Topic: Are "392" later CB750F pistons?

[Shenanigans]

While digging around at at the local yard a set of pistons caught my (currently looking for machined 750 pistons for my 550) eye.

They look almost exactly like what I am looking for with the machined in crown and shorted skirt. Stock diameter Honda pistons, marked 392

They were sitting on the rods of a 750 bottom end.  The only reason I can think of for them being in a 750 is that they were later F pistons machined down to fit in an earlier K head to up compression. Which wouldn't for my application. So does 392 designate that they are "F" model pistons. If not then can I finally rejoice at finding what I have been looking for?

[JohnG]

hi
yes those are CB750F pistons  (392); specifically the F1 (1976).

Sometime in the F1 production run Honda appears to have changed from a slightely domed CB750 style piston to a different design, which you found. Not only is the dome different but it takes a completely different piston ring set  (also 392 designation) which has a noticeably thinner compression ring.

So if you decide to use them, remember you will have to come up with the correct rings -  the older  300-024  will not work.  They are out there, however.

        john

[bwaller]

You will still need to machine the dome & skirt, (moreso the dome on these) as in Gordon Jennings article.

Take a while to measure those pistons, maybe you might opt for a skirt coating afterwards.

[ekpent]

Holy Crap. That is quite a graveyard there,lotsa dead Honda soldiers. Any more pics of that solemn place of rest.Nice you have place to pick around in.

[Jerry Rxman Griffin aka MuthaF'er]

Yes, as JohnG points out, 392 are indeed 75/76 F pistons AND they are 77/78 K pistons as well. My bike is an early 1975 built in 1/75, perhaps the first month they were assembled. Engine serial number CB750E-2501954. You will find that 392 pistons have different domes and different compression ring thicknesses as well.



Both of these pistons have identical dome heights and both of these are labeled 392 near the wrist pins?? The piston on the left is from my 78 750K engine. Honda says the 77/78 K are 9.2 to 1 compression. The piston on the right is from my 75 750F. Honda says it is 9.2 to 1 compression also!! You tell me which piston gives higher CR!! Another of Honda's little white lies. Upped the CR and cammed them in the 75/76F to sell more bikes then lowered the CR and changed the cam profile in the 77/78 K for longevity all the while stating they were both 9.2 to1. The 77/78 K cam comes on 5 degress later but has the same duration.

As far as Honda parts info goes I did a little digging. I have a microfiche from 4/93 and the 1st Edition Honda Parts Catalogue for the 75 CB750F and 76 CB750F1. I also accessed Service Honda's online microfiche.
The early 75/76 F 392 pistons are 13101-392-000 up to engine s/n 2551567 with rings 13011-300-024 by Riken and later pistons 13101-392-020 s/n 2551568 & up with rings 13011-392-004 by Riken. Lower compression with thinner rings.
The 77/78 K 392 pistons are 13101-392-020. Rings were made by Riken 13011-405-602, Nippon 13011-405-601, and Teikoku 13011-392-005. They used these rings in the 77/78 F as witnessed by the -405.
 
The differences in rings are the thickness. The early rings are: top 1.4mm, middle 1.4mm, bottom 2.8mm. The latter rings are: top 1.1mm, middle 1.1mm, bottom 2.8mm.

What all this means is check the engine serial number and measure the rings/grooves before you order new rings. It got my curiousity up so I did the digging. Hope this might help.

I couldn't directly link this next picture. It shows both pistons have 392 on them.

http://i233.photobucket.com/albums/ee84/RxmanGriff/Honda/P1020789.jpg

 

[Shenanigans]

Wow some good info there. I did not recognize that piston profile as being stock. I thought those had been modified. Learn something new every day... I have a set of nice "300" rings so using these pistons would be pointless on multiple levels but I sure am glad I found out here instead of in my motor!

Holy Crap. That is quite a graveyard there,lotsa dead Honda soldiers. Any more pics of that solemn place of rest.Nice you have place to pick around in.

Its Bobs used motorcycles in Phoenix. What you see in this picture is about 1/5 the stuff out there. Its has to be one of the most exciting and depressing places...

[torchmonkey]

Hey Shenanigans, you still have those "300" rings? Would you be willing to part with them?

[Shenanigans]

Hey Shenanigans, you still have those "300" rings? Would you be willing to part with them?

Yeah, I might be able to part with them. I will check the parts pile tomorrow to see if I still have them.

[david 750f]

Jerry,
thx for the info on the 392 pistons. Looks like I missed out on the higher compression pistons by 154 units according to my engine serial numbers!!

[mick7504]

Just adding to Jerry's knowledge base.
These are marked 410 and were pulled from an F2 motor.
Top Groove measures 1.31mm
2nd     "            "      1.22mm
3rd      "            "      2.81mm

[mazingerzeca]

Yes, as JohnG points out, 392 are indeed 75/76 F pistons AND they are 77/78 K pistons as well. My bike is an early 1975 built in 1/75, perhaps the first month they were assembled. Engine serial number CB750E-2501954. You will find that 392 pistons have different domes and different compression ring thicknesses as well.



Both of these pistons have identical dome heights and both of these are labeled 392 near the wrist pins?? The piston on the left is from my 78 750K engine. Honda says the 77/78 K are 9.2 to 1 compression. The piston on the right is from my 75 750F. Honda says it is 9.2 to 1 compression also!! You tell me which piston gives higher CR!! Another of Honda's little white lies. Upped the CR and cammed them in the 75/76F to sell more bikes then lowered the CR and changed the cam profile in the 77/78 K for longevity all the while stating they were both 9.2 to1. The 77/78 K cam comes on 5 degress later but has the same duration.

As far as Honda parts info goes I did a little digging. I have a microfiche from 4/93 and the 1st Edition Honda Parts Catalogue for the 75 CB750F and 76 CB750F1. I also accessed Service Honda's online microfiche.
The early 75/76 F 392 pistons are 13101-392-000 up to engine s/n 2551567 with rings 13011-300-024 by Riken and later pistons 13101-392-020 s/n 2551568 & up with rings 13011-392-004 by Riken. Lower compression with thinner rings.
The 77/78 K 392 pistons are 13101-392-020. Rings were made by Riken 13011-405-602, Nippon 13011-405-601, and Teikoku 13011-392-005. They used these rings in the 77/78 F as witnessed by the -405.
 
The differences in rings are the thickness. The early rings are: top 1.4mm, middle 1.4mm, bottom 2.8mm. The latter rings are: top 1.1mm, middle 1.1mm, bottom 2.8mm.

What all this means is check the engine serial number and measure the rings/grooves before you order new rings. It got my curiousity up so I did the digging. Hope this might help.

I couldn't directly link this next picture. It shows both pistons have 392 on them.

http://i233.photobucket.com/albums/ee84/RxmanGriff/Honda/P1020789.jpg

 

Now I have a doubt with my rings, which I have to order.
My bike is a K7, and the only number listed on the partslist for the rings is 13011-392-004 (STD RIKEN), up to Serial# 2734071, in which mine is included. I measured the top and second rings and they were about 1,1 mm. The difference between my pistons and the ones you posted is that in mine is stamped 392, but the other number is a 8, not a 6, as in yours.
After reading this post, I'm not so sure if I will order the ones that I thought were right (13011-392-004) or the ones you wrote: Riken 13011-405-602, Nippon 13011-405-601, and Teikoku 13011-392-005 

[rohalloran]

Hello,

After doing some searching I have a question which is similar to this post. I recently picked up a 1977 CB750K7 which had a seized engine. Turns out 2 of the cylinders got water in therm and started to rust. The pistons are the same as those in the picture on the left with the higher dome and are .75 oversized. I am not having much luck finding 1.0 oversized pistons (was goign to bore the barrels) but I did find a used barrel and piston set (with standard pistons) which are listed as for a K7 but have pistons which look like those on the right? My question is if I was to purchase these, woudl they fit/work correctly?

Many thanks in advance for anyone's assistance on this as I am confused!

[rohalloran]

The pistons are the same as those in the picture on the left with the higher dome and are .75 oversized. I am not having much luck finding 1.0 oversized pistons (was goign to bore the barrels) but I did find a used barrel and piston set (with standard pistons) which are listed as for a K7 but have pistons which look like those on the right?

I was referring to this image in my post.

[Jerry Rxman Griffin aka MuthaF'er]

Either will work and the ones with the full dome will give you a slight bump in compression.

[HondaMan]

I recently bought a set of new cast 0.5mm oversize -392- style pistons (later 392, with rounder domes) from Britain. They are very nice pistons and rings, and nicely packed for the trip across the pond. They are destined for a K7 I'm rebuilding from its slumber.

[rohalloran]

Either will work and the ones with the full dome will give you a slight bump in compression.

Thanks for the reply. So I could go ahead and get this s/h jug, barrel & pistons and they should bold straight on with my crankcase and heads with no issues? No doubt a light hone and new rings would be required.