Another short stroke

Hi guys,

It's been a long time!
I bought this factory short stroke crank, that came with a letter authorizing its use.
A real piece of history.
But what pistons could I use with it in order to build a road racing bike? The ones I got are trashed. And the conrods seem a bit on the heavy side. Would you reuse these?

Regards

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Ping Jim at JSMotorsport, I'll be buying rods and pistons for my short stroke from him

Yes, I've already been one of Jim customers.
So, if I guess right from what I read, these rods and pistons are waaay too heavy.

I had a doubt about reusing these super strong looking rods.

The short stroke Manx Norton 500 was faster than the long stroke Manx Norton 500, but my short stroke Triumph 500 was faster than a short stroke Manx Norton 500. However it was a piece of shit. If it was balked in a corner, the clutch need to slipped to get back into the power band, and the bike would go sideways. My Seeley 850 is the easiest motorcycle on the planet to race really well. I can just flick it into corner while braking. then immediately get on the gas and power full bore through the corner, under the other guys and with less lean. With most bikes, the rider cannot gas them until they are almost out of the corner. When I built my bike I believed it would be useless, because I knew how most bikes handled. The 850 motor is radically different in the way it needs to be used.
When my bike self-steers it's way around corners, it feels absolutely wrong. However it is very effective. If I gas it when on a lean, it turns in the direction of lean and stays more upright. I used to be very careful with the throttle when racing. Smooth power delivery is better, when you accelerate really hard when on a lean.
I only ever picked-up on this stuff, because I rode a Short Stroke 500cc Manx Norton and I know what they do when you gas them when on a lean. It is the reason they were much faster than most other bikes. The weight distribution is important- needs to be well forward, or you can lose the front.
Of course I might be an idiot and not know what I am talking about.

I worry about racing again, because if I was to get into a corner miles to hot and accelerate hard to get around, the speed would be horrendous. If a tyre lost grip or I twitched, the whole thing might become very spectacular. I now have 6 speeds close ratio instead of 4 close with a high first. So it would be on from the word go. It is probably lucky my wife has banned me.

Xpongebob said:

I bought this factory short stroke crank, that came with a letter authorizing its use.

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I'd be interested in seeing that letter!

WHO "authorized" the use of the subject crank, in WHAT machine/application? (Not to mention why the user would need authorization).

I have to assume something is lost in translation...

I'm away from my shop, but it virtually says the sell and use of this crank is authorized by the factory. I'll upload it when back home.

hosting pict

Xpongebob said:

I'm away from my shop, but it virtually says the sell and use of this crank is authorized by the factory. I'll upload it when back home.

hosting pict

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Ah, that makes sense.

"warranty will be upheld" (more or less)

My Short Stroke (Maney crank) uses JSM rods and pistons. Jim has a suitable longer rod for his pistons in short strokes. They will differ in length to your factory rods, which are designed for the Omega pistons, and of course the original Omega pistons were intended for use with a fully hemispherical combustion chambered cylinder head.

I have an Australian made Fullauto head, so JSM longer short stroke rods and lightweight pistons work fine, assuming your barrels are around standard deck height. (I have 0.020" skimmed off mine)

The only parts I have left of my original '75 built race motor are the pistons, similar to yours, but with a smaller gudgeon pin hole to fit standard rods. Except one of mine has the head of a rather large vale stuck in the top.

The '75 motor was in effect an 850 version of the 750 short stroke. I raced it from '76 to end '79. Thruxton used similar motors for non 750 restricted UK races, at National level we ran mostly 501 to 1000 class.

(whereabouts are you located? I am near Flers in Orne)

SteveA said:

My Short Stroke (Maney crank) uses JSM rods and pistons. Jim has a suitable longer rod for his pistons in short strokes. They will differ in length to your factory rods, which are designed for the Omega pistons, and of course the original Omega pistons were intended for use with a fully hemispherical combustion chambered cylinder head.

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Hi Steve, please help jog my memory. The Omega pistons have a circular squish band, so the head's squish band is different obviously. Is depth of the combustion chamber different to a standard head, e.g.,RH4 ? Is the RH7 (?) head geometry depicted somewhere?

- Knut

Xpongebob said:

I bought this factory short stroke crank, that came with a letter authorizing its use.


[url=https://postimg.cc/RqTQ8DrR] [/URL]

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Can someone in the know please confirm the crankshaft has the shape of a genuine shortstroke crankshaft? I am puzzled by the rim which has been welded on the internal cheeks.

- Knut

Here's the factory 335 gram omega piston compared to the JS 216 gram piston for the fully hemisphered factory short stroke head.


Be sure to put a radius on the crank output shaft to keep it from cracking (see details in the JS race manual).



Here's the heavy factory to JS Carrillo rod comparison

Thanks Jim for these inputs.
I'll get in touch with you when I am sure of what's needed.
Unfortunately, I only have a RH4 head on the shelf. And I need sone barrels as well.

Regards

Xpongebob said:

Thanks Jim for these inputs.
I'll get in touch with you when I am sure of what's needed.
Unfortunately, I only have a RH4 head on the shelf. And I need sone barrels as well.

Regards

Click to expand...

Send your head to Steve Maney for his stage 2 or 3 conversion.

Andy Molnar does fabulous alloy barrels.

That is the strangest looking "short stroke" crankshaft I've ever seen. The stock Norton short stroke 750 crankshaft looks just like a standard Commando crankshaft, except the rod journals are ground and finished at a closer distance from the mainshaft centerline. If you saw one on a bench, you would probably think it was just a standard crankshaft, unless you measured the stroke, or had enough experience with the short stroke to notice the slight difference in the journal location. I'd really like to know the history of your crankshaft, who made it, and why. My fading memory tells me I've seen pictures of a similar welded together Commando crankshaft, but I've been unable to find any, so that might just be another senior moment.

Ken

In rummaging through my files, I did find this picture, that I had labelled as a "Martin Bernert Crankshaft", and which looks quite similar to the one in this thread, except it has had the welds machined flat. Martin Bernert is (was?) a Spanish Norton enthusiast, and at one time was on this forum.



Ken

mdt-son said:

Hi Steve, please help jog my memory. The Omega pistons have a circular squish band, so the head's squish band is different obviously. Is depth of the combustion chamber different to a standard head, e.g.,RH4 ? Is the RH7 (?) head geometry depicted somewhere?

- Knut

Click to expand...

Yes, the short stroke head was chambered fully hemispherical right out to the 77mm bore, and or course, the chambers become a larger volume, not a problem if you were going to use a domed piston. The valve angle was also machined to suit large valves from the blank, so no 're-angling was required'. So in effect there was no squish band in the head, not that Norton thought of it as a squish band anyway. Squish wasn't something that got mentioned much in '70s UK race circles regardless of the way we bandy it about these days. The pistons provided 'squish' by filling the area out to the edge, though the design was to regain compression volume in the shorter stroke engine.

Silly high compressions were also something for dope motors and Triumphs. 10.25 to 10.5:1 was plenty for a 750 or 850 you had to reliably push start each race. When my pistons were subjected to a loose valve head at 7200rpm, I took the head and barrels to Mick Hemmings. The head was repaired and skimmed 0.040", the barrels bored 0.040" and skimmed 0.060". This gave about the same compression. Mick supplied flat top pistons with valve cut outs, But when I tried installing them the rim hit the head. I had a 45 degree chamfer machined off for clearance.

JSM has some suitable pistons, as in his response, and I think AN did have some Omega ones at one time too, but for standard rods. Note, they differ from the Omega pistons they now list for 'standard 77mm 850s. So someone has drawings, I have never seen any, just the parts I personally had. My current short stroke crank is from many, and like Ken says, you would need to know details to tell the difference from his 89mm versions. Thankfully, he stamped it 80.4!

Ken would have been the guy I would have asked your questions of, and he has answered. I have not seen a crank like that, but here in France, they do things differently!

lcrken said:

In rummaging through my files, I did find this picture, that I had labelled as a "Martin Bernert Crankshaft", and which looks quite similar to the one in this thread, except it has had the welds machined flat. Martin Bernert is (was?) a Spanish Norton enthusiast, and at one time was on this forum.

View attachment 117318

Ken

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Martin Bernert sounds a bit like a French name to me, but, around southern France and Northern Spain and Italy

Xpongebob said:

Thanks Jim for these inputs.
I'll get in touch with you when I am sure of what's needed.
Unfortunately, I only have a RH4 head on the shelf. And I need sone barrels as well.

Regards

Click to expand...

Martin Bernert sounds like a French name to me, but in Southern France and Northern Spain and Italy, the names do travel cross border regularly, my friend Virgile Riccomi is from Nice!

Xpongebob said:

Thanks Jim for these inputs.
I'll get in touch with you when I am sure of what's needed.
Unfortunately, I only have a RH4 head on the shelf. And I need sone barrels as well.

Regards

Click to expand...

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