Short stroke Nortons

It seems to me that there should be more short stroke Nortons around. especially in the 750 racing stables. Compared to long strokes they definitely put out more power and rev higher with less piston speed because of the shorter stroke. In the early days they had a tendency be less reliable because of the higher revs but all that has changed nowdays with the availability of heavy duty lower ends and reduced reciprocating weight. Heavy duty cranks are available with the 80.4mm stroke. Note that even with the 80.4mm stroke they are still undersquare (longer stroke than bore for 750s). The shorter stroke of the Triumph is the main reason that Triumphs are competitive with Nortons, otherwise the Triumphs have an inferior Head design and they wouldn't have a chance except for their shorter stroke. Below are some photos of some good looking short stroke parts.

Short stroke components with Nourish crank (owner - Gary R) Photos courtesy of Gary R.


Crank & rods (owner Gary R)



Top view (owner Gary R)



With cylinders (owner Gary R)


Below is what I consider to be a very desirable combination. Its a short stroke crank (80.4mm) with 81mm pistons. This makes it a true 850 short stroke with a shorter stroke length than the bore diameter (828cc actual displacement). This is the motor I would go for if I had the choice.

Short stroke 850 with Maney 80.4mm crank (owner Gus F). Waiting for FA cylinder head. Photos courtesy of Gus F.


Front view (owner Gus F)


Piston tops (owner Gus F)


So where are all the other short strokes out there? Lets see em.
Jim Schmidt

We can all see where this is going.
When can we expect to see what you have been developing ! (??).

Would it be eligible to race in classes where Nortons currently race though ?

Great pics BTW.

P.S. Manx Nortons - single cylinder racers - went to a 'square' engine dimensions 86 x 86 mm back in the early 1950s,
and were sorta successful at the time - the 4 cylinders were starting to rule the roost back then.
There were experiments with much shorter strokes also, and if they'd appeared 10 or 20 years earlier,
they would have blitzed the field. History repeating itself ?

Where I'd like to see this go is to bring more short strokes into Norton racing. Its legal because short strokes were introduced by the Norton Factory. Many racers buy the heavy duty cranks now anyway so why not spend the $ on a short stroke crank? With the larger bore you can install bigger valves and not have the valve shrouding problems that you have with 750 long strokes and big valves. You can get longer rods to make up the distance and steel is a good idea since alloy rods are not as reliable, or you can take the risk with stock alloy rods and shorter cylinders. There are options to be considered but if you want it to stay together then you have to do better than the early Norton short strokes with their stock type cranks that were failure prone.

If you're limited by a cc limit, like in a racing class, a short stroke could pay off and give more power per cc.
If you put a longer stroke crank in that same engine, you lose nothing and gain reliability (but it could put you over a cc limit).

The shorter stroke of the Triumph is the main reason that Triumphs are competitive with Nortons, otherwise the Triumphs have an inferior Head design and they wouldn't have a chance except for their shorter stroke.

O'Rielly :shock: Held the World Speed Record for sixteen years , can'tve been to slow .

Be intresting to see a destoked 750 for 500 class ( Norton ) or a 88 ( 72.6 mm ) stroke 750 , or suchlike . Any Flow figures on the P. R. Heads :?:

Maney Cases and 80.4 Crank, JSM Rods and Pistons, PW3, Iron Barrels, FA head.....

Waiting for.....me to finish it.....

Getting close....but sometimes that last 5% takes a little longer.....

I looked into building a short stroke, big bore, 750 8 Valve Nourish based engine, but stopped racing before I started it!

What would an 81mm bore super short stroke 750 Norton be like (stroke at approx 72.5mm I guess)?

Like Jim says, bottom ends and gearboxes are available now that will stand extreme revs.

For the road though, my thinking is that the longer stroke is no bad thing. So a short stroke 850, whilst being an interesting engineering 'study' would be of doubtful value IMHO.

I did consider a short stroke 850 when starting to plan my rebuild, but decided on keeping the 89mm stroke. I was afraid of changing the feel & sound of the engine I know & love so well. Ultimate power is less important on a road bike compared to reliability & riding pleasure. Never having tried a short stroke Norton twin I don't know what they feel like. Does anyone have both to compare?

I don't think I would consider a shorts stroke for road use, the best Commando characteristics are associated with that 89mm stroke.

Racing is different and where Jim has focused his comment.

For me it was attractive to build a 750, which means in the UK I can run in F750 and 1300 twins, though you would want a larger motor to make a Commando based bike more competitive in either ;-). As I am using an original '75 rigid mount frame, reducing vibes and 'shock' was also important. And if I had not been reunited with that frame, which I originally commissioned from Rickman in '75 I would not have started the project at all. And in '75 that frame had a lot of Short Stroke parts in it, like head, cam and pistons.....

BTW, Norman White told me earlier this year that the most power they ever saw at Thruxton was from a short stroke, in '74.

But hey, I shall be 61 in December....to me being competitive means staying on the same lap as the leaders!, and I think that is no small achievement with any 750 Commando engine compared to a 930 triple....

In UK racing bore and stroke choices are free, as long as you meet the capacity limit of the class. Hence things like 90/95mm ceramic bore singles! Norton did not go above 86mm.

Building a 750 also means I could run in some events in Europe, where the classics are limited pretty much to 750cc.

The US has a better 750 race environment from what I can see. (and lets be honest, there were very few twins above 828/860/900cc were raced in the '70s) I am convinced that as a 750, a short stroke is a good choice there, but very aware that Kenny Cummings does extremely well with an 89mm stroke.

SteveA said:

In UK racing bore and stroke choices are free, as long as you meet the capacity limit of the class. Hence things like 90/95mm ceramic bore singles! Norton did not go above 86mm.

Click to expand...

Thats not strictly true - Nortons experimented with several versions of a 90 bore manx engine.
Although its true they weren't offered for general sale (?).

In the 1980 Ron Woods short stroke was getting around 84 HP on Axtells dyno. Todays racing long strokes get HP in the mid 60s. That may not be a fair comparison but Ron W was able to get at least 10 more hp with a short stroke than with a long stroke. Getting more piston pulse per minute with less pistons speed is the big advantage. It seems obvious that more Norton short strokes should be running in todays 750 races.

One of the more developed Nortons on the race circuit (endurance) is the "Norton Yellow Peril team" (see it on facebook). This is a big bore short stroke and they probably chose it for their own reasons of power balanced with reliability.



Here's the short stroke crank of the Yellow Peril. Note the roundish cam lobes indicating BSA lifters or roller lifters.

jseng1 said:

In the 1980 Ron Woods short stroke was getting around 84 HP on Axtells dyno. Todays racing long strokes get HP in the mid 60s. That may not be a fair comparison but Ron W was able to get at least 10 more hp with a short stroke than with a long stroke. Getting more piston pulse per minute with less pistons speed is the big advantage. It seems obvious that more Norton short strokes should be running in todays 750 races.

One of the more developed Nortons on the race circuit (endurance) is the "Norton Yellow Peril team" (see it on facebook). This is a big bore short stroke and they probably chose it for their own reasons of power balanced with reliability.

Here's the short stroke crank of the Yellow Peril

Click to expand...

What crank is that Jim?

Fast Eddie said:

jseng1 said:

In the 1980 Ron Woods short stroke was getting around 84 HP on Axtells dyno. Todays racing long strokes get HP in the mid 60s. That may not be a fair comparison but Ron W was able to get at least 10 more hp with a short stroke than with a long stroke. Getting more piston pulse per minute with less pistons speed is the big advantage. It seems obvious that more Norton short strokes should be running in todays 750 races.

One of the more developed Nortons on the race circuit (endurance) is the "Norton Yellow Peril team" (see it on facebook). This is a big bore short stroke and they probably chose it for their own reasons of power balanced with reliability.

Here's the short stroke crank of the Yellow Peril

Click to expand...

What crank is that Jim?

Click to expand...

I really don't know much about the crank above, probably a special one off made by or for the Yellow Peril team.

Check out the short stroke offset crank below. This was a one off special request.


The above stortstroke crank went with a special bathtub head with increased squish band width that actually worked.

Fast Eddie said:

I looked into building a short stroke, big bore, 750 8 Valve Nourish based engine, but stopped racing before I started it!

What would an 81mm bore super short stroke 750 Norton be like (stroke at approx 72.5mm I guess)?

Like Jim says, bottom ends and gearboxes are available now that will stand extreme revs.

For the road though, my thinking is that the longer stroke is no bad thing. So a short stroke 850, whilst being an interesting engineering 'study' would be of doubtful value IMHO.

Click to expand...

There was a 650cc Triumph Thunderbird in the late 70s which had about a 75mm stroke crank, I think a 750 built around that would be a good thing. Better a Triumph with the separate cams than a Norton, however the combustion chamber shape would need to be better. I don't believe the Weslake head is much better than the Triumph 2 valve head ( a job for Fullauto ?). I lived with a 63mm stroke 500cc Triumph for 12 years of racing - I love my long stroke, heavy crank 850 - it is superb. I don't enjoy racing with my heart in my mouth. The 500cc Triumph used to rev easily to 10,500 RPM, and with 4 inch megaphones, it was very entertaining. It was the main reason I stopped racing after a big crash in 1973, and rarely raced for the next 20 years. I can still feel the pain.

acotrel said:

Fast Eddie said:

I looked into building a short stroke, big bore, 750 8 Valve Nourish based engine, but stopped racing before I started it!

What would an 81mm bore super short stroke 750 Norton be like (stroke at approx 72.5mm I guess)?

Like Jim says, bottom ends and gearboxes are available now that will stand extreme revs.

For the road though, my thinking is that the longer stroke is no bad thing. So a short stroke 850, whilst being an interesting engineering 'study' would be of doubtful value IMHO.

Click to expand...

There was a 650cc Triumph Thunderbird in the late 70s which had about a 75mm stroke crank, I think a 750 built around that would be a good thing. Better a Triumph with the separate cams than a Norton, however the combustion chamber shape would need to be better. I don't believe the Weslake head is much better than the Triumph 2 valve head ( a job for Fullauto ?). I lived with a 63mm stroke 500cc Triumph for 12 years of racing - I love my long stroke, heavy crank 850 - it is superb. I don't enjoy racing with my heart in my mouth. The 500cc Triumph used to rev easily to 10,500 RPM, and with 4 inch megaphones, it was very entertaining. It was the main reason I stopped racing after a big crash in 1973, and rarely raced for the next 20 years. I can still feel the pain.

Click to expand...

Errr, the Weslake / Nourish head is completely different to a Triumph, in just about every conceivable way, apart from having two cylinders.

The combustion chamber is actually just about the same as any modern 4 valve head, so much so that GSXR pistons can be used on certain bores, and, as I did, bronze skulls made for big GSXR based drag bikes can be fitted into the heads.

Look here: http://www.nourishengineering.co.uk/gal ... d=1&page=1

How can you develop a race motor in which the inlet and exhaust cams cannot be moved independently ? If you are going shorter stroke, the optimum power band is higher so the exhaust is different and the optimum relationship between the inlet and exhaust cam timings are different. Seems to me it is a much longer road it you have to change the camshaft every time you want tray a different relationship. One thing is certain, if the bike is going to be a pig to ride you will find that combination first.

I am well aware of what the Weslake 4 valve head is, Rod Tingate had one many years ago on a Rickman 650 Triumph. He is an excellent tuner and found there was very little advantage in the Weslake head compared with the standard two valve head. The standard two valve Norton head seems to be far superior on the same capacity motor. I think there are two things - the position and angling of the inlet ports, and the squish bands. The Weslake head might look excellent however it's the result which counts. From memory, the inlet ports are nowhere near as steep as on the Commando head. I think the main advantage of the Weslake head is the lower valve mass, and that can also be achieved in a two valve head with titanium valves.

Have to say there are no 2 valve Triumphs running at the front in classic racing in the UK & they haven't for well over twenty years (the time I have been with the club)
Strangely all the Triumph twins running at the front have 8 valve top ends.
The front running twins are all weslakes the odd Norton BMW Ducati

But of course 4 valve heads are not any better than 2 valve Triumphs???????

I think the racing results speak for themselves

Chris

Chris said:

I think the racing results speak for themselves

Click to expand...

Indeed.
Someone tried a 4 valve manx, somewhere back in history, and it gave something like a claimed 65 hp.
i.e. about +10+ hp more than a good/great 2 valver.

But they are not eligible to race??
Since it wasn't done by the factory, and was much later.

There was one racing in the just finished Manx Classic GP, but it was in the same class as the Paton twins.
Bruce Anstey came third overall (but i think first in class) on a McIntosh 1962 spec Manx, but considerably behind the 4 valve.