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Author Topic: Honda CB750F Piston Types  (Read 1997 times)

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Offline RodSOHC750

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Honda CB750F Piston Types
« on: February 09, 2016, 10:01:41 AM »
I'm having difficulty finding 0.5mm oversize pistons for my 1975 CB750F.  I don't have the pistons with me as they were left with a machine shop out of town. I recall the pistons having a flat top and the machinist told me the pistons had a "392" on them. My only source of information is the attached table of Honda 61.0mm pistons.  Some of the information is confusing to me and indicates more than one piston for the F model. I know the number after the piston height (+5) is the dome height (5mm), so a flat top piston would be indicated by "+0".

What is the Honda P/N of the 0.5mm oversize piston I need?  Will a 750K piston work?  I want to maintain the stock compression ratio (9.0:1).
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1979 Honda CB750L Limited Edition
1975 Honda CB750F Super Sport
1972 Honda CL450
1970 Honda CT70

Offline BPellerine

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Re: Honda CB750F Piston Types
« Reply #1 on: February 09, 2016, 12:13:30 PM »
75 f are the same as 77,78k I think,only the 77,78f pistons were diff,should be a 392 pt no,later ones had a 410 pt no.bill
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1978 CB 750K ard and webers
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Offline Kawahonda

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Re: Honda CB750F Piston Types
« Reply #2 on: February 11, 2016, 08:44:56 AM »
That chart says that F0/F1 pistons are not necessarily the same as K pistons.

What a predicament! Is that chart wrong?
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Offline przjohn

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Re: Honda CB750F Piston Types
« Reply #3 on: February 11, 2016, 10:01:10 AM »
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Offline SOHC4 Cafe Racer Fan

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Re: Honda CB750F Piston Types
« Reply #4 on: February 11, 2016, 10:02:10 AM »
CMSNL never is cheap.
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Offline Kawahonda

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Re: Honda CB750F Piston Types
« Reply #5 on: February 11, 2016, 11:36:23 AM »
In Rod's case, I think he decided to go with a slightly over bore rather than an 836 kit.

The cruisin image seller has this for cheap.

Looking at the chart, it APPEARS (like others have been saying) that the F0/F1 pistons have the same heartline and height as the early K pistons. The only difference is slightly different diameters over the year, but since you're over-boring anyways, it doesn't matter.

.5mm oversized:
http://www.ebay.com/itm/69-76-HONDA-CB750K-0-5mm-OVERSIZE-PISTONS-SET-4-PISTONS-INCLUDE-CI-CB750KPS-1-/131588210647?vxp=mtr&hash=item1ea344bfd7

1.0mm oversized:
http://www.ebay.com/itm/69-76-HONDA-CB750K-1-0mm-OVERSIZE-PISTONS-SET-4-PISTONS-INCLUDE-CI-CB750KPS-2-/131588211611?vxp=mtr&hash=item1ea344c39b

I think you've got your answer on this one.
« Last Edit: February 11, 2016, 11:45:41 AM by Kawahonda »
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Honda CB750F Piston Types
« Reply #6 on: February 11, 2016, 11:52:06 AM »
Both of these are -392 pistons. The 'full top' is from my 75 CB750F. It uses standard -300 rings. 1.5, 1.5, 2.5mm. The 'partial dome' is from a 78 K engine. It uses 1.2, 1.2??, 2.5mm rings

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As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline RodSOHC750

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Re: Honda CB750F Piston Types
« Reply #7 on: February 11, 2016, 01:01:05 PM »
My piston top appears like the piston on the right. . . . 75 CB750F.  Oil ring is 3 parts. 
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1979 Honda CB750L Limited Edition
1975 Honda CB750F Super Sport
1972 Honda CL450
1970 Honda CT70

Offline Kawahonda

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Re: Honda CB750F Piston Types
« Reply #8 on: February 11, 2016, 01:32:46 PM »
The cruisin image kit appears to have that safe flat top profile with valve cut outs.

So I assume with the cruisin image kit you'd be at 9.2:1 or abouts, same as stock. You certainly don't want to lose compression. I'm paraphrasing big time here, but I remember Hondaman stating somewhere that 9.4:1 (or was it 9.5:1?) was a good healthy limit for street SOHCs, maybe he can confirm.
« Last Edit: February 11, 2016, 01:34:21 PM by Kawahonda »
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