Topic: 1976 CB750F - Pistons?

[MacM2010]

Hi everyone:

I searched around and I don't think this has been answered before.  Anyway, I have pictures to look at, so it can't be too much of a waste of time. Here is one now:  

So a friend and I have been rebuilding a 76 750F for a couple months, and we're getting close to seeing if this thing will run.  When we opened up the engine, however, we found that one of the pistons had gotten stuck in the cylinder--bad news.

So, we got a new sleeve, got it bored out, got the others honed, we're getting new rings, and we bought a new piston... but there seems to be something wrong.  I've read that the 76 F bikes used different pistons than K bikes, and I also noticed on the fiches that there are two part numbers for pistons for the 76F.  So now I have three pistons that all look alike, and one that... I don't know, maybe it's because it's clean, but it looks off.  It has a raised area on top, and just doesn't look like the rest.  Did I buy the wrong piston?   

Below are pics... can anyone tell me what I've done?
 

[MacM2010]

More pics! (Too much for one post.)

[bryanj]

392 is the correct number for a 75/76 F now there are several possibilities

(1) wrong part in box

(2) wrong engine in bike-- what are frame and engine numbers?

(3) previously rebuit using wrong year parts

(4) McGinty is sticking his nose in

[cafe750]

More pics! (Too much for one post.)

I believe that Honda slightly bumped up the compression on the 75/76 F motors, and that small dome is probably how they did it. I just pulled a piston like the one on the left out of a '71 750k motor that grenaded on me...I can mail you one of them if you would like...

[Porscheguy912]

More pics! (Too much for one post.)

Im just worried that the valve relief on the new on isnt correct.
I sure dont want to get this thing back together only to find that the valve hits the piston in the face.

here are the engine and frame numbers:
engine: CB750E-2540180
frame: CB750F-2020116

both pistons say honda on one side and "392" on the other.
the old ones have a "E" above the 392 and the new one has a "CL" or a "GL" i cant tell. old one has a "4" to the right of the pin on one side and the new one has a "6" to the right of the pin on one side.

other than the markings and the face being different, they are exactly the same. Same ring size and spacing, same holes everything.

[MacM2010]

Mr. Moustache says it for me (he's got the bike in his garage). 

I'd be delighted to have your piston, cafe750.  PM me (or Porscheguy, that might be easier).  Thanks a million!

[cafe750]

More pics! (Too much for one post.)

Im just worried that the valve relief on the new on isnt correct.
I sure dont want to get this thing back together only to find that the valve hits the piston in the face.

here are the engine and frame numbers:
engine: CB750E-2540180
frame: CB750F-2020116

both pistons say honda on one side and "392" on the other.
the old ones have a "E" above the 392 and the new one has a "CL" or a "GL" i cant tell. old one has a "4" to the right of the pin on one side and the new one has a "6" to the right of the pin on one side.

other than the markings and the face being different, they are exactly the same. Same ring size and spacing, same holes everything.

Also, don't forget that with the taller dome on the piston, the Compression ratio on that cylinder would be higher, meaning you would never get that motor to run right with that new piston...pm me with your mailing address, and I'll drop a piston in the mail...

[NWrider]

I think the group concluded it's the wrong piston. The left piston (used one) looks exactly like the ones in my 1977 750F Supersport.

NWR

[Jerry Rxman Griffin aka MuthaF'er]

From my "Honda Parts Catalog 1st Edition" dated 1975:

13101-392-000 Piston *STD*  (*STD* meaning not over sized)

From my "Honda Parts Catalogue CB750F & CB750F'76" (printed from the microfiche) dated April 1993:

Piston (STD) CB750F  13101-392-000 to CB750F '76 engine serial number 2551567
Piston (STD) CB750F '76 13101-392-020 from engine serial number 2551568 

From my '75F engine serial number 2501954 (original owner so I know it's legit):

piston on the left

Your engine appears to be before the switchover ?? All these F engines are supposed to be 9.2 to 1 CR. I would suspect/expect that the difference (other than obvious shape) was different manufacturers between -000 & -020. I would not use the new one simply due to the different shape. Don't think there would be a valve issue. Compression Ratio perhaps?? Keep in mind that whatever you use be sure the weights match. The matching letters on the skirts may be the key. My 75F -392's are "1" while my 78K -392 engine that has your new piston(s) has "6". Now, another possibility: Honda has been known to start off a new model with hotter cams, etc for sales then detune for longevity. Perhaps they decompressed the later F's. I know mine ran like a scalded ass monkey before I blew it up. I had a couple buddies with 500 Kaw triples that tried to pick on mine like they did to K's but got spanked major league.   

[Clyde]

The piston on the right with the raised flat section is a correct piston for a 76 F. I have those in my 76F1.
The raised section increased the compression ratio to 9.2 (from 9.0).

If the other piston has 392 then presumably it must have been made for the F/F1, but I have not seen such a piston in the F1 engines I have owned or worked on. It looks like Honda has used a slight dome rather than the raised section, so maybe the compression ratio is the same and they can be used together.

A K-K6 would have a flat piston, and in the photo the one on the left appears to have a very slight dome, but less than the 77/78F2
Honda changed rings during the 75F model and introduced thinner rings (and hence pistons) which were then used for the K7/K8. But both these pistons use the same rings you note.
The 77/78F models used the thinner rings but had domed pistons due to the changed geometry of the larger valves.

[bryanj]

Those numbers tie up with a 76 750F1, if it was me I would get another 3 and fit a full set