Topic: CBX Pistons - A cheap 811cc hop-up?

[jweeks]

     Last year I had a stock high mileage cb750A needing a top end overhaul. We've all seen the smoke on start-up. Not so bad when it gets up to temperature. Oil weeping around several gaskets. Looking at 836cc kits that may exhaust your budget? The head has to be redone/valve guides checked/valve seals replaced/valves checked and ground. Gaskets for the head and base. I was looking for a lower cost alternative....
     Yes, there are cast piston 836 kits out there. I wanted a little stronger piston, so I read on Axel's old site about using CBX stock pistons for a lower cost hop-up. "Minimal machining necessary" were the words that I read. Went to Ebay and bought 6 used pistons for about $100. Rings? After a lot of searching I bought 4 OEM Honda ring sets for just under $25 each. Gaskets came from one of the SOHC specialist suppliers on the web. Thought that I'd save at least $100 against the Wiseco alternative.
     On paper, it worked well. Got a spare block bored for the pistons. Got the gaskets in the mail. Set aside a weekend to do the overhaul. First problem - the pistons won't fit onto the stock 750 SOHC rods. The CBX rods must have had a taper on the small end of the rods... So, my first trip to the machine shop to get the pistons milled to fit the stock rods.
     Another weekend, second attempt. Everything bolted together OK, but it was a little hard to turn over. Used a fiber optic scope, looked into the spark plug holes, and found polished marks where the exhaust valves lightly hit the pistons. Stock cam lashed at stock clearances. Pulled the motor back apart and found a couple of piston domes just brushed the edge of the combustion chamber when spun over...  So, my second trip to the machine shop for exhaust valve pockets and a 1mm narrowing of the dome to fit the stock combustion chambers. Yes, the gaskets were stock thicknesses.
     The moral to my tale is that you can't believe everything that you read. After two visits to my machinist, I would have been better to have found a deal on a 836 Wiseco kit. Now I have one of the stronger CBX pistoned Hondamatics around. Didn't even try the aftermarket cam I had planned for it. I had that strong feeling that I'd be going back to my machinist for the third time....

                                                                   Jon Weeks   

[Sam Green Racing]

Everything that glitters is not gold. there allways seems to be the odd problem hidden with cheap fixes.
My favourite cheap fix is using cable ties instead of bolts on none structural fixes.

Sam.

[Old Scrambler]

Thanks for the info.............maybe CBX pistons are a cheap fix for an F2 motor with the larger head area!

[bucky katt]

this is the same reason i havent tried the cb350T pistons in my k4 750. i know it gives something like 12.5 : 1 compression.

[bert96]

 Is there a difference between the 350 and the 350T ?


Bert

[HB-1fan]

John

Any chance there are spec differences between european market CBX pistons and ours in the states.  Not knowing where your used pistons came from!!
 Were the deck heights of all autos and regular CB's the same. Maybe AXL forgot to specify about certain origins, or years. Wondering if it was a specific year to year match up between models(CB- CBX). I doubt if AXL would have put it out there, based on just "hearsay"
Or maybe I'm just reaching!!!

HB

[jweeks]

HB,
     Pistons came from a US parts yard. Bike could have come from anywhere. Deck heights are the same. Don't think that anyone had researched the issue that well until I tried. And tried. And tried....


                                                        Jon Weeks 

[Industrial Cafe]

 Is there a difference between the 350 and the 350T ?

Bert

the 350T is a twin with the same size pistons as the 750four, the 350 is a four cylinder with 4 teeny baby pistons.

[Big Jay]

350 Twin pistons are not the same size as 750/4 pistons

[Industrial Cafe]

then why are they used to make the 'easy' 811 setup if they're not the same diameter?  I know the top is different if that's what you're talking about.


edit: if they're not the same, they're not as far off as the 350four piston...   that's the point I was trying to get across.

[Big Jay]

They are oversize to the 750. That's how they make it 811.

[Industrial Cafe]

well i figured that much but-

they're not as far off as the 350four piston...   that's the point I was trying to get across.

without going into any real detail
for the quote below-
 

Is there a difference between the 350 and the 350T ?

I wish I had a fiber-optic cam, I think my motor has had some work (bought it from a member here)

[bucky katt]

Everything that glitters is not gold. there allways seems to be the odd problem hidden with cheap fixes.
My favourite cheap fix is using cable ties instead of bolts on none structural fixes.

Sam.

i carry around 200 zip ties of different sizes on long rides/trips just for that reason.

[754]

Industrial.. hook a mag lite bulb to 2 wires, insulate connections, slip them 2 wires and a pce of solid cooper wire into heat shrink tubing then shrink..... now pull plug , inert lite, put power to the 2 wires, and you will see the pistons clearly..

[Soos]

Being a machinist and enjoying an unusual overbore options, this is good info to me.



I run my cb650 with cb750 ('77) high domes pistons.
Also am getting ready to do another members pistons as well as a set of 750 hondamatic pistons I have as cb650 overbore pistons.

Even got a 66mm set i'm going to drop into a 650 as well.
So far those have been a headache as to the amount of work needed.
I have already pressed out the old liners, bored out the block, and installed cb900 liners.
Still need to bore out the cylinders and have them honed, not to mention the mods I need to do to the pistons themselves.


The sacrifices needed for speed.










l8r

[Industrial Cafe]

Industrial.. hook a mag lite bulb to 2 wires, insulate connections, slip them 2 wires and a pce of solid cooper wire into heat shrink tubing then shrink..... now pull plug , inert lite, put power to the 2 wires, and you will see the pistons clearly..

you told me that a while ago, and I made one for my little maglite, but I got distracted by something and forgot all about it.  think I'll put it to use now that you mention it.
    I still want a snake cam.

[turboguzzi]

gee jon............. when making any kind of piston swap like that, a quick check with plasticine (for piston to head clearance) and light valve springs on just one pot would be recommended.

the CBX is also a four valver, would remove any ridge between the two valve reliefs (if any) to begin with

And yes, cheap often means more work 

[Soos]

And yes, cheap often means more work 

Kinda like when any job starts with the customer saying "All you gotta do is"...


l8r

[jweeks]

Turboguzzi,
     The point was that I had read that it had already been done. Successfully. I did spin the motor over by hand the first time and it did spin. If I was building a motor with tight clearances, I'd use clay and spin over by hand, disassemble and measure. There were no valve notches on the piston from the factory. The valves didn't hit the stock dome on the piston. Using a stock automatic cam, you don't get anything with milder lift to open the valves. This topic was given to help others realize what would be involved in a similar swap.

                                            Jon Weeks   

[bucky katt]

how hard will it to be to hear any detonation/pinging? i have a kerker with the stock baffle, packing removed for a little extra racket.

[Bill/BentON Racing]

Grasshopper... what one hears another does not  Bill

[imabass]

Thanks for the writeup.  I found this old post via google search and I just planned on buying some CBX pistons to do some testing.  Guess it has already been done and no need to reinvent the wheel.

[Big Jay]

this is the same reason i havent tried the cb350T pistons in my k4 750. i know it gives something like 12.5 : 1 compression.

They require quite a bit of machine work.