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Flywheel cracks

Submitted by david_cooper on Wed, 26/09/2012
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Singles

Latest issue of one of the classic bike mags has a part article about restoring a Model 18. The article concludes with a picture of a crack in one flywheel casting - with a remark that this is a common defect.

A couple of questions arise.

Is it common?

How common? If the WD bought maybe 100,000 16H bikes - a 'common' defect must surely have arisen in hundreds - maybe thousands - of them. Did hundreds or thousands of flywheels explode? Or did they crack and nobody noticed? So: either the cracks are rare and (I hope) none of my business - or they are commonplace but don't matter (again - not really my concern)? The crack is from the big end to an oil drilling - not near the edge where centrifugal stresses are highest.

Any experiences welcome! Thanks

David Cooper

Submitted by gordon_johnston on Thu, 27/09/2012

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I had the flywheels crack on a heavily modified Model 50. It seems that Norton's use of cast iron flywheels was OK for general use, but if you want to make an engine rather more powerful, the first thing is to fit steel flywheels - usually modified Ariel. Like wot I didn't...

Submitted by richard_payne on Fri, 28/09/2012

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One of the reasons that the War Department chose the 16H when they could just as easily have had the Model 18 is that it was so low-stressed. The side-valves run out of chuff before they rev highly enough to damage themselves.

I've handled some rough old 16H cranks but never seen cracked flywheels on those. If they're cracked then I'd regard them as scrap.

Mike Pemberton would be the person to talk to about when it is necessary to think about steel flywheels.

Submitted by david_cooper on Fri, 28/09/2012

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Thanks for the reassurance, Richard. And Gordon of course - I don't expect huge RPM...

Anyway - if I find the 16H too slow I'll use something else...and life's too short to worry about engines going bang..

Submitted by mark_zenor on Sun, 30/09/2012

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Hi

I would add the following from my observations. The Model 18 does develop this type of crack quite commonly and there are probably hundreds of them out there running with cracks. I went through 3 different sets of flywheels all from ES2's or Model 18"s until I found a set that that did not have a crack. Is it still in that condition after 2k miles after the rebuild for my 1946 Model 18, I have no idea and am not losing any sleep over it.

I have run this machine regularly and she will cruise at 60 mph with no problems. It's my pleasure to show Commando's what a single can do.

I often consulted with Mr.Pemberton and this was my understanding of his opinion. However if it keeps you awake at night, one of his steel crankshaft would provide a good nights sleep.

Regards

 


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