Topic: Frankenmotor?

[timtune]

I'm rebuilding my 76 750 Supersport and I have a few motors to play with. I want to use the tranny gears out of a 76 "K" and put them in a 78"F". Will this be a plug and play swap?
I want the higher compression F motor with higher gear ratios from a K.

Any problem with this?

Also if you're in Canada where's a good place to get a full gasket set?

[700504]

You'd need to put the F top end on the K lower, and use the K cases.

[Alan F.]

I'm not in Canada, but this place is:

https://www.siriusconinc.com/search_result.php?search=search&make=&model=Cb750+&part=6&partno=&x=52&y=13

[timtune]

Is it not possible to use the F motor and swap out just the gear set? I just plastigaged the motor and found the bottom end to be well within the specs.

[754]

Far as I know you can swap the gears , make sure the output shaft that holds sprocket, is driven by correct gear.  So use the Kboutput with the rest of the K transmission.

[PeWe]

I have a K2 case (US K5 number)
It will get a fresh K7 transmission with different gearing on 4th and 5th gear.
The last gear on outgoing shaft with front sprocket will be mounted on an earlier K shaft that match cases and bike. Up to K6.

I have seen transmissions to be sold without the final gear that must match the transmission.

F, K7/8 are different than K6 and earlier.

[754]

The gear will pull of the K7 shaft and mount on the earlier one, just make sure you got matching tooth count. 

[Alan F.]

Is there an advantage to be gained by changing gearing here?

[scottly]

Is there an advantage to be gained by changing gearing here?

Only if you are trying to optimize gearing for Land Speed Records at Bonneville, like oldscrambler Dennis.

[PeWe]

Or finding a very good looking transmission that happen to come from a K7 to be used in the earlier engine.

[Don R]

 I also am sitting on a very low mile K6 bottom end and a nice F2 backcut trans. The K6 was wrecked at 12K miles and had 10 documented oil changes. 
  It was left with the carbs off and mice moved in to the cylinders with open valves.

[timtune]

Years ago I swapped the tranny gears from two K motors simply by eyeing up wear on the individual gears. Got the motor bolted all back into the bike before realizing the motor was locked. Turns out there was one set of gears with a notches cut into the outer edge of the teeth to denote a set and I had mated one of them to a non notched one.

Regarding the original Franken motor build I'm hearing I'm good to go with the 78 F motor I know to have a solid bottom end and just be careful swapping the gears.

Is there an easy place tp find out all the tranny ratios from various years and models?

[timtune]

If I was content with just the higher top gear from a K (and skip trying to get a lower gear off the line) can I drop an entire 76' K tranny into a 78' F case?

[madmtnmotors]

CB750's thru 1976 used 530 chains. 77/78 CB750's (both F and K) used 630 chains and the front sprocket sits 10mm further out than 76 F and K 750's. The rear swingarm and shock mounts are different too. I think swapping the 78 F top end (head/cyls/pistons) onto the 76 bottom end would be easier...

Don't forget the extra O-rings required at the base of the 78 F cyls and the four acorn head nuts w/copper washers also req'd on the 77/78 F head.

[754]

You can put short out put shaft into a 77 or 78 case.
 But you cannot just swap the shaft  without matching to the gear that drives that shaft.
 So you use the gear pair of early or the gear pair off the late , and set them in to check  while assembling.

[timtune]

Sorry 754 - what is "ysevthem"

[napoleonb]

The gearboxes can be swapped, just the output shaft with its bearings etc. has to be correct to the cases as was pointed out earlier.
You can easily identify the gears just by counting or looking for a "groove" in the gear for the later ones (392 and 410).

There isn't much difference between the gear boxes as you can see in my excel file.

[madmtnmotors]

You can put short out put shaft into a 77 or 78 case.
 But you cannot just swap the shaft  without matching to the gear that drives that shaft.
 So you use the gear pair of early or the gear pair off the late , and ysevthem when assembling.

Sounds like fun... 

i.e. a whole lot of extra work for negligible, if any, gain. I could see if one transmission had issues (excessive wear/damage) and you were trying to make one engine out of two marginal engines, but swapping mismatched parts just for the sake of some perceived gain seems irrational to me, especially when you're not sure of the gear ratios to begin with:

Is there an easy place tp find out all the tranny ratios from various years and models?

I've been able to find satisfactory gearing with simple sprocket changes:



Either go low or go high depending on what you're after.

Sorry 754 - what is "ysevthem"

Looks like "use them" based on the context. 

[scottly]

Is there an easy place tp find out all the tranny ratios from various years and models?

It's in the factory manuals, but you have to drill down through the different supplements. I did this a couple of years ago, and found that the F3 had a 1.986 primary, with a 1.133 4th and a .969 5th. The K models had a 1.708 primary, with a 1.097 4th and a .939 5th. 1st through 3rd were the same for all models, as far as I can tell.

[754]

Either my F motor is geared stinking low , or my tach is way off, and I am running a small tire.. and  I am getting quite sick of poor gas mileage, and revving an extra 1000 rpm all the time.
 Swapping to the Shaft of choice and K primary  gearing  ..aka the last 2 gears.. would fix it.
 Actually I prefer the old final gearing , and swapping between 17 T and a19 T on the front sprocket when I want acceleration or touring performance

[scottly]

The primary ratio of 1.986 of the F compared to the 1.708 of the K has much more effect that the 4th and 5th gears, and probably accounts for a lot of your higher RPMs, Frank. Like Wilbur mentioned, you can adjust the final gearing for the intended use with sprockets, and with much less work...

[754]

I know it's in the final drive compounded by a bad tire choicel. Just mentioning it as it may be the exact opposite of what you  really want .. be careful..
 Cost me huge in gas alone,  probably the worst gas guzzler 750 I have had..

[Don R]

 My F1 750 seemed happier on the interstates with an 18/47 set than the 17 front it came with. If I remember correctly 4800 versus 5200 rpm at 75 mph.  That said, I still hunt for 6th gear on my gl1000 so maybe it's just me. 

[PeWe]

Looking for 6:th gear is normal
I usually do on my K6. 17:48 or 18: 48 same need of another gear

[johno]

Who is the pic of the Valdosta round 2 of ?
That is an awesome pic, I absolutely love it, not the quickest way but the coolest way 
Funny thing happened to me once at an eighth mile track a customer was running a bike I had just finished  and was popping big wheel stands every run, I then had a run on it to see how it was going, later in the meet I was at the hotdog stand when I heard two girls chatting about this Honda in front of me. One of the girls was  saying Johno was faster and boring but Mick was pulling awesome wheelstands, he's great.
HMMMMMM    stuck in my mind, fame or money !

PS did I say that is an awesome pic