Topic: cb750 pistons in a cb650?????????

[nkymain485]

so i recently picked up a 1980 cb 650 custom... picture below  picked it up for 200 bucks...well it burned oil and i have been reading a few tech posts saying how you can bore the cylinders and put in cb750 pistons...well i was thinking of doing this however i haven't found a good post on exactly what needs to be done to the head for them to fit, also while shopping around for a used set on ebay noticed there is a few different kinds of cb750 pistons...so what kind should i be looking for, for the easiest install...all help is appreciated im ready for this thing to be running.

[Soos]

Yes cb750 pistons work in a cb650.



I am running a set of cb750F pistons('77/'78) .
The machining I had to do or have done:

Bore/hone out the cylinders to 61mm.
In my case using "F" pistons, I had to machine the valve reliefs deeper, and remove material from the dome a bit.
Being a machinist, I did the mods to my pistons, but left the bore/hone job to a shop with more specialized equipment.




K ('69-76) pistons require less machining simply due to less dome than the F pistons do.

Get a new set of rings and a standard cb650 honda genuine head gasket, re-assemble and there you go.


If you are quick there is a 4th over set of pistons(62mm) listed on E-bay for the cb750.
61mm=652cc's
62mm=673cc's
63mm=695cc's
64mm=718cc's
65mm=740cc's
65.5(cycle-X pistons)=752cc's
BTW, with 62mm(or larger) you will need a custom head gasket, but might be able to get without re-sleeving the block.
The stocker cb650 head gasket will safely run 61mm pistons, 62mm leaves the same material in the liners as a 65mm overbore in a cb750, so they are safe (IMO) as well.
Over 62mm and you WILL HAVE to re-sleeve.
Cometic will make any bore you want cb650 head gasket.

I have another cb650 that is (VERY slowly) being modified for 65mm pistons.
But with OT and life in general, left over time for the spare motor is limited at best(at the moment)


In general I don't think there are pistons for the 650 that will work as an overbore that do not require machining(but I'm still looking).
I have been interested in KZ650 62mm pistons, but having a hard time sourcing ring sets, so not too interested... yet.





Paulages has a realy good thread on 64mm pistons mods needed in a 550/650 hybrid engine.


l8r

[nkymain485]

awesome really good post...but on the k pistons you say less machining do i need to deepin the valve reliefs and still trim some off the top? or just get it bored out and install em and go

[Soos]

Yes they do need 1mm trimmed off the deck height...
forgot that.


I believe I left the skirt length stock length.




l8r

[Soos]

And yes you will need to cut valve reliefs.

Not necessarily due to a higher lift cam, but the difference between the cb650's valve spacing compared to a stock 750's valve spacing.
And therefore the placement of the valve reliefs on the pistons.



l8r

[Pinhead]

Subscribing for future reference and hopefully experience.