Steel flywheel for Commando

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Another good reason to use a stock cam if the engine is still mostly Norton.
With the 850 and stock cam max power is just below 6 k rpm.
On a bike that makes great power way up high one tends to use that rpm range a lot.
With a stock 850, they will keep revving on up, but there's nothing there for you, other than the big bang!
I have often wondered why Norton switched to larger bolts on the MK3 crank. Obviously to add strength, but was there a high failure rate ffor the pre MK3 crankshaft bolts?
I'm referring to production bikes, road use as intended by the maker.

Glen
 
Do the larger holes encourage cracks or are the cracks there after manufacture?
 
Onder said:
Do the larger holes encourage cracks or are the cracks there after manufacture?
Click to expand...

I don't think the larger holes have any effect on cracks developing. By far the most common location for cracks is at the junction of the drive side mainshaft and the cheek. I have also had one develop a crack at the drive side rod journal, and one at the junction of the timing side mainshaft and the cheek (that one really surprised me). I've never seen cracks develop around the bolt holes.

Ken
 
When I had my 72 re-built I was told the crankshaft had a crack developing on the output shaft at the cheek. So I found another and sent that to him and that one checked out good, He told me he won't do a re-build without checking them. He said he has seen plenty of cranks with cracks in them and they were probably like that for some time. Does it take just one event to crack them?? Kinda scary to think about them exploding the way they do, some of the pictures I have seen are nasty. I think I would probably have a heart attack if that happened to me, I am one of those guys that always listening for noises and that must sound incredible???
 
With my bike, the motor pulls hard from bottom to top. If I let it, it will over-rev extremely easily. I sometimes see 7,500 RPM up-changes. I have always felt it must explode - only a matter of time. My problem is I have come to believe in my motor. When it is at peak revs, it is extremely smooth. But considering what you guys have said, I won't let the young blokes ride it. That way, if it lets go,it is only myself who gets hurt.
 
lcrken said:
I don't think the larger holes have any effect on cracks developing. By far the most common location for cracks is at the junction of the drive side mainshaft and the cheek. I have also had one develop a crack at the drive side rod journal, and one at the junction of the timing side mainshaft and the cheek (that one really surprised me). I've never seen cracks develop around the bolt holes.

Ken
Click to expand...
Wow. A crack on the timing side shaft and cheek. That is surprising. My crank actually had a small radius machined in there from new. The drive side had a perfect right angle. Which is where it cracked.

Going back years the famous Beltdriveman talked about a crack which could develop from a mismatching of the drilling for the sludge trap. I think on the drive side. This then broke through into the big end journal which off course also cracked if reground without a proper radius.

I was surprised and disappointed with my mid 74 Mk 11 850 crank cracking at the drive side shaft cheek section change. It was on original big end size in perfect condition. Treated very well as a street bike and laid up for 30 years of its life. But the original manufacture technique was very poor. No radius whatsoever and frankly unforgivable non quality control.
 
How did you determine it was cracked? Was discovered as part of a disassembly for some other reason?
 
I remember after my big crash at Lydden Hill (engine seized) I went back out at Oulton Park less than 5 weeks later just to get on a bike again. ie the crash, it wasnt me! Came in when I could feel my collar bone separating under breaking for Lodge I was pleased with how I had ridden. However I spent the next two years running round listening to engine noises. I remember running into Gerrards at Mallory Park & getting really annoyed with myself as I'd found I was leaning off the bike & looking at the engine & LISTENING!
I had built the engine I knew it was good. I gave it some stick from then on. I've always watched the revs because it's my pocket that's funding it all & it took three years to save up for the parts to rebuild that engine.
 
Alan L said:
How did you determine it was cracked? Was discovered as part of a disassembly for some other reason?
Click to expand...
Sorry missed this question.

The engine was taken down to install Jim Schmitz longer rods and pistons. A 45 year old birthday present to me and the bike.

I replaced the cracked side with a second hand crank. Had an undercut radius machined and polished. Crank was dynamically balanced.

Together with new isolastcs and following the worlds straightest Commando methodology the bike is now amazingly smooth.
 
Onder said:
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. :-(
Click to expand...
Hi all
I really don’t understand the engine design at all.
Of course I do know the original design was for a smaller, less powerful engine.
I would have thought that when Hopwood first penned the bottom end it would just seem natural to include a centre main bearing (what else do you do with the gap between the two journals ). Hard to imagine there would have been substantial extra expense. Would the extra bearing have created more drag and sapped power?
How was it possible that as the decades went by there wasn’t a ‘circuit breaker’ revision of the bottom end (or entire engine for that matter) to beef the structure up. I’m not sure if the old retort ‘if it ain’t broke don’t fix it’ is really appropriate in this situation as even the most cursory glance down the topic list of this forum suggests that bottom end of the norton is ‘a bit broke’.
To point, I consciously limit myself to below 6000 rpm on my 850. Most other engines I operate don’t really need a lot of care approaching the Red Line, as the motor lets you know it’s time to change up.
From a design perspective, starting with a bullet proof bottom end would seem obvious, especially considering the weight is low down in the middle of the bike. The Trident crank, on observation, appears a fairly obvious approach, be it good bad or indifferent.
interestingly the Lycoming aero engines I operate are equally ancient push rod technology. Their bottom ends and cranks really are bulletproof and give very little trouble, not so the top ends with valves, guide troubles and occasional barrel and head cracking.
With modern CNC production methods and better metallurgy has anyone made either an affordable redesigned crankshaft or possibly even 3 bearing crank and cases that retain the original appearance?
From my distant past studying mechanical engineering I would have thought that if in 1969 they could make a pretty good crank that survived most of the time under most circumstances it would be quite easy to redesign it to lasted indefinitely under almost all situations.
Not a criticism of our fine bikes, more just interested in the thinking that went into them.
Hope this makes sense
Al
 
They were banking on the new engine and when that was a dud no money was available for development or redesign. They hung on until 1975 and were going for a wankel. The enviros pulled the rug out on them and that was all she wrote.
All later British bikes were just old designs hotted up and thrown out the door.
 
I think the crank would have been just good enough to last through to the 850 but for poor manufacturing technique.

My crank cracked at a stress raiser at a change of section which had been machined to a perfect right angle. By the factory. No excuses. Shocking manufacturing methodology

A second issue was very similar. Poorly match drillings for the crank oil feed and oil sump in the crank. This is the one Beltdriveman used to go on about. Led to the crank breaking at the drive side big end. Again bad manufacturing.

Third problem is due to latter repair shops not grinding a proper radius when doing big end journal reprints. This one is not Norton's fault.

Apart from these basically self inflicted problems and keeping rpm under 6800 I think these cranks would have done the job for the street. Just.

Racing not so much.

The gearbox and drum front brake might be argued as another matter however They were definitely past their sell-by date by 1969 even for the street. Not that the disc brake was much better.
 
Last edited:
The whole engine is an abortion. Even when it was first designed as a 500, why would anyone design a cylinder head that was such a fiddle to fit. A modern production line planner would have a fit just thinking about it. The three piece crank was used because it was the only way Norton could make them with the machinery at their disposal.
Having worked in engineering all my life it is just typical of this country, instead of investing in the kit to make the best quality parts possible, we just compromise with the clapped out machinery we've been using for ever. Having said all that the bike I enjoy riding, and for that matter just looking at the most is my 920 Interstate, even though 90% of the powertrain is aftermarket parts as are the brakes, suspension, etc etc.

Martyn.
 
Alan L said:
Hi all
I really don’t understand the engine design at all.
Of course I do know the original design was for a smaller, less powerful engine.
I would have thought that when Hopwood first penned the bottom end it would just seem natural to include a centre main bearing (what else do you do with the gap between the two journals ). Hard to imagine there would have been substantial extra expense. Would the extra bearing have created more drag and sapped power?
How was it possible that as the decades went by there wasn’t a ‘circuit breaker’ revision of the bottom end (or entire engine for that matter) to beef the structure up. I’m not sure if the old retort ‘if it ain’t broke don’t fix it’ is really appropriate in this situation as even the most cursory glance down the topic list of this forum suggests that bottom end of the norton is ‘a bit broke’.
To point, I consciously limit myself to below 6000 rpm on my 850. Most other engines I operate don’t really need a lot of care approaching the Red Line, as the motor lets you know it’s time to change up.
From a design perspective, starting with a bullet proof bottom end would seem obvious, especially considering the weight is low down in the middle of the bike. The Trident crank, on observation, appears a fairly obvious approach, be it good bad or indifferent.
interestingly the Lycoming aero engines I operate are equally ancient push rod technology. Their bottom ends and cranks really are bulletproof and give very little trouble, not so the top ends with valves, guide troubles and occasional barrel and head cracking.
With modern CNC production methods and better metallurgy has anyone made either an affordable redesigned crankshaft or possibly even 3 bearing crank and cases that retain the original appearance?
From my distant past studying mechanical engineering I would have thought that if in 1969 they could make a pretty good crank that survived most of the time under most circumstances it would be quite easy to redesign it to lasted indefinitely under almost all situations.
Not a criticism of our fine bikes, more just interested in the thinking that went into them.
Hope this makes sense
Al
Click to expand...
Regarding the comment on the Trident crank, it's worth noting that a) BSA had far deeper pockets than AMC (Norton) at the time when the design was done, and they ended up buying a very expensive press just to to the crank. This was far beyond what resources Norton ever had. I seem to recall that while Norton was fairly profitable at times, almost all the money vanished into the sink hole that was the rest of AMC, leaving poor old Norton scraping along on old clapped-out machine tools.
Royal Enfield had (I think) the only 3 bearing crank in the Brit industry, and that was not a great success. I suspect that RE had problems getting things to line up with a vertically split crankcase & 3 main bearings.

Bert Hopwood's book is quite interesting in this regard, if you ignore his vendetta against much of the management of the industry (while I do not doubt for a moment that there was much deadwood at the top, Hopwood is rather one sided in his account).
 
SteveBorland said:
Royal Enfield had (I think) the only 3 bearing crank in the Brit industry, and that was not a great success. I suspect that RE had problems getting things to line up with a vertically split crankcase & 3 main bearings.
Click to expand...

Also AMC, with center main bearing cranks in 500 through 750 cc twins.

"AJS and Matchless twin – iconic engines
Where others had a massive central flywheel and big mains in each crankcase half, AMC went for a central main bearing that was squeezed between the two flywheels. This it was claimed would lead to greater reliability and smoothness but in fact it created an undeserved reputation for fragility not truly lost until the introduction of the nodular-iron crankshaft. The so-called ‘noddy’ crank of AM legend did finally lay the bugbear of crank breakages to rest.

The twin engine, in the same basic form, came in 500, 600, 650 and early 750cc forms as well as being used for the production based G45 road racer. Using a twin camshaft – one in front of the engine and one behind, timing could be made extremely accurate and even tuning was possible."

Ken
 
For all their faults AMC designed & built some of the most interesting & beautifully engineered engines in the British industry, including as Ickren rightly points out, the three bearing crankshaft twins. Other notable designs being the vee four Silver Hawk, narrow angle vee twin Silver Arrow, & Porcupine twin race bike.
 
Hi all,
All very interesting.
it’s just so hard to get my head around the fact that a brand as famous as Norton couldn’t afford to retool and redesign the engine, i don’t doubt it’s true But where did the profits go? After all, there were many others manufacturers across the world who designed and built modern machines, presumably without the historical wealth of expertise available to Norton. As Commandos were Bike of the Year (whatever that means) you would think that would provide sales and profits to enable them to recapitalise.
I find BSA even more confusing. Having supplied millions of Lee Enfields, Lewis Guns, Bren Guns and numerous other armaments to the Empire through two world wars, you think they would have been swimming in money.

On another matter, how does the Triumph T140 bottom end compare to the Norton. For that matter, how does the late Bonneville motors compare in general to the Commando in reliability, power, torque and general ease of living with.
regards
al
ps please don’t take my questions as criticism of Norton, I’m just really interested on how such an antique (in the seventies) was such a lovely bike to ride and still remained competitive in the face of extremely modern opposition.
 
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