Desmodromic 4 valve Manx

Snotzo · Apr 15, 2017

Rohan said:
Snotzo said:

but he certainly turned down Harry Weslake's 4 valve head offer

Click to expand...

When was this ?
How ye know such things ?
Are there details of this anywhere.

4 valve heads would have been uncommon in the Joe Craig era,
only Rudge and Excelsior (openly) used such things (all prewar.)(?).

The above information may be found in Vic Willoughby's book, Classic Motorcycle Engines, in the chapter about the outside flywheel Norton.
The article also contains a comparison flow chart of the 2 and 4 valve arrangements as shown to Willoughby during his interview with Weslake.

acotrel · Apr 15, 2017

A question - are the guys who are using titanium conrods running plain bearing big ends or roller big ends ?
I do know this. A friend of mine gave Kevin Magee his start on Ducatis in the 70s. One of his earliest bikes was a 250 Ducati single which had as much done to it as possible - high comp. methanol fuel GP carb., factory race cams etc. At most meetings, it was very fast at first, then it always slowed. When the bike was stripped the only thing he could ever find was too much clearance on the roller big end. If the conrod is titanium and it starts to get hammered, the rod dimensions will probably change.
In Australia we have a couple of guys with grossly oversize G50s - they are not very good. The bigger motor delivers more torque, but the bikes seem to stop being nimble.
I still believe that a 500cc single cylinder motor is probably very limited in how much further it can be developed, than Joe Craig achieved with the Manx. Obviously the Molnar Manx is better than the 1962 Manx, however it is still only marginal.
The simple fact is that oversize motors destroy the whole object of the exercise - the playing field is not level. 4-valve might not be so bad, but if it comes to having the big bill every time you race just to be competitive, it can kill the race class.

Snotzo · Apr 18, 2017

acotrel said:
A question - are the guys who are using titanium conrods running plain bearing big ends or roller big ends ?
I do know this. A friend of mine gave Kevin Magee his start on Ducatis in the 70s. One of his earliest bikes was a 250 Ducati single which had as much done to it as possible - high comp. methanol fuel GP carb., factory race cams etc. At most meetings, it was very fast at first, then it always slowed. When the bike was stripped the only thing he could ever find was too much clearance on the roller big end. If the conrod is titanium and it starts to get hammered, the rod dimensions will probably change.
In Australia we have a couple of guys with grossly oversize G50s - they are not very good. The bigger motor delivers more torque, but the bikes seem to stop being nimble.
I still believe that a 500cc single cylinder motor is probably very limited in how much further it can be developed, than Joe Craig achieved with the Manx. Obviously the Molnar Manx is better than the 1962 Manx, however it is still only marginal.
The simple fact is that oversize motors destroy the whole object of the exercise - the playing field is not level. 4-valve might not be so bad, but if it comes to having the big bill every time you race just to be competitive, it can kill the race class.

Acotrel, your post is so far off my original, you have effectively killed this topic stone dead!
I will however take you up on one point in your post.
The Joe Craig Manx engines at best were making 52 crankshaft horsepower, and very good horsepower it was for the period in which his engines were raced. The Molnar 2 valve re creation Manx engines are regularly seeing better than 55 rear wheel horsepower. Crankshaft compared to rear wheel, that cannot under any circumstances be considered as a 'marginal' improvement.

Rohan · Apr 18, 2017

And Carillo etc have been supplying Ti conrods for quite some years now,
and if there was a problem someone would have noticed by now !
Next, Alan will be telling us that the alloy Commando rods aren't up to the job... ?!!

acotrel · Apr 18, 2017

Aren't Carillo rods used with plain bearings rather than roller bearing big ends ? My comment about titanium rods possibly changing dimension due to hammering was about roller bearing big ends as used in most Manx Nortons and G50s in the old days. I've had titanium bolts neck down when holding brackets and have come loose. It is amazing how quickly it happens. If you destroyed a roller big end in a Manx with a titanium rod, the result might be extremely expensive. Do Molnar Manxes run high pressure pumps and plain bearing big ends ?

Momus · Dec 16, 2017

Mate, one piece titanium rods have an appropriately engineered steel sleeve pressed into the big end bore to accommodate the rollers. Ti has slightly less expansion than steel so they stay put.

Momus · Dec 16, 2017

acotrel said:
A question - are the guys who are using titanium conrods running plain bearing big ends or roller big ends ?
I do know this. A friend of mine gave Kevin Magee his start on Ducatis in the 70s. One of his earliest bikes was a 250 Ducati single which had as much done to it as possible - high comp. methanol fuel GP carb., factory race cams etc. At most meetings, it was very fast at first, then it always slowed. When the bike was stripped the only thing he could ever find was too much clearance on the roller big end. If the conrod is titanium and it starts to get hammered, the rod dimensions will probably change.
In Australia we have a couple of guys with grossly oversize G50s - they are not very good. The bigger motor delivers more torque, but the bikes seem to stop being nimble.
I still believe that a 500cc single cylinder motor is probably very limited in how much further it can be developed, than Joe Craig achieved with the Manx. Obviously the Molnar Manx is better than the 1962 Manx, however it is still only marginal.
The simple fact is that oversize motors destroy the whole object of the exercise - the playing field is not level. 4-valve might not be so bad, but if it comes to having the big bill every time you race just to be competitive, it can kill the race class.

acotrel said:
It is common practice to fit the main bearings into Manx crankcases using steel sleeves and they still get failures. The main advantage in using a Manx in road racing lies in it's reliability. Once there is an increase in power output, reliability must suffer. It takes a lot of money to race one of those 4 valve, large capacity Japanese dirt bike motors in a road race bikes, due to the blow-ups. That is why Sounds Of Singles failed. It is not easy to improve on a 1962 Manx engine. If there was a class for 500cc air-cooled single cylinder four-stroke engined bikes, a good Molnar Manx or Summerfield G50 would be very difficult to beat.

I'd cut the water jackets and weld fins onto a 450 Yamaha MXer. Taken to the same state of tune as the 250 version 80 crank up is no problem. Bored and destroyed they have the same 200 HP per litre capability of many modern road superbikes.

Desmodromic 4 valve Manx

Snotzo

acotrel

Snotzo

Rohan

acotrel

Momus

Momus

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