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Discussion Starter #1
Are these pistons stock 1977 CB 750 (See below)? The pistons are Honda, the tops of the pistons have slight domes, and the top and second ring groove are about 1.25mm wide.

The pistons on my basked case 1971 cb750 engine have flat tops, and the ring thickness matches the new rings I ordered (1.5mm)

I suspect that the pistons in my 1977 CB may be for a 1979 or later. The ring grooves seem to match that of the 1979 and later cb750 pistons.

Can anyone confirm my suspicion?

If these pistons are not stock 1977 should I keep them and try to buy a set of rings to fit or should I swap them for the 1971 pistons?

I'm not interested in installing a 835cc kit, I need to keep cost low.

Thank you
 

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HI Turtle racer
Your honda pistons should have a 3 digit no cast under the pin, you can cross reference this with a part no for yor model at site WWW.csmnl.com its a great reference for lots of classic honda stuff with all part no listed and breakdown diagrams Perhaps someone can id them buy site? good luck.steve.
 

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HI Turtle racer
Your honda pistons should have a 3 digit no cast under the pin, you can cross reference this with a part no for yor model at site WWW.csmnl.com its a great reference for lots of classic honda stuff with all part no listed and breakdown diagrams Perhaps someone can id them buy site? good luck.steve.
 

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Discussion Starter #4
Thanks for the info on the part No location. I'll try checking them today. I know WWW.csmnl.com very well.

I also found some pistons pictures on ebay that seem to confirm my suspicion.

I'm wondering if there is some power advantage to running the later model DOHC pistions in the SOHC engines?

Thank You
 

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Discussion Starter #5
Thanks for the info on the part No location. I'll try checking them today. I know WWW.csmnl.com very well.

I also found some pistons pictures on ebay that seem to confirm my suspicion.

I'm wondering if there is some power advantage to running the later model DOHC pistions in the SOHC engines?

Thank You
 

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There is no real advantage to the pistons other than size (measure the diameter of the piston I think stock cb750 SOHC is 64mm). I would measure the rod length too, because there may not be any advantage to a DOHC piston, but there is a questionable advantage to the rods (which would be 1.5mm shorter than stock) being stronger.

I can tell you why I don't think they are DOHC pistons, they don;t have valve releifs cut in the dome for 4 valves (SOHC engines are 2 valve per cylinder, DOHC are 4 per). Later cb750s did go through some piston and cam changes.

You haven't said whether your 77 cb750 motor is an F or a K. this makes a huge difference since the K and the F motors have differen't pistons all together. You can't freely mix 77-78 F parts with the K motors.
 

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There is no real advantage to the pistons other than size (measure the diameter of the piston I think stock cb750 SOHC is 64mm). I would measure the rod length too, because there may not be any advantage to a DOHC piston, but there is a questionable advantage to the rods (which would be 1.5mm shorter than stock) being stronger.

I can tell you why I don't think they are DOHC pistons, they don;t have valve releifs cut in the dome for 4 valves (SOHC engines are 2 valve per cylinder, DOHC are 4 per). Later cb750s did go through some piston and cam changes.

You haven't said whether your 77 cb750 motor is an F or a K. this makes a huge difference since the K and the F motors have differen't pistons all together. You can't freely mix 77-78 F parts with the K motors.
 

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Discussion Starter #8
The short answer is: these are the correct pistons for a 1977 CB750K.

The model number (392) on the piston is for a CB750 F1 engine.

The 1975-76 CB750F (F1) engine is the same engine used on the 1977-78 CB750K, except with the addition of the later model carburettors. The 1977-78 CB750F uses the F2 engine.

The 1976 and earlier K engine and the F1 engine use different pistons and the rings are not interchangeable.

I do not know if the F1 and F2 engine used the same pistions or not.

Hey Santa, if your out there, I need a set of pistons rings for an CB750 F1 engine for my 1977 CB750K.

Steve & Geeto67 Thanks for the help.

P.S. The stock CB750 SOHC bore is 61mm.
 

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Discussion Starter #9
The short answer is: these are the correct pistons for a 1977 CB750K.

The model number (392) on the piston is for a CB750 F1 engine.

The 1975-76 CB750F (F1) engine is the same engine used on the 1977-78 CB750K, except with the addition of the later model carburettors. The 1977-78 CB750F uses the F2 engine.

The 1976 and earlier K engine and the F1 engine use different pistons and the rings are not interchangeable.

I do not know if the F1 and F2 engine used the same pistions or not.

Hey Santa, if your out there, I need a set of pistons rings for an CB750 F1 engine for my 1977 CB750K.

Steve & Geeto67 Thanks for the help.

P.S. The stock CB750 SOHC bore is 61mm.
 

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quote:

I do not know if the F1 and F2 engine used the same pistions or not.

Hey Santa, if your out there, I need a set of pistons rings for an CB750 F1 engine for my 1977 CB750K.

P.S. The stock CB750 SOHC bore is 61mm.

The 1975 (F0) and 1976 (F1) supersports are essentially K engines with hotted up cams. The pistons are slightly different in that thei increase compression ratio from the earlier K engines. 1977 (F2) and 1978 (F3) Supersports do not share pistons with any other cb750 K, F, or otherwise. They are their own design and meant to work only with the 1977/78 F2/3 heads.

rings should be available through honda for you stock pistons.

With cb750s, the big issue is with the heads more than the pistons, Some heads will gaurantee you hit a valve while others will yeild a low low (like 7.5 or 8:1) comp ratio. You should be able to put almost any piston/head combo on any cb750.
 

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quote:

I do not know if the F1 and F2 engine used the same pistions or not.

Hey Santa, if your out there, I need a set of pistons rings for an CB750 F1 engine for my 1977 CB750K.

P.S. The stock CB750 SOHC bore is 61mm.

The 1975 (F0) and 1976 (F1) supersports are essentially K engines with hotted up cams. The pistons are slightly different in that thei increase compression ratio from the earlier K engines. 1977 (F2) and 1978 (F3) Supersports do not share pistons with any other cb750 K, F, or otherwise. They are their own design and meant to work only with the 1977/78 F2/3 heads.

rings should be available through honda for you stock pistons.

With cb750s, the big issue is with the heads more than the pistons, Some heads will gaurantee you hit a valve while others will yeild a low low (like 7.5 or 8:1) comp ratio. You should be able to put almost any piston/head combo on any cb750.
 
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