Matching an 836 bore on my 750.

[mysta2]

My original intention was to build up my 1975 CB750 Super Sport. In fairly stock trim so that I could have something cool to cruise to work and back with when the weather was good but I didn't want the stress of riding my 360. I started to tear the motor down just because the trans was acting up (no first gear). While I was in there I saw that the oil ring on the #3 piston had broken and had scored up the wall pretty good. Long story short; yesterday I dropped off my 65mm pistons along with the cylinders at the shop. This all just kind of snuck up on me one step at a time so I haven't done the research I normally would have before taking this step, so I now find myself at a loss about how I would go about balancing this mod out. More volume means more air so I'm guessing I'll be playing with jetting to balance that. I want to keep the airbox and at least for the immediate future the 4 into 1 stock exhaust will be staying.

Where do I start? I'm in the middle of reading through Sonriers massive post, and so far finding it very informative.

For more info on the bike and what I'm doing click on my signature.

 

[john83]

I got nothing no knowledge to offer ya Kit, I just want to follow along.

 

[surffly]

The stock air box is a great idea. The SOHC bikes hate pods.
Yes jets will need to be changed for sure, but the stock carbs will flow fine.
The bottle neck in a CB750 is the head. Really money is spent better on porting the head then boring the motor.
What CR are your pistons?
Even the stock header will flow fine. Maybe think about a slip on.

 

[teazer]

Jetting will be fine to start with if the rest of the motor stays stock. Bigger pistons will pull in slightly more air and that increase in air velocity will lift up more fuel. Run it in and see what it's like. You may end up one or two sizes up or down on jets.

Clean up the head while you have it off. No need for huge port changes, just cleanup the seat and bowls and make sure the head is flat

A really mild cam works fine too.

 

[mysta2]

Compression ratio... Something I hadn't thought of previously. I don't know.

This is the kit that I bought.
http://item.mobileweb.ebay.com/viewitem?itemId=140881496503

Another question that I must ask is the difference between a really high quality piston and a cheaper one. Do you think it's possible that these ones that I bought are simply gravity cast aluminum. There did seem to be some sharp edges on them which had me wondering if there was any way for me to "finish" them to a higher standard.

 

[surffly]

Yeah those are standard CR, like was said you should expect jet changes, but really not see any increase in HP.
Sure you might gain a few ponies just based on the extra CCs but not really much.
Guys seem to run them because they are cheaper the the .25 or .5 over OEM pistons when rebuilding.

 

[surffly]

They are standard cast pistons, nothing like the forged aftermarket pistons out there.

Check out SOHC4.net Hondaman talks about what clearances to set these pistons at.
There are a few variations in the castings used

 

[teazer]

IMHO cast pistons are fine in 99% of applications. We have run forged and cast in two strokes and way less sizures with stock cast pistons than with forged. All our 4 stroke race pistons are cast and never had a psiton failure that was a problem with teh casting or machining.

Doesn't mean it doesn't happen, but it's rarely an issue. A few more CCs always helps a little.

 

[mysta2]

Surffly, help with any links to the info from Hondaman? I tried searching without luck but did stumble across a "thoughts of Hondaman" thread with a ton of good info in it. But not the info you speak of. I don't plan on racing this bike and so am thinking (hoping) that this setup will work fine for me. I did discover what seems to be a Yoshimira lightened cam sprocket in my head though so maybe this engine has some stories to tell. Any clues on how to identify a cam?

 

[surffly]

I'll look for the link
Latter F cam sprokets are drilled.
Aftermarket cams are normally marked on the ends.

Post up some pictures

 

[mysta2]

Goodies:

CMC (on the other side is an E and a 2)

Below the speedo drive in this pic is "2187" and above is a D1 on one side and C1 on the other. The only thing I can make out on the end is the letter D and what might be a 9

 

[surffly]

I'll check my yoshi cam when I get home for markings.

That sproket looks like a stock latter one