A question about GT750 engine cases

[dude99]

The bike that I'm basing my restoration project around (a 1975 GT750) has at some point in its life thrown a chain, which has broken and bent the metal around the output sprocket on the cases. I do however have a set of cases from a 1974 bike which I have been told are exactly the same and should be fine to transplant everything from my 1975 engine onto.

However, I was reading the Clymer manual and it specifies that "1st driven, second driven, and kick starter drive gears are selectively fitted" and that if the cases are replaced to the the gears that are supplied with the case. Now I haven't pulled apart my set of 1974 cases yet, but I'm pretty darn certain that there is nothing inside them except a crank, at least that's what the guy that gave them to me said.

How big an issue will it be to just use the gears from my 1975 in the 1974 cases? Has anyone done this? Will not having "selectively fitted" gears cause me any issues?

[Suzsmokeyallan]

I don't see a problem as long as you use the gear set shafts/gears as a complete assembly.
On thing that was apparent when I was doing the GS750 gearbox upgrades is I've noticed the bearings move in the cases with a slight variation to each side of the semi circular retainers. So depending on where they are placed before you torque the cases closed, there is some end clearance variations between the bearing and the gear running against it.
Measure the end clearances with the bearings in both directions to the semi circular retainers and check the shop manual for allowed tolerances.

[jabcb]

Had a complete 1975 GT750 engine rebuild done a few years ago. Ended up having to replace some of the gears.
Looked thru my emails.
The following are taken from those emails.
“After a bit of research, there are several things to consider on the GT750 transmissions. Suzuki used a couple different trans gear sets on the GT750. They color coded the engine cases, and you're only supposed to install the proper color coded matched gears for the cases you have. Your cases are a color code brown, which means …”

“Two references:
1) color coding applies to 1st driven gear, 2nd driven gear & kickstart gear
http://www.oldjapanesebikes.com/gt750/g ... ngine.html
2) increased durability of 2nd gear / Service Bulletin GT-29 (new 2nd & 3rd gears)
http://www.ozebook.com/compendium/techb ... 027-33.pdf”

The link on the oldjapanesebikes.com site describes the color coding. The link no longer points to the info. The info is presumably still there & is just located somewhere else.

I’m not an expert on this stuff, so hopefully others can tell you how important the color matching is.

[dude99]

Interesting. The bulletin says that the change in the number of gear teeth was implemented with frame number GT750-66220. So theoretically my 1975 engine and 1974 cases should share the same gear assembly.

[dude99]

Still thought why would they send new gears with new cases? Unless the newer gears somehow fit differently in the cases. Yet the service bulletin doesn't say anything about that. It gives the impression that you can fit the newer gears to any year transmission.

Although I suppose the simple thing to do at this point would be to split both sets of cases and see if the colours match. There is a table in my clymer manual nothing the correct colour combinations... so we'll see.

[tz375]

There are two different issues here.

Transmission ratios: There were three sets of gears. First set covered through to to 74 L models. Second set had a one tooth extra change in top gears and that was up to #66220 part way through A 75 production, and the final sets had a different second and third gear ratios.

The second issue is manufacturing tolerances and that's where the color dots come in. Fitting an innaproprate set of gears into a set of cases could make the gears too tight or too loose but so far I have not found a combination that didn't work fine.

[dude99]

The second issue is manufacturing tolerances and that's where the color dots come in. Fitting an inappropriate set of gears into a set of cases could make the gears too tight or too loose but so far I have not found a combination that didn't work fine.

Measure the end clearances with the bearings in both directions to the semi circular retainers and check the shop manual for allowed tolerances.

So just so I'm clear here, were talking about the side to side play that the shaft assembly has when its sitting in the bottom case right?

[tz375]

Good question. I have always been under the impression that the stack up of tolerances related to gear diameter and shaft spacing in the cases. Allan thinks it relates to side to side tolerance.

I would just drop your gear set into the new cases and bolt them up and see if the shafts rotate smoothly. If they do, clean the cases up and build the motor.

[dude99]

I would just drop your gear set into the new cases and bolt them up and see if the shafts rotate smoothly. If they do, clean the cases up and build the motor.

That's probably what I'll do then.

[yeadon_m]

I don't really know but had the impression from somewhere that the matching of gear sets was as much to do with backlash and resulting noise than for purely functional reasons. Gears might functionally mesh OK but might whine more or less depending on goodness of fit? either way, and as suggested, were I doing it, I'd be looking for a complete set in good condition and drop that in.
Mike