How to tell 250 and 305 crankshafts apart ?

[Phil-UK]

Apologies if this has already been covered but I couldn't see anything when I did a search.

I'm about to send a CB ( 180 degree ) crankshaft off to be rebuilt as it has at least 1 knackered big end and found I also had a 360 degree crank which I thought could provide spares.
As an afterthought I took measurements of the webs/flywheels and while the width and diameter are the same on both, the size of the 2 holes adjacent to the big ends are different - the 180 crank has @24mm holes while the 360 crank has @19mm holes.

I then checked some other bits left over from a couple of 180 cranks and these have @19mm holes so the difference is ( I guess ) not between 180 and 360 cranks. My assumption is that the holes are in effect balancing the crank and that the different sizes are for 250cc or 305cc engines ... is this correct ?

If it is correct can anyone who has a crank lying around that they know came from a 305 measure the holes please as I don't know the origin of mine and always assumed they came off the more popular ( in the UK ) CB72 250.

thanks

Phil

[Tim Miller]

Correct, the smaller balance holes were used on all CB/CL/CA 72's. All 77's came with the 24mm balance holes. But all the parts are enter changeable.

Tim

Apologies if this has already been covered but I couldn't see anything when I did a search.

I'm about to send a CB ( 180 degree ) crankshaft off to be rebuilt as it has at least 1 knackered big end and found I also had a 360 degree crank which I thought could provide spares.
As an afterthought I took measurements of the webs/flywheels and while the width and diameter are the same on both, the size of the 2 holes adjacent to the big ends are different - the 180 crank has @24mm holes while the 360 crank has @19mm holes.

I then checked some other bits left over from a couple of 180 cranks and these have @19mm holes so the difference is ( I guess ) not between 180 and 360 cranks. My assumption is that the holes are in effect balancing the crank and that the different sizes are for 250cc or 305cc engines ... is this correct ?

If it is correct can anyone who has a crank lying around that they know came from a 305 measure the holes please as I don't know the origin of mine and always assumed they came off the more popular ( in the UK ) CB72 250.

thanks

Phil

[Phil-UK]

Hi Tim

Thanks for the confirmation, the rebuilt crank is meant for a 305 so that's good news.
I was surprised at the difference in the weight of 250 and 305 pistons when I weighed them so having the "correct" crank in must help minimise vibration, I know my current CB72 vibrates more than my mate's at high ( >6000 RPM ) revs, which could be down to the crank thats in it.

cheers

Phil

[CJ]

I was surprised at the difference in the weight of 250 and 305 pistons when I weighed them so having the "correct" crank in must help minimise vibration, I know my current CB72 vibrates more than my mate's at high ( >6000 RPM ) revs, which could be down to the crank thats in it.

Hi Phil:

Since you have it apart anyway.... you may want to check the weight of your two pistons (on their connecting rod is best, but if you don't have the rods off, just the pistons will help). I have had pistons that were in the same engine weigh as much as one ounce diffrerence!

Matching the weight will help reduce vibration.

If you are not comfortable (or familiar) with matching them, most good machine shops will do it fairly inexpensively. If you are doing it with the rods connected, be sure they 'mark' one assembly so if you ever have to take them apart, you will know the correct match up.

Cheers,
CJ

[Phil-UK]

Cheers CJ,

Good advice and as I'm going to take some material off the CB350 pistons to get them closer to the weight of the CB77 ones I'll make sure they weigh the same afterwards.
The CB350 pistons have longer skirts ( below the gudgeon/wrist pin ) and are slightly slipper shaped so I can take quite a bit off and stay within the original 305 piston dimensions.

Phil