Topic: Another Frankenbike question

[disco]

Hi Guys,
I'm currently building a CB750 from a basket case.....a very empty basket at that. When I bought the bike all I got was a frame and what is supposedly a "good bottom end". However upon checking the frame & engine numbers at the hondachopper.com website, it appears I definitely have a 1972 K2 frame . The frame number is 207741X. The motor I'm not so sure about. It is engine number CB750E-254642X. Uncertain as to whether that makes it a K6 or an F1 bottom end?? Should it have a '392' or '341' casting somewhere on the cases?
Anyway I have built up a rolling chassis & now am starting to concentrate on the engine. My question to all the experienced CB750 gurus/sages out there before I invest in wet blasting the engine.....
1. If it is an F1 bottom end....will that work within a K2 frame? ie will the front & back sprocket line up?
2. I have two top ends to choose from. A '74 K4 in quite good condition or a K7/8? in need of a bit more work. Does it make any difference which top end I use?




 

[Retro Rocket]

K7/8 head is virtually the same as F1, all 3 are 392 heads, F1 engine has the same sprocket offset as the K's, so your F1 bottom end will be fine in an early K frame, F2/3, K7/8 have the wider chain line..

[disco]

Thanks Retro! That makes me feel better that the F1 offset will work.
Are you certain about K7/8 being code '392'? hondachopper.com lists K7/8 as '405'? F0/1 they say are '392'.
Anyway, I just checked the 2nd top end I have & found it does have a '392' code cast into the head near the inlet flanges. So will probably use this top end. I have an 836cc kit from 'Çruzin Image' in Japan, although I think it's meant to suit 69-77 'K' bikes. They flatter pistons than the raised dome style used in the F1, but I'm hoping they'll be fine to use? Any idea how much a re-bore & valve job should be in Aust?

[Retro Rocket]

Thanks Retro! That makes me feel better that the F1 offset will work.
Are you certain about K7/8 being code '392'? hondachopper.com lists K7/8 as '405'? F0/1 they say are '392'.
Anyway, I just checked the 2nd top end I have & found it does have a '392' code cast into the head near the inlet flanges. So will probably use this top end. I have an 836cc kit from 'Çruzin Image' in Japan, although I think it's meant to suit 69-77 'K' bikes. They flatter pistons than the raised dome style used in the F1, but I'm hoping they'll be fine to use? Any idea how much a re-bore & valve job should be in Aust?

The 405 410, as far as i know, is the F2 F3 head.... I'm using an F1 disc rear wheel on a K2 at the moment, no changes to chain line..

The costs depend on where you go, I have friends that hel,  so the cost is kind of blurry, it usually involves beer and such.... , My heads are all HIPO heads except for the one Mike Reick has , it will be the hipoest of the lot.... , bought them that way so i'm sorry i can't help with costs...

[BPellerine]

just a small correction the f2/f3 heads are 410 .bill

[Retro Rocket]

just a small correction the f2/f3 heads are 410 .bill

Thanks Bill.... I've never owned a F2...

[Nic]

Any idea how much a re-bore & valve job should be in Aust?

I'm in Oz and I have a really good man for machining, he's expensive but good, you just have to ring around but also need to know the machinist is good and knows about 3 angle valve seats and will do a good 1 thou clearance piston to bore. In the end you must find someone you trust first then the cost should be secondary consideration. You need to know what clearances you want, don't leave it up to the machinist, valve to guide and piston to bore clearances.

[disco]

Hi Retro/Nic,

Thanks for the replies. You're spot on there Nic....the $ pain is soon forgotten, but a lousy job lingers like a bad smell!!

Nic, I see you're in NSW. You're machinist wouldn't be Greg Ball Engineering? I keep hearing that does good work & also that he is pretty familiar with old Honda Four's.

I admit, I'm a total novice when it comes to machining clearances. Can you advise what clearances I should be requesting?

[Nic]

Hey Disco, no, my bloke is in a small town called Bungendore, he use to be in Canberra but has relocated to the good life.
Piston to bore clearance .001" and from HondaMan himself re valve to guige clearances:
Make SURE the machine shop sets the clearance at .0008"-.0012" intake, .0012"-.0024" exhaust, even though they will protest and tell you that you don't know what you're talking about: trust me. This MUST be done, or there will be trouble. Honda originally set the clearances much tighter than even those...
Good luck mate.

[Wobbly]

Are you certain about K7/8 being code '392'? hondachopper.com lists K7/8 as '405'? F0/1 they say are '392'.

This is true: the "405" is the K7 code. But the K7/K8 share parts with earlier models. The F1 code "392" is correct, because they both use the same cylinder head. The cylinder itself, however,  is still the original "300" code.

[Retro Rocket]

Sorry, posted to the wrong thread.

You can delete your own posts...

[Nic]

Sorry, posted to the wrong thread.

You can delete your own posts...

Indeed 

[calj737]

Man, if there's a market for those things, I'm gonna start rebuilding Edsels...