350cc Heavyweight Single Big end

matman · Post by **matman** » Wed Dec 12, 2007 11:29 pm

Hello all,

first of all:
YES, I have searched the forums.
YES, all of them.
YES, I have varied the search code: "big end single", "heavyweight single big end", "big end conrod".... and 5 to 8 others.

But nevertheless I have to ask this question: does anybody offer (after the regrettable loss of Malcolm Saggers) the conversion of the rather unstable 7/8" big-end crankpin to the larger (BSA?) 1" type?
It would be very nice if you could provide me with some hints and details on the engine rebuild I am "looking forward to".

My crankshaft left me after a 130-mile chase with a 1951 Squariel and a BMW R100RS, always between 55 and 70 mph speedo reading... Nothing broke but the crankpin - even the conrod and the piston have survived the breakdown, which only separated the left flywheel (primary drive side) from the rest, leaving the timing side (with piston, valves ect.) without any damage.

Thanks in advance for any advice!

Cheers, Jan

PS: The model is a 1958 G3/LS, engine-# is 58/G3LS 36025.

Biscuit · Post by **Biscuit** » Wed Dec 12, 2007 11:37 pm

Can't really help you Matman, but I'm interested in broken crankpins.

Was your example a one piece or the original two piece?.

Are you able to ascertain that the crankpin was pulled up tight against the flywheels?.

Thanks.

matman · Post by **matman** » Wed Dec 12, 2007 11:54 pm

Hello Alan,

I do not really know the difference between one-piece and two-piece crankpin (you might explain that to me?), but I am sure that the crankpin nuts were tight (they still are!!).

If I take a look at my `58 spares catalogue, I see parts # 017020 (crankpin, 1 x), 000233 (nuts for crankpin, 2 x), 017400 (liner for big end bearing, 1 x), 010395 (cage, for crankpin rollers, 1 x) and 000075 (rollers for crankpin, 30 x). These are the parts involved.

Next time I go to the workshop, I will take the camera with me, get some pictures and post them at the "Photos corner".

Thanks for more input coming, matman

bag-o-nails · Post by **bag-o-nails** » Thu Dec 13, 2007 7:52 am

Have you Looked in the archives

http://archives.jampot.dk/

bag-o-nails · Post by **bag-o-nails** » Thu Dec 13, 2007 8:31 am

http://www.mc-albion.dk/AMC/Technical/S ... hanics.pdf

matman · Post by **matman** » Thu Dec 13, 2007 10:07 am

Thank you, Phill!

Have just received & printed it... will see if it helps...

Cheers, Jan

Biscuit · Post by **Biscuit** » Thu Dec 13, 2007 10:12 am

In the article referred to, Ken (Itma) explains the difference between the original two piece crankpin and the pattern one piece.

The nuts may well appear to be tight due to the interference fit of the pin in the flywheels, it is no guarantee that the pin central portion is pulled up to the flywheels.

I have a theory, and it is just that, a theory that these one piece crankpins break due to not being pulled up to the flywheels, allowing a degree of flexing.

Malcolm Saggers was of the opinion that the BSA conversion was not necessary on the 350's.Edited by - Biscuit on 16 Dec 2007 8:13:16 PM

itma · Post by **itma** » Thu Dec 13, 2007 12:28 pm

It should also be kept in mind that the article is some 20 odd years old and some of my comments in it are no longer applicable, and some of my attitudes have been modified.
I have some two piece pins that need a new outer if it helps anyone out. Some precise engineering wil be need for this.

edited because this s*dding snow is getng in my eyesEdited by - itma on 13 Dec 2007 12:29:29 PM

Charles · Post by **Charles** » Thu Dec 13, 2007 12:55 pm

I believe Malcolm's view was also that the one piece pin had a larger stress relief radius from smaller to outer diameter than the two piece (no radius required) and that unless a chamfer was made on the flywheel faces, often you were pulling the radius onto the flywheel, not the end surface of the bearing surface cylinder. This, of course puts, stress exactly where you don't want it.

56G80S · Post by **56G80S** » Thu Dec 13, 2007 10:42 pm

Just to put in my nose in, I had a crankpin snap, a pattern one piece. The explanation I received was the same as the one Charles has described. I got this response from Alpha Bearings (cries of dismay from some quarters) who fitted an oversize pin for me - but in my naiviety I didn't ask what that size was or the origin of the pin!

But it's still in several thousands of miles of hard use later.

Johnny B