Crankshaft / engine sprocket query - 57 twin

msammels · Post by **msammels** » Tue Jun 03, 2008 3:51 pm

I have managed to fathom what does what and where for 99% of things in my box of bits that was once a 57 model 20, but I am really struggling on the arrangement for the engine sprocket side of the crank.

I have not been able to locate any decent or consistent drawing or photo so hoping someone may be able to point me in the right direction.

The problem is that the spares list seems to show it as a spacer collar/ engine sproket/ breather bolt set up, but other diagrams apparently have some sort of spring arranagement. In my 'assorted/ unknown' box I do have a spring, a cup and an elliptical device (the latter does fit the splines) - but no combination of these seems to make any sense or have any operational capability.

Can anyone put me out of my misery?

Biscuit · Post by **Biscuit** » Tue Jun 03, 2008 4:10 pm

The spring, cup and 'eliptical device' (actually a shock absorber cam) would not be fitted to your bike, assuming it is standard. The spacer (actually an oil thrower), sprocket, lock washer and bolt/breather is all you need.

I am assuming it is a dynamo equipped model.Edited by - Biscuit on 03 Jun 2008 3:15:00 PM

paul knapp · Post by **paul knapp** » Tue Jun 03, 2008 4:24 pm

Mark, the 1956 mod 20 had the 'spring/eliptical device' shock absorber sprocket set up, this was changed for the 1957 models to a splined sprocket that fitted straight on the crank shaft after the oil slinger, the shock absorber being transferred to the clutch hub in the way of rubber blocks.
look here for the 1956 set up.
http://archives.jampot.dk/Book/Spares_l ... s_List.pdf

and here for the 1957 set up.
http://archives.jampot.dk/Book/Spares_l ... s_List.pdf

Paul

Eric · Post by **Eric** » Tue Jun 03, 2008 5:00 pm

The engine cross section upon page 11 of the new illustrated parts catalogue recently sent to all members with the Jampot magazine shows the correct setup for your bike

msammels · Post by **msammels** » Tue Jun 03, 2008 5:24 pm

a haa - joy! now i am starting to see what is confusing things... it seems someone has eroneously picked up some '56 parts, with the exception of the matching sprocket (hence why I could make no sense of the set up).

I do have the '57 factory spares list, which indeed seems to be the same model engine as p11 in the Jampot spares list. What was confusing me is that p10 of this has a diagram of the '56 (at least) model which has the other design!

The only slightly worrying thing is that the splined shaft seems too long for just the parts for the '57. I have 2.25" from the side of the main bearing (the crank is out) to the end, and the sprocket doesn't sit hard up against anyhthing. There is a an oil thrower but it sits flush with the casing. Mine is the mag model by the way.

Any good ideas?

msammels · Post by **msammels** » Tue Jun 03, 2008 7:31 pm

I am thinking that maybe someone has used a pre 57 crank at somepoint and that's what I have now?

The crankcase is stamped 57 and the gearbox is amc with the clutch correct for the year. Short of cutting the crank shorter I guess (assuming the sprockets align) that running it with the 56 set up is not going to affect it too much.

Scaling up the Jampot sectional drawing it seems the correct crankshaft length is more like 1.25".

Don Madden · Post by **Don Madden** » Tue Jun 03, 2008 7:48 pm

The pre-1957 drive side axle is 2-3/4in from the cheek of the flywheel to the end, the 57-up dyno cranks, 1-3/4in. If longer than either with external threads on the end, it is an alternator shaft.

Cheers, Don.

msammels · Post by **msammels** » Tue Jun 03, 2008 9:03 pm

It's becoming horribly clear now - it is 2.75" from the face of the flywheel to the end of the axle (the 1/2" difference I measured before being the bearing), therefore someone has done a contemporaneous repair/ bodge by using a pre 57 crank.

I assume it was used like this because the bottom end was intact when I got it, and I only stripped it as the big ends were gone.

I will have to finish the reassembly of the engine, fit it into the frame along with the gearbox and hope the sprockets line up I guess. If not get a spacer machined up or some other ingenious solution I suppose.

Why are my restorations never simple...

Don Madden · Post by **Don Madden** » Tue Jun 03, 2008 9:31 pm

And the 1957 twin primary outer cover was a one-year only with a short protrusion for the crank end & after that, all were alloy. But there are more of the earlier about so should not be a major problem.

Cheers, Don.

msammels · Post by **msammels** » Tue Jun 03, 2008 10:53 pm

Thanks to all for helping me to solve this conundrum by the way.