Was the 1961 Domi racer 500 a short stroke?

Avoiding the technical discussion in favour of costs and logistics....and preference....

All Maney cranks are 3 piece, lightweight steel flywheel, all 'Nourish' cranks are one piece billet and 'heavy'.....

I know the guy who used to make the Noursh cranks, he retired before Dave sold the buisness. It will be someone else who makes the forthcoming batches now that Dave has also retired and Nourish, under new management, has relocated. Proposed prices undercut Maney. Waiting time should be factored in to both.

There are several 500 'Weslakes' running in UK classic racing, with pretty high rpms being used in most cases (10,000).....There are almost no 500 Norton twins (Domiracers)...and of course above 500cc there are no 600s or 650s, a small selection of 750s (80.4 and 89mm), but mainly now 900cc plus machines with 89mm or 93mm strokes.

Most modernn build Weslakes are running 90 degree cranks and of course separate pipes. Dave used to sell complete engines at a lower price than could be achieved assembling (or making) specialist Norton parts for a 500 to 1007 motor, so all round a Nourish Weslake was a good choice for some....being MUCH cheaper than a Manx or G50 single....

Although one has been seen at Goodwood that is a special case, generally it would take an odd perversion to choose to develop a Domiracer to run in the UK 500 classes today, perhaps a similar perversion to using an ES2 with pretty much Gold Star internals, yes someone is doing it. Particularly difficult choice to make to build a Domiracer when today the 500 Weslake, even in the hands of a talented rider, struggles for places against 90 plus bore Manxes and G50s....

johnm said:

Bernhard said:

quote;

It is often forgotten that Phil Read rode the 500 Dommiracer in practice for the TT, but opted to ride his own 500 Manx in the race.

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And also reported that he chose to ride the Manx because the Domi spat him off in practice due to an abrupt power band.

If you look at photos of the Domiracer you can see why. The header pipes are too short. (My opinion based on 5 years of dyno testing and racing of 500 twin Norton engines.)

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RE; "And also reported that he chose to ride the Manx because the Domi spat him off in practice due to an abrupt power band."

Perhaps the 500 Domiracer also had a sharp power band as it had a cam with .500inch lift on the inlets and Amal G.P. carbs, together with a Manx close ratio gearbox, I should imagine it was a bit of a pig to ride.

Doug Helne used bucket cam followers with longer push rods, Paul Dunstall in his infinite wisdom lighten the cam followers his way;

1968-dunstall-commando-lyster-resto-t17964-30.html

acotrel said:

I'm not aware of what the standard bore and stroke of the stock 500cc dominator might be. I'm surprised that it is shorter than the stock 500cc Triumph (80mm stroke). If it is close to 75mm, I wouldn't change it. So I don't think the domiracer would have been much shorter stroke. It would not make sense to do that. When the stroke of the twin gets down to around 60mm, it all becomes too difficult unless you have four valve heads and 6 speed gear box. When the original domiracer was around, most British bikes only had 4 speeds. The domiracer also had megaphones, so if it was really short stroke, it would be a pig - the results say otherwise.
I recently saw a Seeley with a short stroke 500cc Nourish Weslake engine fitted - that would be a bit exciting. (Even had separate pipes.)

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Curious what you mean by

When the stroke of the twin gets down to around 60mm, it all becomes too difficult unless you have four valve heads and 6 speed gear box

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Reason being I'm working on a 500 twin with 61.5 stroke. :?:

Its a good job that Mr Honda et al didn't listen to this 'advice' and not build his famous 5 cyl 125cc and 6 cyl 250cc GP racers -
that demonstrated that there IS power on the other side of 10,000 rpm.
And even over 20,000 rpm....

Some of em did have a lot of cogs in their box, it must be admitted though !

Rohan wrote

Its a good job that Mr Honda et al didn't listen to this 'advice' and not build his famous 5 cyl 125cc and 6 cyl 250cc GP racers -
that demonstrated that there IS power on the other side of 10,000 rpm.
And even over 20,000 rpm..

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never mind about Mr Honda's multi cylinder DOHC 4 valve per cylinder works specials, they are hardly relevant to the discussion - tell us
about his twin cylinder, two valves per cylinder, pushrod engines

Point taken - but Mr Honda arrived there via 2 valve engines and then going to sohc initially.

But if the Norton 500cc dommie twin had a shorter stroke from its birth than Alan is describing,
surely this has no place in this discussion either ?

If you have a look over at "speedtalk.com" in the advanced engine tech section,they're saying that an engine has most power potential when the bore is 1.5 times or even 1.6 times the stroke.Any bigger bore/shorter stroke makes the combustion chamber too much like a thin orange peel.

At 1.5,that works out to 78mm bore x 52mm stroke for a 497 cc twin.
They'll also tell you that short stroke does not mean less low rpm torque,and other things will affect that.

I don't believe that piston speed is a fair way to compare engine stress.Piston speed hardly matters,but maximum piston acceleration matters.If you halve the stroke and double the rpm,you'll end up with twice the acceleration which is too much stress.You could increase the rpm by the square root of 2 and have no problem.

78 mm bore and 52mm stroke,with power peak at 9000 rpm sounds pretty safe to me.

4 valve heads just allow you to use less cam duration and less lift to make much the same power.Less duration pays off with more torque at low and mid-range rpm.

I feel I ought to contribute here but if I were going to build my ultimate 500 Dominator engine for NZ classic racing the first thing I would do is a lot of research and thinking. So I cannot say what I think right now. Plus I will probabely buy the software metioned by Snotzo to try a few options out.

Much could also be learnt from the best 250s of the past. Aermacchi, NSU, Moto Guzzi, Mondial, Morini etc. There are several good books on classsic motorcycle engines which deserve careful study.

So I dont really know what I would build but there are a few practical issues that come to mind. Unless you are really rich you will have to use major components that are already available.

Obtaining a head. You would probably go the Fullauto route and perhaps try to get one which could be machined and ported for a 500. Some authors would suggest the standard Dommie needs a larger diameter inlet valve and a small diameter exhaust valve. Its unlikely to be the same as a Commando.

My ideas about rpm (bore and stroke) would be constrained around how well you could make a two valve engine breath at high rpm and how to control the valve train. These are probably your limiting factors. With the right pistons rods etc they shouldnt be a limiting factor. Snotz has already pointed out a standard Dommie can go to 8500 using the old 4000 ft/min rule of thumb My Dommie has a one piece Nourish crank and has reved to 8500 as recorded by the onboard video - but I know from the dyno power and torque is dropping at that point. Breathing could be simulated on the computer but simulating a valve train is only possible on very expensive software. As this point you might be best to steal ideas from those who have gone before Beehive valve springs, diferent followers etc. Jim Schmidt would be the sort of man to talk to !

I would try to build an engine that minimised vibration and was very strong and reliable. An ES2 ws mentioned above. There is a ES2 in NZ that beats most Manx Nortons. It won races in the US this year. But even the owners would recognise that it is fragile. It is much much easier to make a machine orignally designed for racing - Manx, G50, KTT etc go faster for longer than the very best modified or developed production engine. But the twin does have an advantage. Its a four valve engine by design. Only a couple of Manx Nortons can match that

Paper or computer designing and simulating can be a very interesting exercise. Either way you don't have to make anything neither are you going to break anything.
X-file draws attention to 'speed talk.com' a site where some very self opinionated types hold forth on all kind of topics relating to the internal combustion engine. There are often some highly informative posts to be found there, but one should bear in mind that all too often posts will be merely an individual's opinion, and should be treated as such. The comment about bore to stroke ratio is interesting however, but before going further we should keep in our minds that this discussion is supposed to be relative to a 500 Norton Dominator in general, the factory Domiracer in particular, i.e a twin cylinder, air cooled, pushrod two valve per cylinder engine.
What the factory built is difficult be be certain. We know the bore and stroke of the 1961 TT machine (s), but what else they had tested prior to the TT, or what they were planning to do by way of development is open to some speculation. Paul Dunstall is reported to have driven away with a substantial amount of parts, Reg Dearden likewise. What each ended up with has never been cataloged as far as I know, so we end up speculating as to what might have been if the factory had continued.

Acotrel favours a 75 mm stroke as the smallest he feels inclined to go with. That would in turn go with a 65 mm bore, which compromises the valve sizes that are possible unless the valve angle were opened up considerably. Rev wise not a problem, but if I may add something extra to my earlier mean piston speed figures, something else emerges of interest. The corrected mean piston speed concept was first put forward by Dr. Frederick Lanchester as a more meaningful way of representing the stresses in an engine. It is simply the mean piston speed divided by the square root of the bore/stroke ratio. In the instance of Acotrel's dimensions, while the mean piston speed at 8250 rpm is 4060 ft/min, the corrected mean piston speed is 3780 ft/min. This was considered to be more meaningful because it took into account a smaller (and theoretically) lighter piston.
To go to the other extreme, 72Combat is working on a stroke of 61.5 mm . If I round it down to 61.4, it means the bore can be a round 72 mm for a swept capacity of 249.99 cc per cylinder. This bore/stroke combination would need to rev to 12000 to see a mean piston speed of 4029 ft/min, and a corrected speed of 4363 ft/min. Not bad - still ok for stresses, but getting into engine speed region where control of the valve train is beginning to look difficult. bear in mind the larger bore will allow larger valves, and for the valve curtain area to allow adequate intake flow the lift needs to be greater than 25% of the inner seat diameter.

The final consideration of a 52 mm stroke with a 78 mm bore that meets the speed talk guru's criteria for a bore /stroke ratio of 1.5 is out on a limb on it's own.
A 4000 ft/min mean piston speed equates to a 12000 engine speed, but the Lanchester corrected figure is 5015 ft/min. In addition, to make this configuration work well, large valves in a 78 mm bore are a necessity, but the means of controlling then at engine speeds in the region of 10000 to 12000 rpm would present any potential builder with some considerable headaches.

The above is all an exercise on paper. Doug Hele, being a clever and resourceful engineer would surely have had ideas for the future, but he may also have felt that the real future needed a different design completely in order to make meaningful forward progress. As we all know it never went any further. AMC bought up Norton, and Doug moved to Triumph. Acotrel is entitled to his ideas, good luck to him. Johm is presently contemplating some future project, good luck to him also, and anyone else for that matter who wishes to do his own thing.

All very well , but the 500 dommie Racer and the 650 / 750 were PRODUCTION derived raceing / development engines , comparison to the D.O.H.C. Manx is intresting ,
performance wise .

IF the lessons learnt HAD been used in the PRODUCTION road machine engines , AND the standard of manufacture was suitable , things would have been interesting .
Whoever , metalurgical limitations - Valve spring tecnology - wouldve meant the service life & maintanance requirements were not acceptable - at that time .

Components count actually means O.H.C. has less componentry , standard of accuracy of manufacture & existing tooling / costs were decideing parameters .

this hidious contraptin - -
aptly named ' Vulture ' was in effect Two peregrines , run Flat out in early guise , theyd throw a rod . inadequate oil & cylinder cooling would have been contributary .
plus other things - griffon development - got in the way . so a promising design - despite service failures - was cancelled .

likely similar constraints on Norton development , ' we cant afford it ' one way or another . So eventually production suffers through lack of investment .



Not as if there wasnt the capeability there . They lacked the thoroughness of the nasty germans , who were also half assed . Motorcycles were seldom

' price No Object ' recreational goods .

A Weslake would be easier to play with. At least the inlet and exhaust cam timings are independently adjustable without regrinding the camshaft.

Matt Spencer said:

Not as if there wasnt the capeability there . They lacked the thoroughness of the nasty germans , who were also half assed . Motorcycles were seldom

' price No Object ' recreational goods .

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The original factory 500 Dommiracer unlike this Alfin barrel which came with a cast iron liner, was sent to Germany to be specially chrome lined in the cylinder bores, as there was nowhere in the UK in the 1960s where this could be done at the time.
The image of the advert of the Alfin alloy barrels might be a good indication of where Norton got them made.
Does anybody know :?:
Where the cam followers tunnel in the barrels also chrome finished? or did they run them straight in the alloy barrels :?:

It wouldn't be difficult to sleeve the cam follower tunnels or use follower blocks as in Triumph motors.