Topic: Difference between 750K5 and 750F engines

[gregs750effer]

Hello,

I recently bought a '76 750F and am now having to rebuild the motor due to a very load chain-scraping noise when running.  Anyway, after looking at this very cool web site I discovered that my motor is from a '75 K5 and not the stock '76 750F.  Here is the number on the motor:  CB750E-2533785.... '75 K5 right?  So now I'm wondering, what is the difference between these two motors?

Thanks for the help!
Greg

[HondaMan]

One quick way to check: the "F" models had different exhaust port fittings. The "K"before the "F" had two studs that held the exhaust "flower" clamps on for each pipe, the "F" did not. Inside, the "F" has higher compression, so the tops of the pistons are domed, not flat.

I think the later K7/8 models had a combination of "F" internals mixed with "K" suspension and styling. Pipes were again different, though.

[gregs750effer]

The exhaust ports have two studs and the pistons are flat tops so I guess that means it's not an F model engine unfortuneately.

Thanks for the reply,
Greg

[KB02]

I have the same thing with my bike: 76 F bike with 75 K engine.
   When it is running, do you have questions about the accuracy of the tachometer as well?

[comcrx]

when rebuilding, could you just put the dome top pistons in the K engine?

[MRieck]

when rebuilding, could you just put the dome top pistons in the K engine?

Yes. The F dome is 1mm higher. The F has several different gear ratios from the K and slightly different cam. Exhaust stud spacing appears the same to me.  I have used a very early K head (1970) on my 75F with no problems mounting a Kerker exhaust. K7/8 heads have different chambers than early earlier K's and F's and use modern groove keepers and their corresponding valves/retainers. Valve diameter is the same.

[MRieck]

One quick way to check: the "F" models had different exhaust port fittings. The "K"before the "F" had two studs that held the exhaust "flower" clamps on for each pipe, the "F" did not. Inside, the "F" has higher compression, so the tops of the pistons are domed, not flat.

I think the later K7/8 models had a combination of "F" internals mixed with "K" suspension and styling. Pipes were again different, though.

The F has 2 mounting studs extending from the port also.

[Clyde]

In Australia the F1 motors ran from 2519919 to 2563999. The K5 ran from  2372115 to 2463966

I have changed heads from one to the other by just changing studs for stubs. The heads are basically the same except for the F2.The F1 has two studs.
The pistons are as Mike states with a 1mm elevated FLAT section (like a mesa), but they are NOT domed (The F2 are domed). I can forward photos of the different types of pistons if you need.

According to my parts books the K7/K8 is virtually the same motor as the F1 except for the final drive shaft. The change in valve keepers only took place on the F2.

Regds Clyde

[MRieck]

In Australia the F1 motors ran from 2519919 to 2563999. The K5 ran from  2372115 to 2463966

I have changed heads from one to the other by just changing studs for stubs. The heads are basically the same except for the F2.The F1 has two studs.
The pistons are as Mike states with a 1mm elevated FLAT section (like a mesa), but they are NOT domed (The F2 are domed). I can forward photos of the different types of pistons if you need.

According to my parts books the K7/K8 is virtually the same motor as the F1 except for the final drive shaft. The change in valve keepers only took place on the F2.

Regds Clyde

Clyde, I agree with everything except the keeper part. The K7/8 use the same diameter valves BUT the keeper groove is exactly the same as the F2. Retainers and keepers are the same and interchangable with the F2 also. The springs are different from the F2. the F2 springs have about 55lbs of pressure on the seat compared to about 40 lbs with the K.

[Clyde]

Mike,
I have no experience with a K7/K8. Most of my experience is with the K0-K2 and F1 apart from a weak moment rebuilding an F2 motor. I know the keepers on the F2 are different as I needed one to complete the motor and it was not easy to get.
I have a Honda parts book for the K7 and it lists the same part number keeper as for the K1-K6/F1. The valves are also listed as the same??
Regds Clyde