Molnar Precision Ltd TGA Commando Parts

These cranks that are breaking are stock 850 cranks in race motors are they not? No members here racing Norton twins are breaking Molnar cranks are they?
 
Broken crank stories here.

Crank Broke . wtf .

O.K. want the failure pictures , preferably with close up of matching sides / break . Can be morbid and explain what happened too . Clearances in Bearings & big ends ( to spec , or outside ) of relevance also . presumeing theyre torsional failures from pics Ive seen .
www.accessnorton.com www.accessnorton.com

I believe the Molnar Norton twin cranks are a fairly recent offering. Molnar items tend to be very high quality.
I would expect that his crankshafts are made to withstand the rigors of racing.

Glen
 
Good intentions are not risk management. It is what happens when the cranks are used in anger which counts. If Molnar has made them and proved them through a lot of racing, the cranks are probably good. The standard 850 crank does not look as though it can withstand racing, but they will still cop a lot without failing. When I first saw my 850 crank, I was disgusted. It is the reason I never really believed in my bike. I did not even run the motor before I filled hole in the counterweight with steel. There is no way that hole could ever be right. And surprise, surprise - the balance factor came out to be 72% - which it always should have been ! You do not have to be a genius to figure out what Norton did. That hole in the crank was not the result of racing experience. It was done to make the bike docile.
If peak revs are 7000 RPM, THAT is where the crank should NOT vibrate.
I believe the 961 Norton has balance shafts. They damp the vibration, but that does not mean the cases are protected. The 270 degree crank might be better, but getting the cam timing right might be more difficult.
 
Schwany said:
These cranks that are breaking are stock 850 cranks in race motors are they not? No members here racing Norton twins are breaking Molnar cranks are they?
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I believe most of the stock cranks had a service life in a road bike before being used for racing. It is the accumulated fatigue which kills these crankshafts. Even if magnafluxed and found to be crack-free,
tiny cracks are often undetected and may grow at exponential speed. Using old crankshafts in racing is a very dangerous venture.

Even Molnar's crankshaft will fail one day if subjected to high loads over a length of time. Strength of the material is just one parameter. Stress level, stress vs. time, and presence of stress risers are much more important.
Alternating stress from bending actions is the worst fatigue action there is, and it's what makes the Norton crankshaft so vulnerable.

- Knut
 
Molnar’s reputation is worth FAR more to them than the few quid they might make from a few cranks. They are very financially successful, they don’t ‘need’ the money from these crank sales.

They are heavily involved in racing at the highest level. You don’t get good at that without knowing your shit !

I think we can rest assured that their cranks will be very much up to the task.
 
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I'm not worried at all about the Molnar crank doing the job I want it to do, and I don't think anyone should be if they are OK with a new 3-piece crank. Besides it looks like new crank options are limited for Nortons.

I won't live long enough to break the Molnar crank, and I'm only racing my inner young self when I ride. lol
 
Schwany said:
Sounds nasty and abrupt. Did you break any of your personal body parts?

I don't race but that kind of thing goes through my mind way too often when I'm out riding an old Norton and that is just on the street. I do like to ring its neck, so if it let go nobody to blame but myself.
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Actually, never broke the (Maney) cases. The 2 across the journal were held together by the conrod.

The one that split lengthways was at the end of the Norwich Straight at Snetterton. I was pulling over 7500rpm and had just passed what was the quickest 750 Rob North at the time, a shade over 145mph on the gearing I was pulling. The engine just locked and I grabbed the clutch and coasted down the slip road
 
Fast Eddie said:
Molnar’s reputation is worth FAR more to them than the few quid they might make from a few cranks. They are very financially successful, they don’t ‘need’ the money from these crank sales.

They are heavily involved in racing at the highest level. You don’t get good at that without knowing your shit !

I think we can rest assured that there cranks will be very much up to the task.
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Yep,

I had a Maney crank in my "standard" 750 motor (i.e 73 x 89) and it has no problems at 8000. The motor makes peak power at 7400, but doesn't drop off after that, so I have an extra 600 in hand between corners if needed. Now it has a billet crank (that weighs 15lbs, and doesn't stall going into a headwind, and makes 62ft/lb of torque) despite what the ex spurts would try to tell you

No, there is no valve bounce due to the cam (and I'm not saying what it is except that it's British made and chilled iron, been in there for 20+ years and no wear) and it has been tested on 4 separate dynos, all saying within 1hp of each other. Oh, and it runs on petrol, like proper racers/tuners do
 
Seeley920 said:
Actually, never broke the (Maney) cases. The 2 across the journal were held together by the conrod.

The one that split lengthways was at the end of the Norwich Straight at Snetterton. I was pulling over 7500rpm and had just passed what was the quickest 750 Rob North at the time, a shade over 145mph on the gearing I was pulling. The engine just locked and I grabbed the clutch and coasted down the slip road
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Good reaction time. Low siding at 145mph would leave a mark.
 
I watched that video of Doug MacRae crashing at Daytona and decided I don't want to do that. I have already had some biggies, but Doug's crash was horrendous when his motor exploded. I love my 850 motor - I do not know how Norton got it so right - it is like the featherbed frame of the Manx. It could not have been invented by accident - but the crank balance factor was not designed for high revs.. If a crank is going to fail through fatigue, what does vibration do to it ? When it runs smooth at the revs you normally use, it might be better ?
I always doubted the validity of the high weight of the 850 crank, but when I got the jetting correct, the throttle response became better. I don't think it is possible to get petrol jetted so closely. Methanol jets flow about twice as much fuel, so differences in needle jet sizes are half as critical. With methanol fuel the difference in jet size between fast and slow are almost nothing. So petrol must be impossible.
 
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IMHO Al it’s really quite straightforward, they’re not balanced to be smooth at 7,000rpm cos they weren’t supposed to be revved above 6,000 !
 
With the parts available today for Nortons, it is not difficult to build a Norton motor that can wind up quick and be broken if it stays up there too long. 6000 RPM is nothing on a modern motorcycle. It is however getting up there on an antique Norton. I've been at 7000+ RPM and like Seeley920 alludes to it was not out of legs yet, but that was on a slightly smaller piston 750 with less rotating mass and not a stock 828. 8000 RPM even in a 750 Norton is asking for trouble I think, unless a lot of additional strengthening steps are taken, and the motor is assembled with extreme care.

I would not build a motor with a heavy crank in it for my own use. That is just me though. I'm accustomed to how they work and what the ride is like. I have not gotten my 750 up to 145mph ever. That is very impressive and probably costs close to $55 per mph to achieve often in a reliable motor. On my next engine rebuild I'm shooting for having a motor as reliable as an Al 850 Seeley, but getting to Al's top speed about 3 seconds sooner sitting straight up in the saddle. :)
 
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I have got a spare set of cases which I have repaired by welding an 8mm aluminium plate to surrounf the drive side main bearing housing. I also have the crank, barrel and head from the same motor. The kid who gave them to me had a Mk2a 850 which he used to thrash mercilessly on the Great Ocean Road in Victoria. His mates had H2 Kawasaki two strokes. The 850 did 8000 RPM easily without tangling valves, until it cracked the cases. My friendly rival raced a 650 Triton against my 500cc Triton and never revved the 650 over 6.500 RPM, but he runs it very high geared and has always concentrated on getting more torque. I rode it at Calder one day and blew-off a good H2 Kawasaki.
For what it is the 850 Commando is excellent, but for that one flaw - the balance factor.
I think the heavy crank is a joke. 'Some things are so bad that they are good' ? On a race track there are sometimes ripples and rises. My Triton 500 used to feel them. My Seeley 850 totally ignores them. When the heavy crank is wound up, it must store an immense amount of energy. The way you change gears can use that to advantage. When I race, I know what revs my motor is doing by the note it emits. I don't need to look at the tacho. I never believed in my 850 motor until I got it going properly. I now think it is really great. Almost unmodified, it is fast enough to be competitive
( I don't think I could race a motorcycle without using methanol - it is good for your motor - stops the heat problem and has unlimited anti-knock. )
 
Seeley920 said:
I had a Maney crank in my "standard" 750 motor (i.e 73 x 89) and it has no problems at 8000. The motor makes peak power at 7400, but doesn't drop off after that, so I have an extra 600 in hand between corners if needed. Now it has a billet crank (that weighs 15lbs, and doesn't stall going into a headwind, and makes 62ft/lb of torque) despite what the ex spurts would try to tell you
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I guess it's not the crankshaft which produces 62 ft*lb by itself ;-) Good result though.
By billet crank I guess you mean a one-part crankshaft. I am curious - who made it? The low weight hints at one of Mick Hemmings' crankshafts ....

- Knut
 
All these goodies and I can only dream in building a Norton with all these goodies being a self funded retired person and cost of living it getting harder to just live a normal life.

When I built my hot 850 Commando/ Featherbed back in the 80s I followed the high performance section in the back section on my manual and did everything on the cheap with talent machine shops in my city, Ivan Tighe cams in Brisbane did the 2S cam grind, crank balanced by a old gentleman that Ivan Tighe put me onto, head work was a stuff up but was fixed at no cost to me, but after that getting a job at a Tec college and 2 very talent motorcycle teacher one who was a Kiwi and work for a Norton race team back in NZ the other older teacher rode British bikes from the early 60s to very experienced teacher and a good machine shops as well.


So was lucky I built my Norton on a budget and it's still going strong and things were cheap to do and buy back then, now cost are so high and spending $10k on my motor is out of my price range, but what I done to my motor when I did all thought years ago has proven it worth for long life and reliability so done something right and the last 12 year major upgrades to make it even better. My motor is quick and torquie without over reving it and with a light bike still does it job great.
It still gets up to and well over the ton if I want to pretty quick but it's the handling and lightness of the Norton that has proved it worth all these years of riding and a good experience ride help who know how to push a Featherbed, it's like riding on rails.
But would love to build a hot 750 Commando motor with all these new bits that were not around in my time of building my Hotrod Norton and before the days of the internet, maybe when I win lotto, well just another dream I suppose lol.
 
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