Topic: Converting a K to F Motor

[TheodoreVH]

Hey everyone, so I recently rebuilt the top end on my 1975 Cb750K engine and I've been riding it for a while but it still needs to have new valve seats. I recently bought a 1977 CB750F engine - not functioning. The 1977 CB750F motor revs to 9500 RPM while the CB750K goes to only 8000. I would like to know what causes this rpm difference. I am planning on swaping the heads off my K-model to the F-model for the bigger valves, different springs, and camshaft (I believe the Cam is slightly different). However I would like to know if there is anything else that creates this RPM difference because I would like my bike to rev to 9500 RPM. I've seen people say that the pistons and crank are different. I currently have 78 pistons in my engine which are the same as the 77 Fs because I wanted higher compressions. Crankshaft I am not so sure about as the part numbers are the same. Obviously I will check valve piston clearances after I swap the head. Please no guesses - I can easily do that myself. Thanks

My main questions - what do I have to do to make the RPM go from 8000 to 9500?

[69cb750]

what do I have to do to make the RPM go from 8000 to 9500?

Open the throttle all the way, long as the rods don't fly through the case your good to go !
More advice, use the 75 cases and sprocket shaft, everything else from the F engine.

[bryanj]

Just head wont work due to bigger combustion space and different oil drain down path, MAYBE cylinders,pistons and head with mods to crankcase, never done it not sure but i do know there are a lot of differences.

Oh and buy the way big end bolts are stronger on the F, or so im told.

[HondaMan]

The crankshaft and rods in the F2/3 engine are different from the F0/1 engine, but they will all work in a given bottom end. Be sure to keep the same rods with the same crank, as they were selected by Honda to match the crank lobe weights when the set was spin-balanced at the factory. While swapping them around will work OK, the end result will be much smoother if you keep the original combination.

The F2/3 engines have stronger rods and rod bolts than are found in the earlier engines (and K7/8). This was done partly because of the higher (9500) RPM capability of the cam and valve train. That said, the K0/1/2 engines were used by midget-car racers to turn 12K RPM or more using stock rods (and stronger bolts), so I have nver been sure WHY Honda chose to use stronger rods in the F2/3 engines...

[madmtnmotors]

The 77/78 CB750 F and K models have what is considered weaker valve spring retainers. The 76 and earlier models (both F and K) have valve spring retainers that are less likely to pull through the spring retainer cap. I would recommend reworking your K5 with stiffer valve springs, stronger valve spring retainer caps, and upgraded rod bolts. The later F model engines (77/78) are very different from the K models, mostly in the top end. If you use the later F model head you will want to use the F model cylinders too. If you understand the differences between the two engines then you will understand why the F cylinders should be used with the F head. The combustion chamber volumes are different too, so your head selection will dictate your piston selection.

EZPZ 

[Old Scrambler]

I use an F2 head on my Bonneville race bike with a K4 bottom and barrels, F3 clutch internals, F3 tranny & drum. You will have to install the 'pipes' in two of the oil-return holes around the studs as shown on the Cycle-X site and in my engine-build thread....Classic Bonneville Racer.

The RPMs are safely increased by the combination of tranny, sprocket ratio, the larger valves, the cam, and your right hand. Reliability may be reduced for extended mileage. The F2-3 keepers and retainers are weak because they are lighter. Heavier spring pressure will only cause more problems. After you have assured proper valve seats, I suggest beehive springs and Ti-retainers. These motors can withstand high-revs when under load........BUT if you miss a shift

[madmtnmotors]

I use an F2 head on my Bonneville race bike with a K4 bottom and barrels, F3 clutch internals, F3 tranny & drum. You will have to install the 'pipes' in two of the oil-return holes around the studs as shown on the Cycle-X site and in my engine-build thread....Classic Bonneville Racer.

Yeah, there are workarounds, but I couldn't bring myself to delve too deeply into all the possibilities without having to explain all of the subtle differences between the two engines. Then there's the frame itself that will either dictate your choice of K5 or F2 output shaft or require accommodation for the longer F2 output shaft if that is used.

Hence my advice to merely swap over the head, barrels, and pistons with upgraded rod bolts and be done. If the OP can live without revving it to the moon he could forego the rod bolt upgrade. If the pistons are the seizure culprit in the F motor then sleeving the exposed oil returns in the F head would permit use of the K5 barrels and pistons but would also result in a loss of compression due to the F head larger combustion chamber.

Although I have to tip my hat to Old Scrambler, he's done ALOT more HIPO mods than I'll ever do! 

[PeWe]

Only the tachometer will not determine the power you can use.
Revving a stock engine over 8000rpm will not give any extra worth the wear.

You can modify it with bigger bore, ported head with stronger springs and hotter cam.
My CB750 engine is very modified, max power around or just before 9000rpm.
No need to rev it more.
8000 enough too, only a few hp less

[HondaMan]

One other thing: if you use the F2/3 valve springs, opt into the CycleX valve retainers. They are cast and are stronger than the OEM version and will prevent the valve pull-thru of the keepers that casues the dropped-valve destruction.