Was the 1961 Domi racer 500 a short stroke?

Bernhard

Joined
Apr 20, 2011
Messages
5,773
Country flag
There is information in Roy Bacon’s book of the Norton twins that the 500 works Domiracer with the alloy barrel had a non standard crank with shorter con rods than normal.
According to Paul Dunstall, none of the engine parts were interchangeable with a Dominator. (Which is to an extent, true?)
So where they a short stroke motor?
 
' my information ' :lol: says 99 type crank at 88 stroke .Being Journals on 99 are larger than 88 therefore ' 99 based ' .
Obviously the rods have 99 size for shells also .
 
88 mm would make it an ultra-long stroke ?!!
So your info must be flawed.

The 99 wasn't 88 mm either, guess again ??

P.S. Don't know about all models for all years, but the parts book says 88 and 99 conrods were the same.
 
No , Thats INCHES . :wink:

' Motor cycling ' October 5 , 1961 . P 699 etc This is the I.o.M. machine , 285 Lbs Wt . 66 Bore x 72.6 Stroke , 11:1 C.R. 52 b.h.p. @ 7.200 r.p.m. ( note the correct punctuation of the era ! :D )

other ' information ' suggests no internal engine parts were interchangeable with production items . Theres a 500 Domi raced in N.Z. currently , stuff on U tube .
Wonder how many Tech. representives took delight in missleading gullible journalists .

Was something ' elsewhere ' i cant lay my hands on , regarding the Crank / rods . IF the ' SS ' or 650 was out then , with the Larger big end journals , it was refering to these being utilised to give more (or some)
overlap on the webs , for obvious reasons. Im sure Rohan will enlighten us . :? 8)
 
I have read a lot of material on the Domiracers and it all said they were the standard Model 88 bore and stroke. I have never seen even one part of a sentence that ever mentioned anything else being tried. You will find talk about the cylinder and rods being shorter on the latest type development engines as used for that 100mph lap at the Isle of Man.

Most all the photos you will see of Domiracers will show them with standard looking engines, cast iron cylinders and standard valvegear. The engine with the fancy short alloy cylinders and eccentric rocker adjustment is mentioned along with that 61' Isle of Man race and that is it. It was entered in a GP race after the 61' Isle of Man but it rained and the bike crashed out. In 62' a couple of Domiracers were sold, one to Rudi Thallhammer and one to Luigi Taveri's tuner, but when you see a photo of Thallhammer's bike the engine in it looks completely standard from the outside. Another thing you don't hear a lot about was that Domiracers were built not only with the lowboy frame, but also built and raced with the standard Manx frame.

In 1962 word came down that Bracebridge street was closing and everyone jumped ship before the year was out ending the roughly three years Doug Hele had spent getting the Domiracer to where it was.

Dunstall ended up with the Domiracer stuff as everyone knows. I have a copy of a letter written from Dunstall to a couple of racers in the USA, dated December 1972, ten years after he had received the stash of works twin parts, stating that Dunstall had one works-type Domiracer engine left that he was willing to sell, that had a short alloy cylinder, eccentric rocker adjusters and Commando-sized rod journals and matching rods and a larger than stock drive-side crank bearing He wanted 200 pounds for it.

By the time the of the above letter, the Domiracers and works parts had been beat on the track, blown up and mixed and matched and scattered to the four winds for ten years and like all 500 four strokes completely obsolete in the face of current two strokes.

So I would not worry too much about replicating the 61' Isle of Man bike, it was the exception, not the rule as far as Domiracers go. Thallhammer entered his in a number of GPs and races and did well and a lot more than the factory bike did, and his with the standard looking engine and a Manx frame, not a lowboy.

The three works racing twins sent over the the United States in 1962 had an externally standard looking engine besides the Gp Carburetors, but on the inside they had the large rod journals with special rods and bucket tappet-needle bearing cams with pressure oil feed. To appease AMA rules they used the slimline street frame.
 
Its perhaps also worth pointing out that there are 2 different versions of the "lowboy" ?

The original manx lowboy was decidely different - the front forks didn't have a top yoke, just a very sustantial lower yoke. This bike still exists somewhere, ClassicBike or someone featured it.
 
Thalhammer got his- a pukka works engine- from Doug Hele. Spoke to him at a hillclimb a couple of years ago. He confirms all the castings were different from production items. So forget your replicas- what you will end up with is a standard racerized Dominator, not a Dommiracer.
Joe Seifert/Andover Norton
 
Whatever sort of Domiracer Thalhammer got, here he is with it, photo from Mick Walker's book. Definitely a k2f style mag, not the 2mtt based one, standard featherbed frame, no eccentric rocker adjustment, standard looking 8-fin cylinder except note how close the top cylinder fin is to the bottom head fin, something has been skimmed to raise compression. You can see the high-pressure plumbing for the needle cam and rockers and the racing ball-bearing tach drive. This engine and exhaust looks close to the engine in the three race bikes sent to the USA in 1962, which was the year Thalhammer picked his Domiracer up at an Isle of Man race.

The three twin racers sent to Berliner's in 1962, were entered in the Canadian National Gp in early 1963, one of them winning. All three bikes had cylinder sealing problems and ruined the skimmed cylinders. For the next race at Laconia they had to use standard non-skimmed cylinders, and still did well anyway winning the two supporting races. This information given by Kenny Hayes, one of the Berliner riders at Laconia.

The castings on Thalhammer's Domiracer may not have been interchangeable with stock items not because they were from different molds and cores, but because they were machined to accommodate different internal parts, which was the case with the 88 racers sent to the United States.

The Domiracer was also built and raced in 650cc form. So again, the fancy Domiracer with the lowboy frame and top-notch development engine does not seem to have seen as much light as ones built from much more standard parts. There is another photo in Walker's book of Reg Dearden sitting on a Domiracer. Dearden supposedly got his hands on ex-works Domiracer stock along with Dunstall. The bike Dearden is sitting on in Walker's book looks even more unremarkable than Thalhammer's.

Was the 1961 Domi racer 500 a short stroke?


Was the 1961 Domi racer 500 a short stroke?
 
O K, so I wasn’t clear on what I made my suspicions on, let me give you quotes from the book.
The 500 88ss
bore x stroke is 66 x72.6 mm
650 is ……………….68 x 89 mm

1, Crankshaft strength had been increased by larger dimensions and using larger main bearings. The plain white- metal big-ends (and small ends) were also upped in diameter, while the length of the light alloy connecting rod was reduced. With a compression of 11:1, the full skirt pistons were both shorter and lighter than standard components.

2, A cylinder head from the new six-fifty Manxman, later to be marked in the UK as the 650SS was employed because it used a steeper inlet port downdraught and wider splaying of the exhaust ports for increased cooling.

3, Dunstall, on the works 500 Domiracers.
The Domiracer really are specials and nothing from a standard engine is interchangeable. The con rods are much shorter than standard and consequently is the cylinder. The 10:4 pistons have much shorter skirts and the flywheel is cut away to allow piston clearance at the bottom of the stroke. The crankshaft too is ‘special’ but the measurements are as for a 650.

Re no 1 and 3, unless I completely don’t understand engines how do you use shorter con- rods while maintaining the stock 500, 72.6mm stroke?

Re no 2, a 650 head on a 500 means they used 650 pistons? Right? It is only a 2mm bigger bore, is it not? I myself have a 500 engine with 600/650 pistons inside it.
So how does one fit shorter con- rods and larger diameter pistons that has, either, a crank cut away for piston clearance or pistons cut off from the bottom of the skirt for same reason?
I realise that unless I can get hold of a works 500 engine with the alloy barrel then this question might remain unanswered.
 
beng said:
Whatever sort of Domiracer Thalhammer got, here he is with it, photo from Mick Walker's book. Definitely a k2f style mag, not the 2mtt based one, standard featherbed frame, no eccentric rocker adjustment, standard looking 8-fin cylinder except note how close the top cylinder fin is to the bottom head fin, something has been skimmed to raise compression.

The three twin racers sent to Berliner's in 1962, were entered in the Canadian National Gp in early 1963, one of them winning. All three bikes had cylinder sealing problems and ruined the skimmed cylinders. For the next race at Laconia they had to use standard non-skimmed cylinders, and still did well anyway winning the two supporting races. This information given by Kenny Hayes, one of the Berliner riders at Laconia.


The Domiracer was also built and raced in 650cc form. Quote.

The 500 barrels were skimmed to raise compression about .030 thou removed for each 1 set higher compression.
The two pics are of both the earlier wide-line and the latter slim-line
Dunstall obtained 3 lowboy frames, at least one he fitted the 650 engine into.
Click to expand...
 
Bernhard said:
Re no 1 and 3, unless I completely don’t understand engines how do you use shorter con- rods while maintaining the stock 500, 72.6mm stroke?
.
Click to expand...

Making a shorter stroke engine with Nortons using forged crank components could have been quite difficult, since the forgings were a standard size and not easily altered ?

If they did use a std 500 crank and shorter rods, the skirt would need to be reduced to avoid the flywheels.

Just some random observations, wihout having seen inside one of these engines.
No need for torrents of abuse if they need any corrections....
 
Re no 1 and 3, unless I completely don’t understand engines how do you use shorter con- rods while maintaining the stock 500, 72.6mm stroke?

Conrod lenght has nothing to do with stroke. (except that the engine has to physically fit together of course - hence the shorter pistons and modified flywheel). A Commando conrod at 5 7/8 inch is shorter than a standard street 500 Dominator rod at 6 inches.

Standard Dommie rod stroke ratio is 2.1:1 where as a Commando must be down in the 1.8 ??? area. 2:1 would be considered a pretty high ratio.

I doubt anyone who can answer this question is about today but I remember reading in press reports published during that TT meeting that there were several engines on the Island anyway and none were the same. So the answer may be yes and also no.
 
johnm said:
Re no 1 and 3, unless I completely don’t understand engines how do you use shorter con- rods while maintaining the stock 500, 72.6mm stroke?

Conrod lenght has nothing to do with stroke. (except that the engine has to physically fit together of course - hence the shorter pistons and modified flywheel). A Commando conrod at 5 7/8 inch is shorter than a standard street 500 Dominator rod at 6 inches.

Standard Dommie rod stroke ratio is 2.1:1 where as a Commando must be down in the 1.8 ??? area. 2:1 would be considered a pretty high ratio.quote]


Thanks for clearing that up.
Click to expand...
 
beng said:
Dunstall ended up with the Domiracer stuff as everyone knows. I have a copy of a letter written from Dunstall to a couple of racers in the USA, dated December 1972, ten years after he had received the stash of works twin parts, stating that Dunstall had one works-type Domiracer engine left that he was willing to sell, that had a short alloy cylinder, eccentric rocker adjusters and Commando-sized rod journals and matching rods and a larger than stock drive-side crank bearing He wanted 200 pounds for it.
Click to expand...

When Dunstall says the crank measurements are "as for a 650", he is talking about the rod journal size, they used 650 rods, which are the same dimensions as 750 or 850 rods. In the above letter I quote, since the letter is from the early seventies, he makes the reference to Commando rods instead of the 650, because by then the 650 was history.

As far as the engines having larger main bearings, in what way were they larger? 35mm is quoted for the drive side bearing in the above letter, and nothing is said about the timing side at all, and this is certainly for the shaft not the O.D., there would have been a lot of options for machining the drive side for bearings when accommodations for the alternator and production machining did not have to be made.

The 650ss and 88ss head are the same and interchangeable between the two bikes for 1962-onwards, the head worked with both 66mm & 68mm bore engines.

A racer that lives close to me took a 750 crank, whacked off the shafts and bored holes the correct distance from the rod journals for the 500 stroke, then he pressed in new shafts and tig welded them on the inside to hold them in place. Now he has a 500 crank using the 1 3/4 rod journals. He shortened his cylinder to match the rod length, and that is how he made his "Domiracer".

Replicating the top-shelf development twin engine with it's alloy-nikasil cylinder, 2mtt magneto and eccentric rocker gear is going to be a lot of trouble and expense for anyone, and I would applaud their effort to do so.
I just pointed out the things I did to put out there that to have a good Domiracer replica all that trouble is not necessary. That since some Domiracers had engines that were built up using mostly tweaked and re-machined production parts it is possible to do the same now and be historically on the money and in their power range.

Also when racers blew their works twins up, especially after the works closed after 1962, they raced them with whatever production parts would let them get back on the track.

About the 650 Domiracer that I believe Derek Minter used on short circuits, Doug Hele described it as a "tuned roadster engine". Heinz Kegler had a standard appearing 650ss to ride around while he was working for the experimental department at Norton which had a light flywheel, raised compression, big carbs and a special cam with radius lifters he said was tested at just under 130mph. Another tuned roadster engine......

So lowboy frame, Manx frame, or slimline, iron or alloy cylinder, eccentric or standard tappet adjusters, 2mtt or k2f mag, etc.. Choose what you like and what you can get your hands on and whatever you put together the chances are excellent that the factory did it too and also raced it.

Was the 1961 Domi racer 500 a short stroke?
 
WHO is Buildeing ONE ?

Raceing IS Cubic Dollars ( unless youre cunning or privilaged )

Any Twerp could getta Coustom Crank , stroke to order .

THe ' artical ' Actually mentions the ' 650 ' Journals .

Sid Mizen's Dunstalll Domiracer, lowboy frame , had a breather through the left mainshaft . NOW , theres a Thing .

M/Cycle weekly , and so on , ' scooped ' all sorts of inane drivle ( probly cost em a few beers) & had photographs ,
of the latest goings on , and conjecture . To fill their pages .

Most Likely ' Motorcycle Sport ' had the pictures of internals , perhaps.Though back 61 Mayve been Blue or Green Un.
 
So in respect, I can build a 500 engine with Commando rods, and a crank specially built to accept these larger diameter big end journals with larger roller mains bearings?
Doesn't seem impossible.
 
Larger main crank bearings would be more trouble than they are worth now. Guys are building 750-1000cc Norton twins with more horsepower and torque than a 500 will ever see and they are using stock-sized superblends.

The 1 3/4" rod journals are a great idea for a model 88 racer though. The stiffer the crank, the less it bends and the less stress it will put on the main bearings and cases. I even thought with that short a stroke there would be room for 2" rod journals, bearings for that size would be easy to find, small block chevrolet's using them in the sixties for one....
 
Nortons experimented with the BBC (Big bearing crank) on some manxes. Quite large main bearings according to the pics, 5" seems to ring a bell.
Didn't proceed, and didn't see much said about it, but lack of any benefits had something to do with it. Rotational speed within the bearing must have been enormous, but lightly loaded presumeably, or couldn't have worked. ?
 
more farticals on this to post , so hold your horses .

Pretty sure one of the weeklies had a pic of Hele's desk with pistons & maybe head sitting on it , around the time of the
I.o.M. , probly worth finding , valve size details etc .? But I have not that .

The Idea was to get ' overlap ' or less overhang , in the flanks , between the mainshafts & the Crankpins . Therefore
more ridgid And Stronger .

Will get this other tripe I have posted this week I should think , Though Id be surprised if you havnt seen it before .
Youve looked up Domiracer & Dunstall on search here ( though some entity changed it to DUSTALL , so key in that )

Wondering if the Matchy rockers are worth purloining , as there Eccentric Adjuster type .
Gib Boggle , the Type that did the Computor Analisis for Freeth's MacIntosh WhizzBang swing arm ,
Had a shed full of debri centuries ago ( 1978 ) one of the clear out clearance contraptions was his 500 Engine .
He'd been looking at the Domiracer & thought the Std. Worked 500 he'd built wasnt going to get the result .
I presume he had a cache of info on the worls 500 , if youre leaveing no stone unturned .

Youre looking at around a 8500 rpm ceiling to emulate the I.o.M. machine , I think .
Was definately a item that featured in the blue & green mag. as there weekly , and therefore short for content at times,
The tech interviews / reviews covered the mechanical side fairly thoroughly at the time, with pictures at times .
 
Matt Spencer said:
Wondering if the Matchy rockers are worth purloining , as there Eccentric Adjuster type .quote]

Eccentric rockers for 500 Domiracer where one of a kind, they were longer than Matchless rockers, so factory /home made.
Click to expand...
 
You must log in or register to reply here.
Back
Top