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Author Topic: 750 pistons in a 650 block?  (Read 1090 times)

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Offline tourmax

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750 pistons in a 650 block?
« on: May 04, 2022, 02:11:48 PM »
I know it's a possible thing with a bit of piston reworking.

Seems the most desirable pistons to start with is the 77-78 F pistons, due to their raised compression and the 650's (CV heads) hemispherical chambers.

But, they're not easy to come by used and if you go new.....well, $$$$.

From reading old threads, it seems the 69-77 pistons give a lower compression ratio if used in the 650 block.

I'm wondering how low? Stock? Lower than stock?

I'm considering punching the block on my 82 650 out to the 61mm 750 pistons and need options to look for. As I mentioned, the 77-78 F pistons just don't seem to turn up very often....
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Offline gschuld

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Re: 750 pistons in a 650 block?
« Reply #1 on: May 04, 2022, 03:07:39 PM »
« Last Edit: May 04, 2022, 03:13:49 PM by gschuld »
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Offline tourmax

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Re: 750 pistons in a 650 block?
« Reply #2 on: May 04, 2022, 04:09:33 PM »
If I recall threads in it, the kz rings are neither inexpensive nor easy to source..
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Offline Old Scrambler

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Re: 750 pistons in a 650 block?
« Reply #3 on: May 04, 2022, 05:10:53 PM »
I have a set of used (18,000 mi) F2 pistons that I removed from a motor with a bad head. They are buried in storage but may have the rings and pins.  Pics and price by email if your interested.       
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Offline gschuld

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Re: 750 pistons in a 650 block?
« Reply #4 on: May 04, 2022, 05:27:11 PM »
If rings are available, would 62mm bore work?  Do the 650 sleeves have enough meat to get to 62mm and retain .080” wall thickness?

George
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Offline scunny

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Re: 750 pistons in a 650 block?
« Reply #5 on: May 04, 2022, 05:32:35 PM »
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Offline Tracksnblades1

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Re: 750 pistons in a 650 block?
« Reply #6 on: May 04, 2022, 06:18:07 PM »
Might look at Honda 750 dohc pistons.
I’m thinking they have a high pin hole 1.035 vs 1.140 compression heights ..?
Check to verify.... 750dohc rods are 1.5mm shorter than 750 sohc. 900dohc are 1.5 longer...
64.5mm CBX pistons have 15mm pins and are lightweight...
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Offline gschuld

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Re: 750 pistons in a 650 block?
« Reply #7 on: May 04, 2022, 06:26:07 PM »
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Offline tourmax

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Re: 750 pistons in a 650 block?
« Reply #8 on: May 04, 2022, 06:43:37 PM »
Well, damn the torpedoes! I’m going in!

Decided to take the risk and grabbed a fistful of used cb750 dohc pistons and we’ll see how it goes I can machine them in my home shop, so lots of development time available at the low low cost of my time.

If it works, should kick the cc’s out to 657.

I’m also following a couple used sets of cb900 dohc pistons. They’re 64.5 mm and should punch it out to 729 cc’s with the use of some cb750 sleeves in the cb650 block….
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1989 FJ1200, 1983 Yamaha Venture (Vmax conversion), 1985 VF 750F Interceptor, 1982 CB650SC, 1988 Corvette convertible (Z52), 1983 Mustang GT, 2009 Mini Cooper Clubman.Couple more lying around but this is long enough already!

Offline Tracksnblades1

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Re: 750 pistons in a 650 block?
« Reply #9 on: May 04, 2022, 07:05:02 PM »
Might consider using custom base gaskets or double to adjust excessive piston protrusion…
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Offline gschuld

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Re: 750 pistons in a 650 block?
« Reply #10 on: May 04, 2022, 07:11:04 PM »
I’ll donate a set of factory sohc cb750 sleeves if you need them.

I have a few sets on the shelf.

George
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Offline Don R

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Re: 750 pistons in a 650 block?
« Reply #11 on: May 04, 2022, 07:44:31 PM »
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Offline grcamna2

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Re: 750 pistons in a 650 block?
« Reply #12 on: May 04, 2022, 08:18:19 PM »
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Offline Tracksnblades1

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Re: 750 pistons in a 650 block?
« Reply #13 on: May 04, 2022, 08:40:18 PM »
Borrowed a picture of a std Honda CBX piston comparing compression heights with a std Honda CB750 piston.
Credit the Satanic Mechanic..

eBay has a set of 6 Honda CBX lightweights @ std bore..

https://www.ebay.com/itm/403632563141?hash=item5dfa6023c5%3Ag%3ACO0AAOSwH9Vibc2v&LH_
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Offline tourmax

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Re: 750 pistons in a 650 block?
« Reply #14 on: May 05, 2022, 03:50:17 AM »
Anyone who wants to follow this, I will be continuing it on my thread in “projects”.
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1989 FJ1200, 1983 Yamaha Venture (Vmax conversion), 1985 VF 750F Interceptor, 1982 CB650SC, 1988 Corvette convertible (Z52), 1983 Mustang GT, 2009 Mini Cooper Clubman.Couple more lying around but this is long enough already!
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