Topic: Anyone heard of using CA77 pistons in a CB750 motor to boost the CR?

[Jonesy]

(Hopefully this is considered "high performance enough to qualify here.. if not, I apologize..)

This may be timely after reading the thread about using CBX pistons...

While at a bike show this weekend, I got into a chat with another member of the VJMC who worked for a Honda dealer in the 70's and did lots of high-performance work on CB750's. Mostly the usual stuff, like porting, 836 kits, etc. But what caught my interest is when he mentioned that one popular mod was to use pistons out of a CA77 Dream 305 to boost the compression ratio. However, you need to bore the 750's cylinders a bit to get them to fit.

Anyone else heard of doing this, or tried it?

[Old Scrambler]

Have not tried that combo..........just some info.......the CA77 as well as the CB77 and CL77 models have a 60 x 54 mm bore and stroke.  Various compression ratios and ring designs were used between 1959 and 1967.  The later years, from about '62 onward, have model '275' pistons with 3 chrome rings. The piston top is mildly domed, more than a standard 750 piston. New standard-sized pistons are somewhat hard to find but 1, 2, 3 & 4th-over sets of two with rings, pins, and clips are usually available on ebay. Most are in Honda boxes.  Beware of early CA77 units with wider oil rings and a 4th ring below the pin. 

[Sam Green Racing]

(Hopefully this is considered "high performance enough to qualify here.. if not, I apologize..)

This may be timely after reading the thread about using CBX pistons...

While at a bike show this weekend, I got into a chat with another member of the VJMC who worked for a Honda dealer in the 70's and did lots of high-performance work on CB750's. Mostly the usual stuff, like porting, 836 kits, etc. But what caught my interest is when he mentioned that one popular mod was to use pistons out of a CA77 Dream 305 to boost the compression ratio. However, you need to bore the 750's cylinders a bit to get them to fit.

Anyone else heard of doing this, or tried it?

Danny, the only down side to using CA77 pistons is the fact they are cast. If you're looking to raise the compression, you're obviously looking for performance, why use cast when forged with higher comm pression are available, just my 2 cents.

Sam.

[kos]

Why would one use those old design pistons? Makes no sense at all. What your VJMC man might have been thinking was that in the beginning we used to use cb350 pistons with valve  reliefs cut into them and these were the first big bore 750 kits. More modern, better rings, pins same size and deck height right on the money.

That helped Pop's Yoshimura get started in USA!

KOS

[Jonesy]

I figured you guys would know this stuff (forged vs. cast, etc). I wasn't planning on doing it, but it sounded intriguing as I've never heard anyone mention doing it before. Plus, I've never seen CA77 pistons, so I have no idea if they are even practical. It sounds not.

I guess this was my SOHC4 "Snopes" thread- was this urban legend or true?!

[bucky katt]

i've looked at pros and cons of several different oem pistons from other models for my 750. cbx pistons will take the least amount of work to get them to work.

[bwaller]

The forged pistons I have need to run more "loose" than cast because of greater thermal expansion. The consensus here is the other way around... on average you can run tighter tolerances with cast pistons.

As for performance one can't argue that most OEM stuff is cast, but for durability forged pistons are stronger.

Simply 2 more cents worth.

[syth82]

Forged pistons are much stronger than cast, which is the performance advantage. However they are more prone to thermal expansion, and thus need more clearance. This can cause a little more wear and noise from piston slap, but you wouldn't loose any real performance from the wider pison to wall clearance.

[NitroHunter]

I've had several sets of forged pistons that required entirely different clearances depending on their composition. One light weight, high silicone set called for .002", but another very stout low silicone piston needed .004"+ minimum. Of course the blower pistons were subject to much higher temperature and pressures.
Same bore, different applications.

[Big Jay]

There are basically two materials for forged pistons. 4032 high silicon ( Wiseco ) amd 2618 low or no silicon. I usually set Wiseco 836s up between .001" and .0015". Slightly more than an oem cast. 2618s a little looser.

And yes, a forging is much stronger than a casting.