Author Topic: Big-Bore CB750 pistons in a CB350 Twin?  (Read 27047 times)

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Offline 754

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #25 on: December 16, 2007, 04:25:49 PM »
I think there is a limited need for mid grade rods from stock cores. Only need to be good for 95 hp and 12,000 rpm.

Check them
Shot peen them,
 bush them
&
source and fit better bolts.


we got cores.. man do we have cores..!!!
Maker of the WELDLESS 750 Frame Kit
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My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline Big Jay

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #26 on: December 16, 2007, 05:00:35 PM »
Here is why the some of that stuff is still out there. The bike guys quit doing abything but 836 kits for several decades. The only ones that did any sohc stuff, and single handly keep the limited aftermarket demand going, were the three quarter midget racers.

That motor ruled in TQs for a long time, until Don Vesco figured out the GSXR750 suzuki.

Since then, there has been a small vintage demand, but nothing like it was. The people who make the cam billets, Engine Power, would have to cast several hundred cores, finish them all but the lobes, then put them on the shelf and wait for the cam grinders to call. I guess it took so long to turn over the last batch that they won't do it anymore. Inventory needs to turn several times a year.

As for rods. The breakage was always in the beam. Bolts were not the weak link, altho I remember putting some car bolts, maybe 289 Ford in some Honda rods. That is why no one ever made a mid grade rod. There were only 836 street guys who were not hurting them much, and the alky TQ guys who needed Carrillos.

One could probably get good rods made in China for about $12.00 each, but wopuld have to charge 10 times that much for them because thay would have to sit on them for ever.

I might know where there are some 970/1000 pistons. I will check.

Jay


Offline JLeather

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #27 on: December 16, 2007, 05:05:39 PM »
Jay,
  You don't have, or know where there are, some forged stock-CR or less CB750 pistons do you?  For my turbo build I was trying to keep the CR near stock, like ~9.0:1 or less, but I want forged slugs for the strength.  Got any turbo-pistons floating around your shop?

Offline DarkRider

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #28 on: December 16, 2007, 06:26:14 PM »
Now we know that Jay offers stuff for the 750 but does he have much for the 500/550?
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Offline 754

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #29 on: December 16, 2007, 10:42:08 PM »
JL if you find used pistons and really want to go this way bore and fit them, clay your pistons.. assemble and turn over carefully  then cut the plasticene clay, and see what you need to take off.. might be able to die grinder them....cheap method.

Other than running used this is gonna cost a lot if you go new...
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline paulages

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #30 on: December 28, 2007, 12:39:42 AM »
JL if you find used pistons and really want to go this way bore and fit them, clay your pistons.. assemble and turn over carefully  then cut the plasticene clay, and see what you need to take off.. might be able to die grinder them....cheap method.

Other than running used this is gonna cost a lot if you go new...

had to do this with my +3mm 750 pistons to find valve and piston clearances for the 650 head. if you get enough clearance to run the motor through, i'd also recommend measuring your CR before doing any cutting, so you know both factors you are working with and can cut accordingly.

i just bought a pile of 350T parts. this thread is making me want to turn some pistons down to put in one of my 750 engines. one of the guys in my shop has a set of yoshi 811 pistons i could model from. hmmm...
paul
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1976 CB550 (590cc) road racer
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Offline 754

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #31 on: December 28, 2007, 09:56:39 AM »
Just thinking.. 360 twin slugs would probably make an 860? or be good overbore for 836 sleeves..
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline paulages

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #32 on: December 28, 2007, 10:00:32 AM »
Just thinking.. 360 twin slugs would probably make an 860? or be good overbore for 836 sleeves..

think jay mentioned that the pin diameter changed with the 360.
paul
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1973 CB750K3
1972 NORTON Commando Combat
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Offline crazypj

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #33 on: January 17, 2008, 01:34:59 AM »
You can also fit Wiseco XR200 pistons and pins to CB350. Dont remember what c/r works out at or other mods necessary
 I think Suzuki GS750 has 15mm pins and 66mm bore as well. (I have some used pistons and think its what they are out of)

PJ
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Offline CBGBs

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #34 on: January 17, 2008, 03:06:26 AM »
I bought aftermarket pistons from www.loudfastugly.com last year.
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Offline JLeather

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #35 on: January 17, 2008, 07:02:38 AM »
AH HA!!!  I knew someone had to have just simple, inexpensive, overbore pistons to clean up my engine.  Thanks man...

Offline crazypj

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #36 on: January 17, 2008, 11:25:09 AM »
If you get GS pistons/block you can take liners out and have pistons ready matched to them.
Only need to bore block and shrink them in then
 I've done CJ250 wih GS1000 pistons/liners (391cc)
You may need to shorten liners, I forget, was a while ago
PJ
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Offline Big Jay

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #37 on: January 18, 2008, 06:01:39 PM »
You can also fit Wiseco XR200 pistons and pins to CB350. Dont remember what c/r works out at or other mods necessary
 I think Suzuki GS750 has 15mm pins and 66mm bore as well. (I have some used pistons and think its what they are out of)

PJ

GS750 is 67mm with a 16mm pin.

Jay

Offline paulages

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #38 on: January 19, 2008, 11:17:03 AM »
When we made those 836 kits we used 4th over 350 twin pistons. The 350 piston had quite a dome on them, and we cut a lot down to make a 10.5:1 750. So I would think that any 750 kits you found would result in low compression in the 350.

There used to be a very nice big bore kit available for the 350 twin. I believe they were 67.5mm. We bought hundreds of them and made the first 915cc kits out of them.

I believe they came from Hap Jones. Don't know if they are still in business.

Jay

hey jay- i'm assuming you just used the 350 pistons as models for the new castings, then? i looked at a yoshi 811 piston and a stock cb350 piston side by side, and there is clearly not enough meat in the dome to cut down the 350 piston enough to result in usable compression. too bad, as i have enough sitting around and it might have been a cheap way to make an 811 kit for my 750.
paul
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1976 CB550 (590cc) road racer
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Offline Big Jay

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #39 on: January 19, 2008, 02:42:25 PM »
We used the 350 pistons and cut them down.

Jay

Offline paulages

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #40 on: January 19, 2008, 04:53:15 PM »
We used the 350 pistons and cut them down.

Jay

really? well that's encouraging. by my measurements, the 350 dome is about 4mm taller (if i remember right) than the yoshi. it didn't seem like there was enough material in the 350 slug to allow that much cutting. i don't know what the yoshi CR is, but i'm guessing 10.5/1 or so. seems like removing the shrouding in the head could help drop the CR too.
paul
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1972 NORTON Commando Combat
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Offline JLeather

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #41 on: January 19, 2008, 05:20:05 PM »
I didn't think they cut them all the way down to 10.5.  I was under the impression the yoshi kits were very nearly 12:1.

Offline mkramer1121

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #42 on: January 19, 2008, 05:48:21 PM »
Nope, the Yosh 811 pistons I had, using 350 slugs, were 10.5:1...Rather flat topped really.

Offline crazypj

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #43 on: January 19, 2008, 08:23:46 PM »
GS850G is 69mm with 16mm pins.
 If you had bushed rods you could probably get new bushes or ream old ones to 16mm.
PJ
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Offline paulages

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #44 on: January 20, 2008, 11:32:11 AM »
Nope, the Yosh 811 pistons I had, using 350 slugs, were 10.5:1...Rather flat topped really.

funny you say that, 'cause your old pistons are the ones i was comparing to. i share my shop with the guy who bought them.
paul
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1976 CB550 (590cc) road racer
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Offline mkramer1121

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #45 on: January 20, 2008, 12:13:57 PM »
 ;D

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #46 on: January 09, 2009, 01:52:09 AM »
Hi all.  I just bought a set of the Hap Jones S12 pistons for my CB350 .    Unfortunatley I only got one set of rings for one piston .  Does anybody know where I can get another set for the other one please.

Cheers,  Ken.

sharp21

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #47 on: December 22, 2009, 08:51:19 AM »
So did JLeather ever get the pistons done? 
For the curious, here is a great spot for replacements, or even high comp pistons:
http://www.bore-tech.com/CB.html
S.

Offline crazypj

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #48 on: December 22, 2009, 09:44:08 AM »
You can also fit Wiseco XR200 pistons and pins to CB350. Dont remember what c/r works out at or other mods necessary
 I think Suzuki GS750 has 15mm pins and 66mm bore as well. (I have some used pistons and think its what they are out of)

PJ

GS750 is 67mm with a 16mm pin.

Jay

Thanks Jay,
 In that case  I don't know what they are out of only they are Suzuki pistons.

PJ
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Offline Seamus

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Re: Big-Bore CB750 pistons in a CB350 Twin?
« Reply #49 on: December 22, 2009, 02:24:38 PM »
I have these in my 750, 64mm diameter and marked up as ART pistons. Always assumed they were 350 pistons.

Am I right??