Topic: Cb750f case interchangeable with other models?

[Rydog400f]

Hello. I butchered the top case of the bottom end on my '78 cb750f while trying to remove the cylinder head studs and was wondering if I can just get another set of cases from any model and throw the guts in them? Thanks!

[Don R]

The transmissons are different at the output shaft bearing, what year do you have?

[Rydog400f]

The transmissons are different at the output shaft bearing, what year do you have?

I have a 1978 cb750f

[bryanj]

You will need 77/78 cases or you could use a complete bottom end from an earlier bike with transmiion and clutch if you can get a gearbox sprocket as the later ones are located differently

[Rydog400f]

You will need 77/78 cases or you could use a complete bottom end from an earlier bike with transmiion and clutch if you can get a gearbox sprocket as the later ones are located differently

Will both  K or F model 77/78 work?

[HondaMan]

You will need 77/78 cases or you could use a complete bottom end from an earlier bike with transmiion and clutch if you can get a gearbox sprocket as the later ones are located differently

Will both  K or F model 77/78 work?

No. You need the 77/78 F2/F3 cases, which are usually black. There are some minor, but important, differences between the F2/3 and K7/8 bottom ends that can give you some grief unless you are real good at measuring lots of things, installing (and even finding?) tranny shims, etc. Also, the clutch covers are different between those 2 lower ends.

[Rydog400f]

You will need 77/78 cases or you could use a complete bottom end from an earlier bike with transmiion and clutch if you can get a gearbox sprocket as the later ones are located differently

Will both  K or F model 77/78 work?

No. You need the 77/78 F2/F3 cases, which are usually black. There are some minor, but important, differences between the F2/3 and K7/8 bottom ends that can give you some grief unless you are real good at measuring lots of things, installing (and even finding?) tranny shims, etc. Also, the clutch covers are different between those 2 lower ends.

Thanks a lot Hondaman. I'll keep going on the rebuild and keep an eye out for those cases. Machine shop quoted me $500-$1000 to remove 3 studs plus a helicoil so I figured I might as well just get different cases.

[754]

 I got a set of 78 f cases and a crate for them but unfortunately  I too have 3studs stuck.
 And I am in Canada. .  Might be worth thinking about.

 I think you can get your studs out with an EDM machine if they are not bad, for less money than your quote.

Hondaman what difference between f and k in those years, is it not just maybe the clutch ?

[Rydog400f]

I got a set of 78 f cases and a crate for them but unfortunately  I too have 3studs stuck.
 And I am in Canada. .  Might be worth thinking about.

 I think you can get your studs out with an EDM machine if they are not bad, for less money than your quote.

Bondsman what difference between f and k in those years, is it not just maybe the clutch ?

How much of the studs is still exposed? Mine are down to the case.

[754]

Mine are high enough to weld nuts on  but myself welding the nuts kept coming off.
it's hard to get the heat into the stud and I don't want to hurt the case, the first 10mm or So does not touch the case, so poor heat transfer.

Search EDM tap or stud extraction. They will burn in a hex hole.
 If they are short, easier to heat up. Then they put an allen wrench in them and turn it out.
 Or the burn it out to minor dia, the  the rest falls out when you run a tap in.

[Rydog400f]

Mine are high enough to weld nuts on  but myself welding the nuts kept coming off.
it's hard to get the heat into the stud and I don't want to hurt the case, the first 10mm or So does not touch the case, so poor heat transfer.

Search EDM tap or stud extraction. They will burn in a hex hole.
 If they are short, easier to heat up. Then they put an allen wrench in them and turn it out.
 Or the burn it out to minor dia, the  the rest falls out when you run a tap in.

Also, assuming I find another set of cases. Should I try extracting those studs as well or not? How many rebuilds have you done without taking them out and didn't have head leaks?

[754]

 I only cut these to lessen shipping cost.
 Lost the first sale.
 Second the the guy wanted studs out, so I tried.... I think they will come but not with my welding skill.
 So I lost that sale..
 Still have them,  not sure what I wil, do......... but at the time , i just had to walk away.

[Rydog400f]

I only cut these to lessen shipping cost.
 Lost the first sale.
 Second the the guy wanted studs out, so I tried.... I think they will come but not with my welding skill.
 So I lost that sale..
 Still have them,  not sure what I wil, do......... but at the time , i just had to walk away.

Right but you've rebuilt a few cb750's right? Do you always remove the studs?

[754]

No, not unless i was building a very hi performance motor.
 I ran 836 without.
 But shipping to say Mass from here with studs off is still about 95 to110 bux.. expensive.
 Cheaper in the US though.
 I looked up  edm stud removal. Plenty of places  around LA and maybe north of there. 

[Rydog400f]

No, not unless i was building a very hi performance motor.
 I ran 836 without.
 But shipping to say Mass from here with studs off is still about 95 to110 bux.. expensive.
 Cheaper in the US though.
 I looked up  edm stud removal. Plenty of places  around LA and maybe north of there.

If you leave in the oem studs do you usually have leaky top ends?

[754]

While they are not a bad idea to have, they can be leak free without.
 What displacement ?
 Everything is fine and easy if they come out easy..

[Rydog400f]

You will need 77/78 cases or you could use a complete bottom end from an earlier bike with transmiion and clutch if you can get a gearbox sprocket as the later ones are located differently

Will both  K or F model 77/78 work?

No. You need the 77/78 F2/F3 cases, which are usually black. There are some minor, but important, differences between the F2/3 and K7/8 bottom ends that can give you some grief unless you are real good at measuring lots of things, installing (and even finding?) tranny shims, etc. Also, the clutch covers are different between those 2 lower ends.

Hondaman. Have you successfully rebuilt top ends without removing the original head studs and achieved no head leaks?

[PeWe]

I used original studs on my CB750K6 with 836, torqued head with 20.6 ft lbs (28Nm). It came a little oil at the top after a while that could have been leaking out via the cam studs or rubber coins.

I guess it could have been leak free with MLS head gasket as it has now. Both surfaces must be smooth and flat, MLC rivets outsiide mating surfaces. If not drill small dents in both sides for the rivets. 8mm drill, depth 1mm max.

[Bankerdanny]

You will need 77/78 cases or you could use a complete bottom end from an earlier bike with transmiion and clutch if you can get a gearbox sprocket as the later ones are located differently

Will both  K or F model 77/78 work?

Rydog, I might be able to help you out here. I picked up a '78 F rolling frame and engine to get the front forks. The engine is locked up and I haven't stripped it down yet, but if you aren't in a big hurry I should be able to get it apart by mid February.

[HondaMan]

You will need 77/78 cases or you could use a complete bottom end from an earlier bike with transmiion and clutch if you can get a gearbox sprocket as the later ones are located differently

Will both  K or F model 77/78 work?

No. You need the 77/78 F2/F3 cases, which are usually black. There are some minor, but important, differences between the F2/3 and K7/8 bottom ends that can give you some grief unless you are real good at measuring lots of things, installing (and even finding?) tranny shims, etc. Also, the clutch covers are different between those 2 lower ends.

Thanks a lot Hondaman. I'll keep going on the rebuild and keep an eye out for those cases. Machine shop quoted me $500-$1000 to remove 3 studs plus a helicoil so I figured I might as well just get different cases.

Holy cow! I get about $150 for that! 

[HondaMan]

You will need 77/78 cases or you could use a complete bottom end from an earlier bike with transmiion and clutch if you can get a gearbox sprocket as the later ones are located differently

Will both  K or F model 77/78 work?

No. You need the 77/78 F2/F3 cases, which are usually black. There are some minor, but important, differences between the F2/3 and K7/8 bottom ends that can give you some grief unless you are real good at measuring lots of things, installing (and even finding?) tranny shims, etc. Also, the clutch covers are different between those 2 lower ends.

Hondaman. Have you successfully rebuilt top ends without removing the original head studs and achieved no head leaks?

Most of the time! I have had my own 750K2 apart 5 times, still original studs, no leaks. It's all about getting the correct O-rings and making sure the seal faces are clean on reassembly. I use only Vesrah gaskets on my own, and most of the rebuilds I do. PM me for the 2 O-rings that you need for the oil passages when you get there: the correct ones to match the head gaskets do NOT come in the kits today.