using non CB72 pistons on a CB72 engine (not bigbore)

jensen · Post by **jensen** » Sat Aug 15, 2009 3:01 am

Hi everyone.

I have a cilinder from a CB72 ready for reboring. does anyone have any experience with using modern and very light pistons from honda 125 CC machine like a cb125 2005 or so ?

Honda has a lot of low displacement machines, does anyone have some experience in that area ?

Jensen

jensen · Post by **jensen** » Thu Aug 27, 2009 2:59 am

I guess there aren't any easy retrofits possible.
I'll use hap jones for once to see how they're doing.

jensen

davomoto · Post by **davomoto** » Thu Aug 27, 2009 3:40 pm

Jensen, I see lots of Honda CL $ CB 72 pistons and rings on Ebay.

davomoto

jensen · Post by **jensen** » Thu Aug 27, 2009 4:27 pm

Hi,

Thanks Dave, I have a lot of CB72 pistons and ring sets (probably over 15 complete sets in all different sizes), old types and new types, includings rings and pins.

But the question was if there where modern pistons suitable for the CB72. The original pistons are very heavy, and I don't know where to start taking off material. A lot of material is in the top of the piston, but where do I take it off ?

Hap Jones pistons are a little lighter, but have these pistons the same quality as the Honda pistons ?
I also have MC pistons (yellow box with black and white arrow print) these are very light too.

The thing is that I'm busy (re)building an engine where I take things a little further.

This engine will have a flowed head, bigger carbs, lighter valves, higher compression.
I don't want a bigger bore, but I do like lighter pistons for higher revs, I'm aiming on at least 11.000 rpm.

But anyway, thanks,

Jensen

CliffC · Post by **CliffC** » Thu Aug 27, 2009 5:55 pm

Jensen, I have a couple of bikes with engines that were modified by Pop Yoshimura that I ran in the early 60's. What Pop did to lighten the pistons was use different size drill bits and a deburrer bit in the top of the pistons and around the pin area. Its been along time but if I remember correctly I was turning both the 72 and 77, 12 to 12,500 with out a problem. CliffC

jensen · Post by **jensen** » Fri Aug 28, 2009 12:26 am

Hi Cliff,

Thanks for the info. I assume that these where high compression pistons.
I'll go for a compression ratio of 10.0 to 10.5 or something like that,

I have a few shot pistons and I'll start with these to try how far I can go.

I also tryed to make the skirt less thick by turning them out, but the loss of weight is almost nothing. Just enough to make sure that the pistons are weighting the same within 0.5 gram.

Do you happen to know if these pistons in your engines have the same skirt length ?
Todays pistons have very short skirts, means ofcourse less friction.

jensen