73 ISDT 125 Crankshafts / Center Cases

[Ron]

Hi Guys,

I'm getting ready to start on my engine and noticed of the 3 junk crankshafts I have, 2 of them have a 1/2 diameter (wider) flywheels and the one is a full diameter flywheel.

I selected a set of center cases and packed the others away.
I hadn't noticed if the crankcase area is smaller in some center cases than others.

Are they? Do I need to match up the crankshaft to the appropriate case set? Is there a performance issue here?

In other words, do I have to unpack my shed again.[xx(] LOL

Thanks for anyones help,

RonW

[Gavin Housh]

Ron, the Sachs motors could come with two sizes of crank. The early models 1251/5A and 1251/6A had the larger diameter crank webs. That would be bikes up until some time in 1972. The later model 1251/B and 1251/6D (6/D motors not sold in Penton bikes, but found in others such as Monark) motors had the smaller crank. These motors could be found in the 1973-75 bikes. These two different motors and cranks are not interchangable. The early larger crank fits in the cases with the bigger space for the crank to fit in. The late model motor has sort of a second sealing wall of aluminum cast into the cases to make up for the smaller crank. The reason for that is to make the crank volume the right size to achieve the compression ratio that was designed into the motor. Some riders prefer the bigger crank because it has more flywheel effect and is easyier to ride. Others prefer the small crank because of it's faster revving with a few more HP. It is a little harder to keep on the pipe and requires you to be in the proper gear at all times. Deciding between the two is a tough decision. I have two 5/A motors and one 6/B motor. they both get the job done, but the way you ride them is a bit different. Also you should look at weather you will use the bike for MX  or trail riding. Hope this helps some and I'm sure the other guys will be able to give some more info. Gavin

[Ron]

Gavin,

Thank you for your response. I dug out the other cases I have and see the extra webbing you mention.

I did seem like a awful lot of room for that little crank.

I think I'll go with the smaller one. I'll have to have the crank rebuilt which ever one I use.

I'm probably to big to ride a 125, so keeping it rev'ed up might be eaiser with the smaller crank.

I plan to ride a Gran Prix on it and then maybe some classic motocross.

Thanks again for the info,
RonW

[Doug Wilford]

Ron;
Make sure who ever rebuilds your crank knows the crank pin(rod pin) is tapered.   The slabs have to be pulled off the pin.

[Ron]

Hi Doug,

Thank you, and all the guys that read and respond to us wantabe mechanics.
 
I would never have known the cranks wheels must be pulled. I would have treated it like any other Japanese crank and tore up all kinds of things.

I really appreciate your input.

RonW