Topic: Changing 750 gearbox

[Tim2005]

Just a quick check- I've got the motor out in order to repaint the frame, and am now planning on swapping the gearbox. Has anyone done just a gearbox swap recently? My box has done over 140k miles and is very clunky, and both 4th and 5th are often hard to get into cleanly.  Last time it was apart there was quite a bit of wear everywhere but nothing was beyond spec, just everything was getting close to it. It is an F1 motor. A few years ago I picked up a whole F2 gearbox, with all selectors etc, which is in extremely good condition so this seems an opportunity not to be missed. I believe the F1 and F2 gearbox are identical- if anyone disagrees please shout! The rest of the motor is running well and in good condition, top end, chains, valves etc., so no need to touch that at all. 

I've not had the 750 bottom end apart for 10 years odd, but as I remember it I should be able to leave the whole top end intact, then just:
- take off all the clutch/alternator/points stuff and gearchange mech,
- remove the bolts from the top of the crankcase,
- then turn the whole motor upside down,
- remove the sump and all the bottom crankcase bolts,
- then lift off the bottom case complete with all the selectors and 1 shaft of gears
- swop all the gearbox parts over and reassemble in the reverse order of the above.

All I remember being awkward is re-engaging selector forks as the bottom case is lowered into position on the top. Have I forgotten anything else?


Thanks

[Tim2005]

Just been getting on with this tonight, removed all the side components and ready to split the cases (n.b. you have to take the starter motor cover off too to find a bolt).

Then rechecked the replacement F2 gearbox and noticed the output shaft is different to the F1 one, as below. Presumably this is the later type with central bolt fixing. The old F1 one does not have the step or the centre bolt, it is designed for the plate-type sprocket mounting, which I prefer.

Looking at this http://www.hondachopper.com/engine/engine_mods/modifications.html I cannot spot any other difference except this final drive shaft. (although there a discrepancy between the way it describes the ratios for 4th & 5th gears, I can count teeth when they are out, but will keep the F2 gears anyway)

Can I use the F1 final drive shaft with the rest of the F2 gearbox?

[SOHC4 Cafe Racer Fan]

I have not done the swap yet, but I have a Cycle X back-cut transmission to put into my next 750 build, since it is going to have a bit more grunt.

[Don R]

 The F1 had the smaller chain, I believe the F2 is 10mm wider at the sprocket and had the big chain.
   I believe I have the same issue, I have a backcut F2 trans for my dragbike, I rat holed a 77K cushdrive just in case.

[tweakin]

You should be fine with the Final drive and new gear box.  I just added replaced the trans on my K1 with a back cut race trans over the Thanksgiving break, quick and easy job.

[flybox1]

Hardest part is getting the engine out 
Check/replace: final shaft oil seal, crank oil seal behind the advance unit, shift shaft oil seal, oil passage seal between the cases (19.4mX2.8mm)

[Retro Rocket]

The F1 had the smaller chain, I believe the F2 is 10mm wider at the sprocket and had the big chain.
   I believe I have the same issue, I have a backcut F2 trans for my dragbike, I rat holed a 77K cushdrive just in case.

Don is absolutely correct, F2/F3and K&/K* all have a 10mm wider output shaft, so if using the later shaft you'd need the later sprocket carrier which is also 10mm wider....

[scottly]

Can I use the F1 final drive shaft with the rest of the F2 gearbox?

That's a good question. The K7/8 F2/3 crankcase is also wider where the final drive is, so you will have to use the F1 piece. You're are going to have to count the teeth on the two meshing gears to see if they are compatible.
 

[Tim2005]

Yes, done it last night- no problem keeping with the the F1 shaft, and despite the hondachopper link above indicating ratio changes I counted every gear and they all have the same number of teeth.