Topic: honda 550 big bore pistons 61mm HC pistons in a stock bore 750?

[hondaman750]

has anybody ever put 61mm high compression pistons from a cb550 big bore kit into a cb750 stock bore ? i would like to try and get some feedback has anybody ever done it ? im wanting to leave my K8 stock bore but get a good bit more power out of it if possible mainly because of the lack of funds and lack of reputable machinists in my area and help would be great!

[BPellerine]

check to see if f2/f3 pistons will fit not sure myself but they are higher comp.bill

[HondaMan]

Subscribed: this is an interesting possibility! If you have the 550/61mm pistons, stick a wrist pin into one, then also into a 750 piston, to see how the crowns and skirts match up. The Gentleman's Express 550 conversion uses these the other way 'round...

[hondaman750]

thats what i was thinking people are using 750 pistons in their 550s why would the 592cc kit work in a 750 they are both 61mm

[hondaman750]

Subscribed: this is an interesting possibility! If you have the 550/61mm pistons, stick a wrist pin into one, then also into a 750 piston, to see how the crowns and skirts match up. The Gentleman's Express 550 conversion uses these the other way 'round...

   the only problem is i dont have any 550 pistons laying around or know anybody with one :/ in theory i would think that it would work

[HondaMan]

Subscribed: this is an interesting possibility! If you have the 550/61mm pistons, stick a wrist pin into one, then also into a 750 piston, to see how the crowns and skirts match up. The Gentleman's Express 550 conversion uses these the other way 'round...

   the only problem is i dont have any 550 pistons laying around or know anybody with one :/ in theory i would think that it would work

I have been getting a bunch of e-mails lately about a 61.5mm big-bore piston kit for the 550 that is $120 on eBay, from the same folks who make the nice 836cc low-compression piston kits. These work well in the street bikes I've built with them, so I suspect the 550 pistons would work, if the crowns and skirts will fit. This would also allow some "bore room" of .5mm for the post-1975 F engines that could then receive a stock-ish bore size, with high-dome pistons.

Hmm...might have to hazard a set?

[hondaman750]

Maybe worth a try

[hondaman750]

The only thing I can think of being different is the valve reliefs in the piston maybe the 550 ones are smaller?

[HondaMan]

The only thing I can think of being different is the valve reliefs in the piston maybe the 550 ones are smaller?

That's what I was thinking, easy to solve, though.

[classic bike tuner]

the 550 biggbore pistons are stock dimensions exept bore  so flat top
and the 550 pistons have a lower pin-crown height than 750 pistons
with 550 - 61,5mm biggbore pistons  in your 750 you have lower compression
if u use 750 F2 you end up to high [1-14 ]

[hondaman750]

On the other hand does anybody know of a place a can find stock bore  high compression pistons about 11.5.1 or so?

[750essess]

M3 racing has 62mm wiseco's on their site. Doesn't say compression.
http://www.m3racing.com/products/parts.html#CB750

[classic bike tuner]

u can use boldor rods  these are 1mm longer than stock 750  and are a bit stronger too need to machine piston for good squis area [ 0,8mm ]and meybe valve pockets

if your stock compression is 9,0 you end up 10,2 with bol rods

[hondaman750]

Is anybody out there running f pistons with a k head?

[EnginebyAdam]

u can use boldor rods  these are 1mm longer than stock 750  and are a bit stronger too need to machine piston for good squis area [ 0,8mm ]and meybe valve pockets

if your stock compression is 9,0 you end up 10,2 with bol rods

If you were to use cb900 rods there are a few things to look at. The cb900 rods are considered weaker than the cb750 dohc rods, but it is all a matter of opinion. The moderator on the cb750 custom site ran NOS on his stock bottom end cb900 with no problems. I shot peened my cb900 rods and have confidence in them.

The one must do upgrade for the cb900 rods is to replace the stock rod bolts with cb1100F rod bolts. The stock cb900 rod bolts have a spiral pattern in the bolt, and have failed.

[BPellerine]

I think the 78k head has a little bigger chamber than earlier so maybe not so high compression .you could use a thicker head gasket or thicker base gasket to lower it some if you want.would be a budget way but don't know about the power increase.you could drop in the f3 cam if you have the clearance .any of the head experts should be able to tell you if it is poss.bill

[hondaman750]

I'm already running  #41 grind webcam I'm just looking for more compression

[BPellerine]

well I guess you could research the f2/3 option.bill

[hondaman750]

Anybody else have any ideas? on how to bump up CR on a stock bore bike maybe milling down the head a little bit?

[hondaman750]

does anybody happen to have a 550 piston laying around that they could donate? for scientific reasons of course doesnt even need to be good could be from a blown bike as long as it can have measurements taken

[twatermelon]

A stock or 61mm piston? What measurements do you need?

[hondaman750]

Looking for wrist pin to crown and wrist pin to skirt and such

[bwaller]

Here's a pic of a 750 (std #300) piston vs 550. The difference is dramatic. The CB550 61mm big bore pistons compression height is nearly identical to the stock 550 piston.

I can send you a CB500 piston if it helps. The only difference is 56 vs 58.5mm for the 550. Otherwise they share exactly the same dimensions.

[hondaman750]

Then I wouldn't really gain anything on CR then

[PeWe]

u can use boldor rods  these are 1mm longer than stock 750  and are a bit stronger too need to machine piston for good squis area [ 0,8mm ]and meybe valve pockets

if your stock compression is 9,0 you end up 10,2 with bol rods

If you were to use cb900 rods there are a few things to look at. The cb900 rods are considered weaker than the cb750 dohc rods, but it is all a matter of opinion. The moderator on the cb750 custom site ran NOS on his stock bottom end cb900 with no problems. I shot peened my cb900 rods and have confidence in them.

The one must do upgrade for the cb900 rods is to replace the stock rod bolts with cb1100F rod bolts. The stock cb900 rod bolts have a spiral pattern in the bolt, and have failed.

Good info! I have a set of CB900 DOHC rods for future project. 900CC cylinder that I might bore and buy newer 915cc pistons instead of old flat Henry Abe 67.5mm pistons.
About the lenght. I double checked my rods in the garage. CB900 DOHC rods are 1.5mm longer than CB750 SOHC. I saw the strange bolts too.
Fine tuning the CR with gaskets. I purchased an 836cc head gasket from Yamiya. This is 0.5mm  thicker than the Cometic fiber gasket.
Edit:
Note! The bearings are still in the 900 rods