Thread: Ordering the correct piston

I have 3 ct's and have notice that the pistons are different. The distance from wrist pin to dome varies.

Is there a difference in stroke to accommodate the different wrist pin/dome dimensions?

Can anyone give details as to the stock configurations.

I would like to order 3 rebuild kits for my winter tear downs and would like to make certain I am getting the right stuff.
1971 K0
1972 HK1
1974 K3

Thanks in advance

The differences are in rod length; compression height of the piston had to be matched accordingly. Something here isn't quite right. Motors of pre `81 vintage all had the same rod length and used the same piston. Sounds to me like a previous owner did some parts mix & match. If that's the case, then you've got some detective work ahead of you. Don't be surprised to find at least one of the three cranks with the big rod bearings worn way beyond spec. 3-speed cranks are NLA and will usually set you back $150, not including shipping, to rebuild. About $200 will buy a new crankshaft and one of those highly regarded 12V CDI conversion kits from ebay...money well spent if you have worn crank in any of those motors. Sorry to digress, but the point is to settle the crankshaft questions first, then source the matching piston. Dratv has a good explanation of rod length changes & specs and should get you headed in the right direction.

Just to clarify:

All 3 of my machines should have same rod length/same piston.

And there are 2 stock rod length differences?

If one of my bikes is different, someone must have swapped in newer parts?

And thanks for the info I will take a look on Beatrice Cycle

Later models had longer conrods and different pistons to compensate the compression. As far as I know, the different crankshaft came with the model change to CDI ignition and 12V alternator (at least over here, where Honda introduced the newer engine later - 1988). There is a little "European history" behind that change. If you are interested, just ask.

You can't mix and match old piston/new conrod and vice versa. Either you have very low compression (doubt it will run at all) or the piston collides with the head (conrod length difference is 3mm).

If you indeed have a different piston with a different compression height running in the older type engine, I reckon its from a different model or an aftermarket part.

Thanks MFRO. Good info.
I will be dissecting these motors this winter and now I know what to looks for.