change in pistons

[mattmf01]

Is it ok to change the pistons in a 78 cb750k to pistons for the 69-76 years? Rebuilding the motor and can find a better deal on them.

[Re-run]

I can't say for sure but they should work. Compression might drop slightly as the k7/8 used the F pistons.

[mattmf01]

does that affect the horsepower or??? what would that change?

[TOOLS1]

There is no difference in the K model pistons from 69-78. However the 77-78 F uses different pistons. Also the A's used the same piston as the K's
TOOLS

[Re-run]

From my digging, the f got a different piston which was carried to the 77/78k. Now, the shape my have been the same, maybe just different rings. I do seem to recall the 77/78k having thinner rings I think.

[TOOLS1]

The 77-78 K did not use the F piston. If it had, they would have had to use the F head also.
TOOLS

[Piute]

      From what I've learned("77"F2)
  My bike has domed pistons,there-for heads are domed in to fit ,Being the reason back in the day some would put in bore kits (sure would have bore)but have lack of compression due to extra space in these heads,valves are different.There is a write up in my build(piute putt) for exact sizes of stk..
  Less compression has less torx,torx is bottom end get up N go. Now I'm not all knowledge on were exactly were stk torx and H.P. is on every bike model Yr.(dyno sheet in post all will be diff.).Just mine witch is in my build easy to find just put ?(words) in search,has been post for easy find,bit long like this,

[Re-run]

Well in 73, the piston was changed to have 3 piece oil rings. a larger piston pin circlip and a different head.

In 77, K7 - carbs w/accelerator pump, shorter intake rubbers, new final drive shaft, sprocket w/center bolt, wider chain line (as F2), final drive 15/41, new clutch, new camshaft (as F1), new pistons (as F), compression ratio 9.2/1. New transmission: 4th gear 1.133 i/o 1.087. Final drive gear 50 teeth i/o 56 (as F1), primary drive 1.985 i/o 1.708
The F had this, F1 - new camshaft, carbs w/additional idle air. New pistons, compression ratio 9.2/1.

The 77/78 engine WAS the f/f1 engine.

I still have not found exactly how the pistons were all different but I haven't looked overly hard yet, but they did change in some way.

This info comes from a book called CB 750 - Die Geschichte einer Legende, german of course but this was translated.

cmsnl shows different part numbers too. Now part numbers do change over the years but I am sure that the rings are different in certain years.  

And the research continues...

[TOOLS1]

This link should give, you all the crossover information, you would need.
http://www.vintagecb750.com/products/4/engine/75/pistons-ring-sets
TOOLS

[Re-run]

So, I have found the 77/78K did use the f/f1 pistons. thes pistons have a higher dome than the earlier K pistons. So while 69-76 pistons should work fine, compression would be lowered and to regain this, the head should be decked.

[TOOLS1]

So your saying the 77-78 K has the same head as the F also. I have not had any 77-78 engines apart, but the 836 kits say they will work in 69-78 engines.
TOOLS

[TOOLS1]

I found this on Cycle-X. Seems to explain the 77-78 K pistons.
cyclexhondacb750firstandsecondoverpistons Cycle X   1st (0.25)  &  2nd (0.50) Piston kits
                  ( 0.25  in stock, 0.50 in stock )

Introducing our Cycle X  ( 61.25 & 61.50 )  piston sets.
Super high quality ( exact OEM spec's )
Our pistons are better.
Why ?
3 piece oil rings.
Chrome top ring ( a must for today's fuels )
All Cycle X pistons are now manufactured with a anti-friction   " Tin coating ".
This coating has been used for 30 plus years on pistons with great success.
And............ they cost less money.

Note:
These pistons are manufactured with the 69-76 dome height.
77-78 K models had slightly smoother combustion chamber so 77-78 K model pistons had a slight dome on the piston.
Because the dome height on 77-78 K pistons is very small, our pistons can be used without power loss issues.

Price:  199.00          # ENP-005
 (4) Pistons, rings, wrist pins, circlips.
SIZE OPTIONS

TOOLS

[Re-run]

The 77/78 engine is pretty much the F engine while the f2/3 has a different head.
The wiseco kits are made to fit all the years, the early pistons will fit all the years too. The issue is the drop in compression.
So you could take a 71 piston and get a 9.0(number out of thin air) in its engine. Move it into the f/f1 or k7/8 and the compression will drop some. Put it in the f2/3 and it drops even more. But it will work.
You deck the head just a bit though, and you can get it back.

[Piute]

   
   SEARCH "f2 piston"  
http://honda-cb750-s.456789.n3.nabble.com/K-series-vs-late-F-series-td3580295.html

Piute Putt...............................Dec ....

The F2/F3 not only has intake valves that are 2mm bigger in diameter than the rest of the CB 750 models, they also have a bigger (approx. 5cc) larger combustion chamber. That means that, with a given piston kit, you get less compression than advertised in a F2/F3 engine. Some manufacturers (Wiseco) listed different kits for K and F models. Dynoman has 836cc kits especially for the F2 head. The F1 head has (almost) the same C/R like the K models.
                                        Just a Lil " Mo in FO "........Piute N Judose........

 FROM DYNOMAN AD:
                  As the 77-78 Honda 750 F2 has an 18% larger combustion chamber
                  to compensate for the larger valves, through the years many
                  people have installed the standard 836cc (ex: Wiseco K836)
                  kits for the 69-76 engines only to discover that the
                  compression was too low to give them the extra power that they
                  were after.
                  Dynoman offers our custom 836cc piston kit for the F2. It has
                  the correct dome to raise the compression to a true 10.5:1.
                  These pistons are state of the art diamond cut on CNC machines
                  in the USA from 2618 aluminum forgings. This is the same alloy
                  used in many prostock pistons. This process makes them lighter
                  than some of the cast pistons used in the 70s, but much
                  stronger. They also come with a brushed dome to smooth any
                  sharp edges.
                  The piston features large valve pockets for the 31 and 34mm
                  valves used in the F2 engines. They also have
                  contact reduction grooves above the top ring for pre-ignition
                  control, and accumulator grooves below the top ring for better
                  ring control.
                  They are the same bore size as the standard Wiseco K836cc kit,
                  so they are a drop in for any F2 with the standard K836 Wiseco
                  kit already installed.
                  Made in the U.S.A....reposted by Piute



         Kits for F's  http://www.dynoman.net/engine/pistons/custom.html#F2
  others just have to look harder

[Re-run]

Yup, you must get higher domed pistons for the f2/3. The f/f1,k7/8 just need a slight decking if using the earlier pistons, Their pistons had just a bit of a dome to them.

[mikeyboy1]

new to this site. so if I got this rite-I can use pistons for a 78K in a 76F? sure hope so as I've ordered them

[TOOLS1]

Yes. However you will want to run high octane gas.
TOOLS