Topic: Crankcases - F2 versus K8

[NickO]

Thanks to everyone one here who is so passionate and knowledgeable about these bikes and has helped me thus far! It looks like I may be a very regular contributor here over the next few weeks as I endeavour to get our 836 race engine build completed.

It would appear that one of our donor engines is a USA K8 model, as it has engine no. CB750E - 3012***? I'm almost certain it originated from the USA. It is so long ago since I brought it I had forgotten to check the engine number. I had assumed it was an early K series!

If this is correct, can I assume the crankcases will be the same as the F2, (with the exception of the colour), and so will be good for our build?

As usual guys, thanks in advance for all help and advice received, (including regular contributors - you know who you are  )

[Old Scrambler]

I'm not one of those experts but I know the F2 bottom case is deeper than standard K cases.......just not sure about the K7-8 cases. All else should be the same regarding the drive-sprocket bearing race.

[lordmoonpie]

I'm not one of those experts but I know the F2 bottom case is deeper than standard K cases.......just not sure about the K7-8 cases. All else should be the same regarding the drive-sprocket bearing race.

Is that the only advantage of an F2 set of cases then Dennis for racing - deeper sump?

[754]

 The oil pan may be a bit deeper but I think the cases are the same... but check the cylinder oil drain area...

[jaguar]

it thought that the idea behind running the F2 case was that it had a better "double" bearing on some part of the trans.

but i thought recently it was talked about that the bearing could be added to any case?

also buy F2 you mean 1977 cb750F?

[Old Scrambler]

I have all of the different years in my stash of motors.......never really examimed the oil return areas.  The K7-8 and F2-3 have the longer counter-shaft with wider bearings to support the extended sprocket to clear the wider rear rim/tire.  The cases are slightly deeper in the pan but have significantly deeper fins for added cooling. I know they have issues mating to standard K barrels because of different oil returns but am not expert to mate them. Next time I play with my motors I'll check the pans for differences in the fins. 

[jaguar]

so did we get a ruling on what the differences are and the pros and cons of the F2 case?
we are talking about a 1977 cb750F right?

[jaguar]

nothing?
ive heard it talked about before but never anything for sure

so whats the difference and why are they better?

[NickO]

Jaguar,

The later cases have a double bearing on the gearbox sprocket shaft. This can be seen by a small "bulge" in the cases around the bearing.

Nick

[jaguar]

only on the 77F case?

also i thought that someone said that the bearing can be used in any case?

[NickO]

only on the 77F case?

also i thought that someone said that the bearing can be used in any case?

It appears that this modification/upgrade is on the F2, F3, K7 & K8 cases.

[Sam Green Racing]

It won't be a problem in your case Nick as your custom chassis will take into account the offset of the longer output shaft with you running the very wide rear wheel.
Your only problem is whatever top end you are useing, will it mate up to the later bottom end.
You need to talk to Mike about the oil returns.

Sam.