Steel flywheel for Commando

I think I might be correct in presuming the Commando fly wheel is usually made of cast iron ? I was wondering if anyone makes a steel replacement for it.. I would like one without the balancing hole bored in the counter weight. ( As would be used in an Atlas 750). My 850 crank has the hole filled with a threaded steel plug which is held in with blue Loctite, and the ends of the thread are punched. If it ever came out, during a race,something very nasty might happen. Normally I would not worry, but I am thinking of allowing one of my mates' sons to race the bike.
I don\'t think a billet crank is necessary for a Commando. The crank bolts are 3/8 inch, from memory. Triumph 650 3-piece cranks are held together with 1/4 inch bolts and do not let go.

A dude in Germany makes steel centres/counterweights (iirc)
The MkIII has 3/8 inch fasteners, before that 5/16 inch.
A good deal of the pictures of broken parts seem to show a failure radiating from the bolt/stud holes, so perhaps a good reason to not only prep the holes and faces (countersinking etc etc) but not do that wishy washy not using a torque wrench.

The German dude requires your crank sent to him, he then makes and fits a steel flywheel and assembles the crank. He doesn’t sell the flywheel separately. In fact he seems a bit offended if you ask (or he did when I did).

My understanding from posts on this forum is that the cast flywheel is the weakness in the crank and can break up without warning.

Bolt up cranks just never inspired any confidence with me. Goodness, even Royal Enfield Interceptor had a one piece crank.
But I suppose failure comes from over enthusiastic use.

I have a steel flywheel in my 750. My friend who raced Commandos in the period snapped a couple of cranks but never grenaded a flywheel. Used to rev the nuts off them.
Ken might know more?

I always imagined that Steve Maney started by making steel flywheels before progressing to complete crank manufacture!!

Anybody know if that is the case?

I wonder if Molnar will do a 3 piece? If so, you could ask for a flywheel...the issue is likely to be that they aren't all the same...variations in width and bolt hole size as mentioned.

Nigel, I wonder if the German Dude's reluctance was due to the fact he wanted to see your flywheel before he made a piece of scrap?

SteveA said:

I always imagined that Steve Maney started by making steel flywheels before progressing to complete crank manufacture!!

Anybody know if that is the case?

I wonder if Molnar will do a 3 piece? If so, you could ask for a flywheel...the issue is likely to be that they aren't all the same...variations in width and bolt hole size as mentioned.

Nigel, I wonder if the German Dude's reluctance was due to the fact he wanted to see your flywheel before he made a piece of scrap?

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Yes, Steve started making flywheels only and progressed to cheeks later.

Dunno if Andy is making Commando cranks or not. Does anyone have any info on this?

I suspected that German dude didn’t want to sell only a flywheel as he'd lose control of the quality of the finished product, so my feeling was that his reasoning was ’honourable’. I just didn’t want to ship my complete crank back and forth like that though.

Onder said:

Bolt up cranks just never inspired any confidence with me. Goodness, even Royal Enfield Interceptor had a one piece crank.
But I suppose failure comes from over enthusiastic use.

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Maybe, maybe not, AJS /Matchless 650 cast iron cranks were reported prone to breaking, c.i. is really not recommended material because it is classed as a brittle metal.
Heavy dropped forged is slighty better. . . . . .

On Weal's web page, he does mention using ground 9mm bolts to replace the standard items, so it's never going to be a fit-at-home job.
He does sound as if it's a serious bit of kit though, and will also balance the assembly if you send pistons and rods with the crank bits.

Even with a steel flywheel a Norton crank is still prone to breaking. I've broken two & the second one nearly cost me a very serious, possibly fatal accident. Steve used to have original Norton cheeks x-rayed & said that many showed cracks.

Matchless said:

Even with a steel flywheel a Norton crank is still prone to breaking. I've broken two & the second one nearly cost me a very serious, possibly fatal accident. Steve used to have original Norton cheeks x-rayed & said that many showed cracks.

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Yes unfortunately many Commando cranks have cracks. Especially at the change of section at the drive end. No radius.

I have seen 3 flywheels explode. I had to dodge one at Lydden, which (luckily) only took out everything below the cylinder head, broke the gearbox casing, engine plates, smashed through the 2-1 exhaust and took a chunk out of the rear tyre. Luckily it was a Mk4 Seeley frame, or it would have gone through the frame too. (this was the bike that Jamie Waters bought later)

I had a steel flywheel in one of my motors for a few years, the flywheel remained intact, but I broke 4 cheeks before I finally gave up and bought a Maney one, followed by a few billet ones.

If your bike is going to sit in the garage and just be an internet talking point, standard is fine...a steel flywheel won't stop it breaking either.

As someone else pointed out, that's why Steve stopped doing just flywheels

People have been making steel flywheels for Commandos since at least the '70s. Here in SoCal the typical process back then for building a sturdier crank for flat track racers included a steel flywheel, with higher quality 3/8" bolts replacing the original 5/16" fittings, with at least two of them reamed to fit, and the outer halves crack inspected and shot peened, as well as turning a radius into the cheek at the mainshaft. That worked pretty well, but it was still possible to end up with a broken crank after hard racing use (ask me how I know ). The first wrought steel flywheel I used was a direct copy of the orignal cast one, made from the original Norton drawings, but machined from a billet. A bit later, several other folks started making them from steel plate, with bolt-on weights. They are pretty simple to make. I think I still have drawings of the ones made by Ron Fraturelli. As already pointed out, Steve Maney started out making just the flywheels, and fitting them to stock halves that he had Tuftrided. The first one-piece crankshaft I know of were the ones Ron Wood had made by Moldex for his flat track bikes, but I've also seen references to the factory race team experimenting with a one-piece crank back in the '70s. Dave Nourish was also offering his one-piece cranks back then, and I'm sure there were at least a few one-offs made by racers. Eventually others started offering them, including Falicon and Rotational Dynamics. There have probably been others that I just haven't run across. I think that by now an upgraded crankshaft of some sort is pretty much essential for a serious Norton race engine, at least one built to the power levels now available with all the modern trick stuff available for the aging Commando design.

Some years back I started a thread here on Commando crankshafts that included info and pictures on some of the ones mentioned above. I'll see if I can find it and post the link.

Ken

This is the link


Ken

From what you guys are telling me, I am right to be concerned about a crank failure. Sounds like a billet crank is the sensible answer. I will either have to find some money, or only ride the bike myself.

acotrel said:

From what you guys are telling me, I am right to be concerned about a crank failure. Sounds like a billet crank is the sensible answer. I will either have to find some money, or only ride the bike myself.

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I agree that it can look kind of intimidating, Al. But on the other hand, there have been many successful Commando racers using standard crankshafts. I raced for several years with the stock engine that came in my Production Racer. I always kept the revs below 7200 rpm, and I think that helped a lot. I also tore the engines down after each season of racing and had everything crack inspected. At least some of the broken crankshafts I've seen in race bikes were from guys that thought it was just fine to rev them to 8,000 rpm. I didn't start breaking crankshafts until I'd put a lot of racing miles on them. But once I started down the horsepower rabbit hole, I began to see failures. I never blew one up, but junked several when magnaflux inspection showed cracks, mostly at the output shaft, but also once at a rod journal. There's a lot of difference in the stress in a 60 rwhp 750 race engine versus a 90+ rwhp 920.

When I first started letting others ride my PR, I fitted a rev limiter set at 7200 rpm. I'm pretty sure that saved me from some engine disasters. I had one rider who swore he never ran it above 7,000 rpm, but kept complaining that it was missing at high rpm. Actually, it was just hitting the rev limiter.

Ken

marinatlas said:

Norton Produkte

Norton Vincent HRD WEAL Hartmut Weidelich Triumph Commando

www.britishclassicbikes.de

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WEAL Norton is a member on here but only now I put two and two together. Nice interesting web site.

lcrken said:

I agree that it can look kind of intimidating, Al. But on the other hand, there have been many successful Commando racers using standard crankshafts. I raced for several years with the stock engine that came in my Production Racer. I always kept the revs below 7200 rpm, and I think that helped a lot. I also tore the engines down after each season of racing and had everything crack inspected. At least some of the broken crankshafts I've seen in race bikes were from guys that thought it was just fine to rev them to 8,000 rpm. I didn't start breaking crankshafts until I'd put a lot of racing miles on them. But once I started down the horsepower rabbit hole, I began to see failures. I never blew one up, but junked several when magnaflux inspection showed cracks, mostly at the output shaft, but also once at a rod journal. There's a lot of difference in the stress in a 60 rwhp 750 race engine versus a 90+ rwhp 920.

When I first started letting others ride my PR, I fitted a rev limiter set at 7200 rpm. I'm pretty sure that saved me from some engine disasters. I had one rider who swore he never ran it above 7,000 rpm, but kept complaining that it was missing at high rpm. Actually, it was just hitting the rev limiter.

Ken

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Ken,

I agree strongly with both your posts. The standard crank may just be sufficient with rpm limited to about 7000, radiusing of the output shaft after magnafux check plus carefully assembly with quality hardware.

After I discovered a crack in my otherwise perfect 850 crank I and my friends radius and either polish or shot pean the output shaft on any rebuild including street engines.

Serious racing requires a billet or Maney type crank. I saw several destroyed Commando race engines in NZ from over revved engines breaking cranks.

I have a Nourish crank in my 500 norton race bike. I bought it from him nearly 20 years ago now.

As street bike guy I keep the revs to 6k max. Usually 5500. Now I don't feel like so much of a wimp. And I wish
I had the crank crack tested when I had it out last winter. :-(