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Model 18 Crank pin

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Model 18 Crank pin

Posted by john_cole1 at September 14. 2018

I have had the crank pin come loose in the flywheel on my 1930 model 18. Fortunately without any damage, some years ago I managed to buy a NOS crank pin by Alpha marked N3. Would this be a suitable replacement. It is sealed in a opaque plastic kind of coating which I do not want to split if it is not suitable. Hence I have not been able to measured it. Also is the flywheel repairable. Thanks for any advice. John.

Re: Model 18 Crank pin

Posted by patrick_breitsch at September 14. 2018

Hello,

 

I attached Alpha's catalog page for Norton big ends.  Their listing suggests a N1 for your year machine.  N3 looks like later year?  Good Luck

http://www.alpha-bearings.com/Big%20end%20Catalogue/Norton-600.htm

 

I see a note at top of listing as to the difference being shank diameter between the two numbers.

Re: Model 18 Crank pin

Posted by john_cole1 at September 16. 2018

Previously patrick_breitsch wrote:

Hello,

 

I attached Alpha's catalog page for Norton big ends.  Their listing suggests a N1 for your year machine.  N3 looks like later year?  Good Luck

http://www.alpha-bearings.com/Big%20end%20Catalogue/Norton-600.htm

 

I see a note at top of listing as to the difference being shank diameter between the two numbers.

Re: Model 18 Crank pin

Posted by john_cole1 at September 16. 2018

Patrick many thanks for that, I shall cut the wrapping and get measuring.

Re: Model 18 Crank pin

Posted by richard_cornish at September 17. 2018

Hi John,

   Your 1930 Model 18 should have a 1" crank-pin. This was used in subsequent engines until part way through 1933 when I believe there was a manufacturing problem and this was increased to 1.02" (20 thou oversize). All these big-end assemblies remained the same for the next 30 years until production stopped. This is the difference between the Alpha N1 and N3.

   I think your bearing dates from the 1970s/80s as this type of protective covering was used then, although it could be later. You should be able to or get your flywheel holes enlarged to the later size but this needs to be done to the upmost accuracy. The hole is parallel bored and the crankpin has a very slight taper and this needs to start to bind when no more than a third of it's length is pushed into the flywheel at normal room temperature. Ideally this should be machined with both the flywheels locked together or to use a mandrel to set-up each flywheel and might be best to take to an engineer who understands the application and tolerance required. When you fit the new pin you could try warming up the wheels to 100c so you don't damage the new surface.

Regards,  Richard.

Re: Model 18 Crank pin

Posted by john_cole1 at September 18. 2018

Hi Richard,

Many thanks, I now know exactly what's needed to be done. Hopefully there will not be too much damage to the flywheel. The tapping started when I was 300 miles away in the South West of France, got back to Surrey OK! No wonder the assembly is so heavy and robust.

Regards John.

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