I bought a NOS Alpha crankpin set for my M21 and was surprised to find it has 12 extra rollers and no cage. The old box bears code B-14. The assembly was encased in that hardened gel that Alpha used and there was no sign of tampering. The crankpin had the typical Alpha oil holes and driveside (nearside? I can never keep that straight) shaft key. I assembled it with a spare cage and it fits fine, now installed and trued.

A few years ago I bought a ZB34GS project from an old chap who had sent it to the Motherland for rebuild and never fired it up. Upon inspection the valves were poorly seated and there were enough bodges and shortcuts in the work to make me strip it down. The rod did not turn cleanly on the pin. Sure enough, there was a new Alpha crankpin with crowded rollers and no cage. Considering the overall quality of the work I considered it another bodge, as did Av Tenkarian, my local BSA guru. He installed a cage and lightly honed the race to fit, likely in high dudgeon the whole day as he'd never seen a BSA with crowded rollers.

Can these cranks run reliably with either set-up? Is the B-14 set for a pre-war model or a V twin?

Jeff (Rhode Island, USA)

email (option): jjbandoo@aol.com

My alpha recept for the work on my bsa v twin crank says "B15".
I can't help with the question of cages.

Hi Jeff..Alpha do, or at least used to, manufacture big ends for these models in both formats..the 'crowded' type being a cheaper option as the manufacture of the cage wasn't called for.
I've come across a few engines fitted with old crowded roller assemblies and they seem to run OK, at least in standard engines. Quite a few British singles used a big end of this type as standard.
For tuned engines I think the caged bigend is more suitable and a needle roller assembly even better if you want to use the revs, as the differential surface speeds between the pin and outer race are reduced..Ian

email (option): ian@wright52.plus.com

Vincents used to use crowded rollers on their bikes, this was ok untill people started to tune the engines when the rollers would bunch up and skid, then they had to revert to a caged bearing. Not a major problem on an M20 I would think.

Hi Ian..Yes, I have read that about Vincents as well..and I guess it will apply to any crowded roller big end. 'Skidding' of the rollers is a limiting factor even in a caged assembly using, say, 1/4" diameter rollers, due to the difference in diameter between the pin and outer eye. At a given engine speed the surface speed of these two components is not identical and the rollers cannot 'roll' on both surfaces simultaneously...So a certain amount of 'skidding' is present by virtue of the design.. Fitting needle roller assemblies reduces the differential and thus the amount of skidding, allowing the use of higher revs before the effects become problematical...Ian

email (option): ian@wright52.plus.com

That need to skid a bit because of the different bearing surface length inner to outer has always helped me not condem a big end just because of tiny verticle play, i bet more than a few perfectly good big ends have been changed.
All this is horrible reading for me though as im about to (once i find some forks) put an M24 on the road ie m20 big end with alot more going on up top. They were prone to big end failure aswell :O

Hi PJ..If you have a standard caged big end it won't be a problem in the M24.(unless perhaps you plan to race it). I have tuned B33s past the power output of the M24 and for road use they aren't problematical. The big end set up is also essentially the same in a DBD 34. For more extreme tuning (revs) I always use a needle roller set up....Ian

email (option): ian@wright52.plus.com

No intention of racing, its not going to weigh in like a road spec bike though & it has the unique to 1938 correct cams :-)

Hi Ian,

I am about to build a long(er) stroke engine.(M21 flywheel)
I plan to finish a nice bottom end, and then see if I would do it 600 or bigger vol.
(Currently running original 500)

What kind of needle roller set up can I use?
Do you have a bearing model? do I need to lathe the bigend pin?..

Thanks,
Noam.

email (option): noam10@gmail.com

Hi Noam..The needle roller assembly isn't really necessary for an M21 however big you make it..The standard set up will reliably do all the miles you will want to...I get the needle roller big ends specially made..they are a complete new big end assembly...
Ian..

email (option): ian@wright52.plus.com

Thanks Ian.

Very nice and impressive bearing !
Is it more expensive than “Alpha”?..

2 more questions, if I may..

1. Do you think putting the original basket, but with 12 long, double-size rollers will be better,
Or there is an advantage to the split- in the middle 24 roller for the bigend? (May be oiling, not blocking the bigend oiling nipple )
it looks more steady this way..

2. Last time I balanced and straightened the flywheel alone, on a vice, using few basic tools,
And all runes fine.
Would it be better to balance professionally, on a lathe?

Thanks,
Noam.



The last one I'v done.

email (option): noam10@gmail.com

Hi Noam..Dave Plumb uses the Harley one piece rollers to refurbish BSA big ends and they seem to run without any problems so I would say that is an option. Also, apart from the roller diameter the needle roller type are essentially doing the same thing.
Building up the flywheels in your workshop at home using your own tooling is also fine as long as you are achieving the correct results and the finished assembly is running within the laid down limits...after all it's more about the result than how you get there.
I'll mail you some information on the needle roller conversion..Ian

email (option): ian@wright52.plus.com

Thaks Ian !

Noam.

email (option): noam10@gmail.com