Was the 1961 Domi racer 500 a short stroke?

As anyone would know that had a Matchless cylinder head in their hand, but I have distilled the most useful information from their paragraphs:

Rohan said:

seems to ring a bell. didn't see much said about it, must have been presumeably, ?

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Matt Spencer said:

Pretty sure But I have not that . I presume . I think .

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Just like to point out that I have never posted anything about eccentric rockers,
And can't even recall where Ben is getting those quotes from.

CB450 Hondas have eccentric adjustments in their DOHC cam gear.
Quick and easy to adjust. (the adjustment, and lock, is all external)
That torsion bar valve spring arrangement is really something.
Rev to 11,500 rpm in the early versions too. ?
Later versions were 'only' 9000 rpm, anyone know why ?

Now why did someone change their quote from this to this. ?
Because they didn't know their technical Norton history, AND had posted rubbishing it ??

I think larger main crank bearings are a waste of time.

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Larger main crank bearings would be more trouble than they are worth now.

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No idea why this subject has been changed.
Just to let you know I have a set of Matchless rockers and their Eccentric shafts, but these were not designed to fit a Norton cyl. head.
I do not know how I could use them in a Norton head that has been machined out to accept Norton rockers-unless I was to fill the holes in with alloy weld first.

Any chance of a picture, so we can see what you are working with ?

Rohan said:

Any chance of a picture, so we can see what you are working with ?

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Err….No, not at the moment.

Why does the total lack of detail not surprise me. ?

Rohan said:

Why does the total lack of detail not surprise me. ?

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Because I don’t have a DVD camera, or any way of downloading photos.
You always like to jump down other people’s throat, don’t you Rohan?

Well the answer to the question at the start of the thread is no, Domiracers were not short-stroke but had production bore and stroke dimensions. All the known books on Norton and quotes by factory people and riders back that up.

Good luck on your Domiracer project and if you get a chance to please keep us up to date on your progress. It would be nice to see someone make a replica of the fancy Domiracer engine with the 2mtt mag, eccentric rockers and short alloy cylinder.

Someday maybe that last short-rod, eccentric-rocker, alloy cylinder engine that Dunstall had for sale in December 1972 will pop back up out of the basement, garage or trash heap it was directed to four decades ago and we will all get to take a closer look at it....

Indeed.

Bernhard said:

Because I don’t have a DVD camera, or any way of downloading photos.

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Why not just say that, instead of making an evasive mystery of it.... ?

Unable to see if this was the actual 500 sold to Dunstall by Norton or a later one. As Dunstall never sold the original works engine it has very likely got the standard valve gear. The frame looks like a lowboy :?:


http://www.flickr.com/photos/59728935@N07/6064662146/


this pic shows the works engine; note; oil feed to needle roller cam e.t.c.

http://www.etmoteur.fr/images/domiracer ... umn_93.jpg

Quite a gathering there of Domiracer etc pics pinched from all over...

http://www.etmoteur.fr/norton_domiracer.htm

beng said:

Well the answer to the question at the start of the thread is no, Domiracers were not short-stroke but had production bore and stroke dimensions. All the known books on Norton and quotes by factory people and riders back that up.

Good luck on your Domiracer project and if you get a chance to please keep us up to date on your progress. It would be nice to see someone make a replica of the fancy Domiracer engine with the 2mtt mag, eccentric rockers and short alloy cylinder.

Someday maybe that last short-rod, eccentric-rocker, alloy cylinder engine that Dunstall had for sale in December 1972 will pop back up out of the basement, garage or trash heap it was directed to four decades ago and we will all get to take a closer look at it....

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There is a pic of the eccentric rocker pins lower down on this photo sheet –next to the yellow number plate with the number 296 ;

https://www.facebook.com/media/set/?set ... 123&type=3

Eccentric Shaft in Rocker Arm; I should point out that these are not for a Norton, but maybe the Matchless?
http://www.bing.com/images/search?q=+ec ... edIndex=20

http://www.bing.com/images/search?q=mot ... tedIndex=0


Has anyone figured out how the bucket cam followers were made :?:

There is a pic of the eccentric rocker pins lower down on this photo sheet –next to the yellow number plate with the number 296 ;

https://www.facebook.com/media/set/?set ... 123&type=3

Eccentric Shaft in Rocker Arm; I should point out that these are not for a Norton, but maybe the Matchless?
http://www.bing.com/images/search?q=+ec ... edIndex=20

http://www.bing.com/images/search?q=mot ... tedIndex=0


Has anyone figured out how the bucket cam followers were made :?:[/quote]


If you look at the pic of followers and pushrods shown in the facebook site that you gave, you'll see the followers are similar in shape to standard ones, but the pushrods seat on the bottom of the follower, not on the top. 'Bucket' is a somewhat misleading description because most assume it to refer to a round follower, but in the Domiracer it was merely used to describe a subtle change to the pushrod location which meant the follower had to be hollow - or bucket like.

The change to a 650 type crank for the I.O.M 1961 TT was to use larger diameter bearings and stiffen the crank thereby. The timing side ball bearing was also replaced by a larger roller bearing type, and upon stripdown after the race it was found to be breaking up. That and the rear tyre fouling due to fling at high speed no doubt contributed to the Domiracer's rather dissapointing showing, but surely says something for Tom Phillis' ability to coax it home into third place.

My short stroke 500cc Triumph was built out of 650 parts by a Triumph factory worker and was in existence in the early 50s. It had a 63mm stroke billet crank, the long 650 rods and 12 to one comp 650 pistons which gave an effective comp of 10 to one. On a big circuit it was great, on a short one - too nasty. The longer stroke domiracer would probably be marginally better due to the better torque. If I wanted to build one, I wouldn't use Norton crank parts, but a billet crank, and I would not go below 75mm stroke. The limiting factor is always the valve gear. If it can't keep up at 10,000 RPM, the exercise becomes expensive. I think you would need a good reason to build a fast 500 twin. In Australia it is a pointless exercise - the class for the Molnar Manxes does exist, however Seeley G50s are excluded, so the whole thing seems pointless to me if you are only beating old dungers. I raced my Triumph for 12 years and only twice started in races with a 500cc upper capacity limit, and they were full of late model two strokes. I would love to race a 500cc domiracer, however the build cost and value for money in the racing I would get, makes it a non-event. I would never go there.

acotrel said:

I wouldn't use Norton crank parts, but a billet crank, and I would not go below 75mm stroke.

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Thats a LONGER stroke than the 500 Dommie had. !
Why on earth would anyone do that ??

Rohan said:

Thats a LONGER stroke than the 500 Dommie had. !

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You're right.
66mm bore x 72.6mm stroke.
The wives' tale about short stroke engines not having strong torque at low rpm is 95% BS.There are car engines with 3" stroke that make maximum torque between 1200 rpm and 1800 rpm.A short stroke engine just doesn't need as much cam duration at the same rpm as a long stroke engine does.

With the same cam duration they're both happy at the same piston speed,which means more rpm for the short stroke engine.Cams that were happy in a 650 triumph engine at 4800 rpm wouldn't have been happy until 6200 rpm in that short stroke 500 with 63mm stroke.

The cams were much longer duration however less lift. The timings we EO 90 BBDC, EC 95 ATDC, IO 90 BTDC, IC 93 ABDC and were very large base circle. It seemed impossible to me however the cams worked, the motor had nothing below 5,500 RPM, and with 4 inch megaphones it was dangerous to ride with the high first gear on tight circuits. It doesn't do me any good to even talk about that bike these days. I still remember the horrible anxiety I had every time I raced it. The Seeley 850 is nothing like that, you just hop on and give it big handfuls of throttle. By the time I eventually got around to selling the Triumph 500, I was just glad to see it go. There was never a class for it, and it was a nasty piece of shit. It was faster in a straight line than a good 500 manx, however I would outride anybody on it, with the manx. Short stroke 500cc twins are bullshit. I believe even the Paton is nasty.

So sounds like you have nothing to contribute to this discussion ?

Doug Hele got the Triumph Daytona (500cc) at Daytona percolating at 140+ mph, so there are folks who know what they are doing.
We diverge from Domiracers, however....

Doug Hele was involved in the Domiracer, we should note though.

What would YOU know about this subject ? If you want a thrill build yourself a short stroke 500cc twin cyl. race motor and try and race it. My Triumph would do the 140 MPH speeds on a big circuit, the trouble is that most race circuits also have tight parts and you have to choose whether you will gear the bike high or low. Then you find yourself over-geared in tight corners with insufficient torque. Many times I've slipped the clutch to get back onto the power band, and ended up going sideways. Buy yourself a manx, and you will beat most 500cc twins.
As a friend of mine used to say 'BE TOLD'!