seen it, this is a great thread on a pretty trick short stroke 750 build.
Ken
this was a monster bike, unfortunately Herb's "left the game" altogether and sold most all of his spares
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If I wanted more horsepower by raising the rev limit, I would make a one-piece billet crank without reducing the stroke. I always thought my 850 motor was an insane design, however it turned out to actually work extremely well when used with close ratio gears and high overall gearing, in a frame which suits its power delivery.
To my way of thinking about motorcycle race motors, its design never made sense.
It doesn't matter what the stroke is, because everything else gets adjusted to suit it. However a peaky motor on tight circuits can make life more difficult. The strength of my Seeley 850 is in the way it can be used in corners, not in the way it performs in drag races down straights. It comes out of corners much faster than other bikes, so does not need as much top end.
The bikes I used to race against were as quick as and similar to Kawasaki Z900 s. I could keep up with them easily down the straights because of the difference in speeds as we exited corners.
The Japanese bikes of that era, did not really get going until the Suzuki XR69 arrived.
To my way of thinking about motorcycle race motors, its design never made sense.
It doesn't matter what the stroke is, because everything else gets adjusted to suit it. However a peaky motor on tight circuits can make life more difficult. The strength of my Seeley 850 is in the way it can be used in corners, not in the way it performs in drag races down straights. It comes out of corners much faster than other bikes, so does not need as much top end.
The bikes I used to race against were as quick as and similar to Kawasaki Z900 s. I could keep up with them easily down the straights because of the difference in speeds as we exited corners.
The Japanese bikes of that era, did not really get going until the Suzuki XR69 arrived.
