Topic: CB750 - Pistons

[Clutch Cargo]

Did the design of the piston (and the part number) change during the life of the 750cc SOHC?

A follow up question woud be - did the part number for the piston ring set change at any point during the life of the 750cc SOHC.  I show that the standard ring set is 13011-300-024. Is this recomened for all years?

Mike

[volz1fsu]

The F pistons were taller dome height than the K models but the Rings for all 750s up to '78 were the same. Bore increased 1mm in '79 with the DOHC models. All '69-'78 models had the same bore, 61mm.

[CB750F2]

    I can partly answer your question. Yes the pistons did change over the years. I think that there were three changes - 300,392,410. I have examples for models 392 and 410 and they are different. The 392s are slightly domed and the 410s are more domed. I have a 78 F2 which is fitted with 410 pistions - this is the standard for this model. I see my parts book for the F2 lists type 392 rings for F2 engine serial numbers up to 2612797 and 410 rings for F2 engine numbers 2612798 and above. I have not seen examples of the 300 type piston but I believe they are flat.
   So beware! Pat from Nth Qld Australia

[Clyde]

The first piston (300) has a compression ratio of 9:1 and an almost flat top. The 392 piston (392 is the model no for the 1975 F and 1976 F1) has a slightly higher compression ratio of 9.2:1 which was achieved by change of piston (not so much domed as a raised platform) and the 410 piston (1977 and 1978 F2) is 9:1 but because of the larger valves is highly domed as Pat says.
There are actually two 392 pistons-one for thin rings and one for thick rings.
The F and early F1 pistons used the same rings as the K0-K6 which are a slightly thicker ring. Honda then changed to a new ring (early F1), which was thinner and used that for the later models- K7, K8 and F2.
The part no you quote is good for the K0-K6 and F.
The ring set for the later type is 13011-392-004
I think there maybe a fifth piston as I believe the Automatic had a different piston with a lower compression ratio. From my parts book it appears that the Automatics use the later (thin) rings.
I am not sure what the change is that Pat is referring to at 2612798, which is about the end of the F2 and the start of the F3. My book also lists a change  which refers to the 410 number, but I think it was to do with oil control ring as there was no change to the piston. Maybe some one (Terry with the immacuate F2) can help.
Regds Clyde

[bwaller]

Not much further to add Mike except that # 13011-300-013 were earlier and used a one piece oil ring. Further to the discussion and maybe you can tell me Clyde I have a cherry set of 61mm pistons marked Haindst 600cc (although probably should be 592) I discovered in a 550 engine I ended up with. Unfortunately the oil ring land is thinner than the early 750K's piston but the top two 750 rings fit. Does anyone have a piston that uses the 13011-392-004 rings that could give me a measurement of the land width?

Thanks   

[Clyde]

Forgot about the early one piece oil ring set. They were used with the 300 pistons.
I can't help with identifying those pistons.
In fact I have a set of Kawasaki Z1 pistons which are marked HAINST and Z1 995. I have been trying to identify the manufacturer as I have them on eBay as we speak.
I have a used example of the pistons above so can take some measurements. I guess feeler gauges would be good enough?

[bwaller]

That would be fine to get me in the ballpark.

[volz1fsu]

I have a cherry set of 61mm pistons marked Haindst 600cc (although probably should be 592) I discovered in a 550 engine I ended up with.

Probably wouldn't want to sell those pistons, would you? Have to ask.

[bwaller]

Sorry man they should fit nicely in my own engine!

[Terry in Australia]

Sorry man they should fit nicely in my own engine!

Be vewy, vewy, careful mate, a friend of mine and I spotted a set of new Dunstall high comp pistons for CB750 (or so we thought) back in the early 1980's going cheap, so my mate bought them, and they rattled like buggery.

Turned out they were big bore pistons for a 500 or 550 (can't remember which, but they'd probably work on either) He took 'em out and they are still sitting on the shelf in his workshop. We miked 'em up, and they were something like 60.90mm, which apparently was common for the "600" kit, and while the engine didn't smoke, the piston slap was quite terrifying.

Just make sure they are in fact, 61mm, or you'll cry yourself to sleep, ha ha! Cheers, Terry.

[volz1fsu]

At least with my "600" big bore, I aquired stock 750 pistons and bored the cylinders myself.  No worries there.  Had to hone by hand though which takes some skill not to taper the bore or take out too much material.  Luckily the hard stuff is all done.

[bwaller]

Thanks guys,  I'll be vewy vewy careful.

[Clutch Cargo]

Thanks to all for the responses.

I have the 300 - flat top pistons and have no desire to bump up the comp ratio - so I'll stick with them. ( I just want to get approx 140 psi from the buggers)

From the above posts, it seems like there are two ring designs for the # 300 piston and the 13011-300-024 is the early design with a one piece oil ring.

Can anyone advise the part number for the later ring set (standard) with the two piece oil ring design and should I avoid the early design in favor of the later design?

Mike

[bwaller]

Mike,
The latest offering is #13011-300-024. This standard set contains the three piece oil ring and that's what you should use. The older set was -013.

[Clyde]

It looks like the oil ring land is the same for all three pistons (300, 392 and 410)
Using a digital vernier I make it 2.85mm.

The thinner compression rings measure 1.18mm and the thicker rings measure about 1.45mm.
Regds Clyde

[bwaller]

Thanks Clyde, I've got a machinist on the case looking at increasing the land width to use a 750 ring.

[pav]

HAINST might stand for Henry Abe. They were a Japanese aftermarket supplier of big bore kits back in the 70's.