Shortstroke Build

comnoz

comnoz

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Dec 10, 2008
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After rounding up all the parts here is the first step of the shortstroke 750 roadrace engine build.
This is a 83mm bore 69mm stroke engine. Here is the custom crank. It was built before piston and rod weights were known so we had it built with plenty of extra metal to work with. The crank weight is near the weight of a stock 750 crank.

[video]https://youtu.be/PQ5dqktsWrQ[/video]
 
I love your movies. Always educational. Question: When you need to remove say 20 gms, how do you know how much you are machining off at the time?
Jaydee
 
jaydee75 said:
I love your movies. Always educational. Question: When you need to remove say 20 gms, how do you know how much you are machining off at the time?
Jaydee
Click to expand...

I can weight the whole crank to see how much I removed. Beyond that it's just an approximation and I hope I error on the not enough side.
 
I was impressed by the magnets staying on at an indicated 1200 rpm. as a turner I have seen a few things shall we say leave the machine rapidly over the years at much slower speeds :oops:

Is there a reason why you milled flats on the crank as apposed to drilling holes (that you more normally see) ?

Did you make the crank? It does look good.
 
I really enjoyed this video. Thank you. Now folks can understand why these processes are not cheap.

I would love to see a factory crank on the balance machine with no modification.

Wait, maybe not. I'll be tearing it apart!

This will surely help higher sustained revs but how much rpm? I assume more modification to the rest of the engine would be in order in a shortstroke application as well right?
 
Awesome to think that's only 1,200 rpm! Would love to see it spinning at 7,500. :shock: Great stuff, thanks Jim.
 
toppy said:
I was impressed by the magnets staying on at an indicated 1200 rpm. as a turner I have seen a few things shall we say leave the machine rapidly over the years at much slower speeds :oops:

Is there a reason why you milled flats on the crank as apposed to drilling holes (that you more normally see) ?

Did you make the crank? It does look good.
Click to expand...

Rare earth magnets. I have not had one come off -but I don't stand in the line of fire either. They work better than the clip on alternatives I have seen.
I did not make the crank.
I don't like to drill holes, they create stress risers. Of course holes are easier.
Shallow dimples with a drill are fine if they're in a safe place.
 
Will you be running a 6 speed box behind that engine ? With that stroke, getting enough torque might be a problem. I don't think I would go below 75mm stroke. My old Triumph 500 was a 650 stroked down to 63mm. In some ways it was very good, however riding it gave me a lot of anxiety due to it's power characteristics which were very much all or nothing. It was all too bloody hard. With the torquey 850 motor in the Seeley, it is all too easy. I'm certain getting the complete package right is the answer. The power characteristics, handling and gearing all need to be in sync. With short stroke, there is not much leeway.
 
I think some people add up the components of their bike to create an expectation. You can fit the best motor into the best frame and still come up with a dud. Tritons are an example of that. A good 500cc Manx will beat most 650cc Tritons. It is about the whole package. I think the featherbed Manx was developed over 12 years of TT racing. It amazed me how the one I rode was so right.
 
Well shortening the stroke will not reduce the torque directly,

Of course the rest of the tuning and porting to take advantage of the rpm capability of the short stroke, will reduce the midrange torque.
 
My mate had a road going Rickman with a shortstroke motor fitted , what a great responsive motor that was 8)
 
Nice one Jim!

This must be the shortest stroke 750 Norton ever isn't it?

Are you using a Maney 1007 style set up for cases and Barrels to allow for that bore size?

Hope you keep the thread going Jim, I'd love to follow this build right through...!

However, I also hope this project will still allow you to do other work... especially heads... especially mine... !!
 
Is this the crankshaft for the Dawg Chaser ? Were tuning here in Mudville also.

John Ellis
AHRMA 5X
F750
 
Jim, how do you bring the barrel height down once you've shortened the stroke? Just mill off the top?
 
Amazing stuff as usual Jim.

How does the system measure the balance? Are there transducers in the fixture?
 
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