Topic: Cruizin Image 836 mods 77 CB750F1

[martin_uk]

Just considering whether to modify 392 head to go with 65mm pistons.

I mainly went with these pistons as the original bores were so badly corroded plus they are a good price.

Performance is not the issue but dont want to spend too much and happy with same power as original 

Havnt finalised exhaust, but may be a 4 into 1 Motad.

Has anyone tried fitting the 836 pistons without enlarging combustion chamber or ports?

Suggestions on carb set up also appreciated ?

Thanks

[SOHC4 Cafe Racer Fan]

Just considering whether to modify 392 head to go with 65mm pistons.

I mainly went with these pistons as the original bores were so badly corroded plus they are a good price.

Performance is not the issue but dont want to spend too much and happy with same power as original 

Havnt finalised exhaust, but may be a 4 into 1 Motad.

Has anyone tried fitting the 836 pistons without enlarging combustion chamber or ports?

Suggestions on carb set up also appreciated ?

Thanks

You'll get a bump in torque and hp with displacement alone.  A port job will really wake up your motor.  It's the most worthwhile performance upgrade.  Who doesn't want a little more on tap? 

[PeWe]

This thread discuss these pistons that many use as it is, no other upgrades.
http://forums.sohc4.net/index.php/topic,89779.0.html

[Jerry Rxman Griffin aka MuthaF'er]

I'd speculate that most big bores use the head as is. I did so with my 812. Kinda like a double bump in compression. My thoughts are anything over 836 then open up the chamber squish to match the bore. At that point you may as well do a GOOD head job with porting at the least.   

[PeWe]

I was surprised that most of the pistons are made for std chambers, not opening them. Having a head that is opened with squish and pistons adapted for it will get lower CR when changing pistons of any reason.

It must give more power with wider combustion chamber, same as the bore, right? Or will the area of the head covering the bore work as a squish area?
I talked to an old CB750 biker/hoarder/dragracer with RC stuff from the 70's that he had helped a friend using these cheap pistons, deepen the valve pockets and use them with RC327 cam. Future will tell if it will keep together.

This forum is very addictive and I have recently got thoughts about 72mm pistons and how the engine should sound...  I blame the domino effect caused by next upgrade, better rods when I'm too worried about +80whp and +9000rpm's.

[martin_uk]

Thanks guys very helpful 

[HondaMan]

I suggest milling the head (F0 and F1 heads) about 0.5 to 0.8mm when switching to the 836 pistons.  This will put compression a bit over 9:1 again. Also, consider getting (or making) a slotted cam sprocket and advancing the cam a few degrees. This will make the most of the bigger bore's torque by loading the cylinder better and by reducing overlap losses to help cool it off.