Failed XS750 Final Drive (Images and video link)

[3Phase]

Assembly Note

So the XS1100 FD had to come long before the XS850 FD.

True that, and some breakers have already raised the prices on their XS850 final drives, too.

It's also possible that Yamaha made some of the last XS750s with heavier final drives or that they put a few of the XS750 style final drives on some of the early XS850s; sort of like they did with the 1980-style carburetors with the crossover passages that need rubber plugs.

I just don't know and it's not worth the headache thinking about it.

Take a good look at any final drive before you buy it and check it out before you slap it on your bike and ride off into the sunset together!



In the, "Better late than never!" category there is one sort of important note for when the final drive is removed to grease the drive shaft and input coupler splines:

I've measured and set up five Yamaha final drives so far and the fine manual says to torque the nuts that hold the final drive on the swingarm to 30.4 ft-lbs. From what I've seen that's not just a guideline, they really do mean it because the final drives were assembled and set up at the factory with that torque value holding the pinion assembly in the pumpkin along with one or more shims to set the pinion depth.

If the nuts are too loose the pinion assembly will be too far out by up to a couple of thousandths of an inch. The gears will be 'loose' and the final drive can 'walk' on the swingarm.

If the nuts are too tight the pinion assembly will be too far in by up to a couple of thousandths of an inch. The gears will be 'tight' and the final drive housing can be distorted enough do permanent damage or just enough to put it ever so slightly out of alignment with the centerline of the rear axle.

Uneven torque on the nuts will cause the pinion assembly sit crooked in the pumpkin by up to a few thousandths of an inch. What happens after that will depend on which nuts were tight, which were loose and by how much....

You do NOT! need to go to three decimal places to set the torque or use a dial indicator (unless you already have one and know how to use it) but try to get as close to the spec as you can. A little loose is better than even a little too tight but either way can screw up the final drive.


No, that's not what happened to the XS750 drive that is the subject of this thread. That drive was kept as close to factory spec as possible and it failed because it was designed for an XS750, not an XS1100.

[DGXSER]

I am far from the expert, but I do know that the type II came in 79 on the 750. I was not aware the 850 was any different from the 750 Type II until Scott posted this.

[3Phase]

I am far from the expert, but I do know that the type II came in 79 on the 750. I was not aware the 850 was any different from the 750 Type II until Scott posted this.

I've seen pictures of XS750 final drives for sale on eBay that show the heavier gear/support collar like the XS1100 and XS850 final drives.

The sellers, a lot more than just one, could all be wrong about what bikes the drives really came from or there was some cross-pollination between the '750 and '850.

I haven't put my hands on one of the heavy-duty '750 drives so I don't know if there are any differences between a beefed-up '750 drive and an '850 drive.

[DGXSER]

It is very common for folks not to have any idea what year the bike was, even the "professional" parts folks that sell on fleabay. My current 81 Special was listed as a 79 when I bought it.

The only external visual determining factor I know to identify the Type II XS750 FD is the phillips head screw holding the drive coupling in place. So I am interested if you have had one of those that did NOT have the beefier setup. My theory being that there is only two, the Type I and the Type II with all XS850s getting the Type II, since the Type II on an XS750 would have been a one year only design if the beefed it up for the XS850. And I really do not think the 850 had much more torque or HP than the 750.

[3Phase]

It is very common for folks not to have any idea what year the bike was, even the "professional" parts folks that sell on fleabay. My current 81 Special was listed as a 79 when I bought it.

Yes, my '80G was registered as an '81 and that made it so much fun to work on during the first week or two after I bought the bike and for a little while after I bought the service manual!

I'm sure that getting the year wrong is part of the problem over on eBay but so many breakers/sellers getting the displacement of the bike wrong in addition to the year is less likely. At the very least the people buying engine parts from the misidentified models will be in for big surprise when they try to put '850 pistons or cylinders on their '750s.

The only external visual determining factor I know to identify the Type II XS750 FD is the phillips head screw holding the drive coupling in place. So I am interested if you have had one of those that did NOT have the beefier setup.

It's your lucky day. Someone here started a thread with a video and some pictures of a Type II 750 final drive with the weaker, dished, ring gear:-

Failed XS750 Final Drive (Images and video link)

My theory being that there is only two, the Type I and the Type II with all XS850s getting the Type II, since the Type II on an XS750 would have been a one year only design if the beefed it up for the XS850. And I really do not think the 850 had much more torque or HP than the 750.

My theory is that there are two types of Type II XS750 final drives: Type II with a dished ring gear and Type II (b) with a stronger ring gear.

There will probably be a few small but important differences between the '750 and the '850 drives but in the end it's all about II (b) or not II (b).

For example: the Yamaha XS750 Shaft Drive manual specifies 4 m-kg (29 ft-lb) of torque for the swingarm to final drive nuts on both the Type I and Type II final drives so that's how they were set up at the factory. They should all have been set the same way in the service bays at the dealerships but there's really no way to know one way or the other.

The XS1100 manual calls for 4.2 m-kg (30.4 ft-lb) of torque and that is how the XS1100 final drives were set up at the factory. Again, they should have been set the same way at the dealership after a final drive service or repair but there is no way to know.

I have no idea what the XS850 manual says but at this time I don't need to know and I don't want to know!


So far I'm in in the unfortunate position of having purchased two final drives with problems that required replacement parts and setup. The '750 final drive was and the current '850 final drive is set up using the XS1100 torque values for the swingarm/final drive nuts.

It's probably way more information than anyone wants to know but I had to juggle/adjust the shim stack under the pinion assembly of both final drives to keep the pinion gear at the correct depth with the XS1100 torque value on the swingarm/final drive nuts.

[natemoen]

Ok finally got around to checking the supposed 750 FD that I have laying around. Sell said it came off a 1979 750. It does appear to be the "beefier" FD. It does have the y-2 cast in it.

[3Phase]

Ok finally got around to checking the supposed 750 FD that I have laying around. Sell said it came off a 1979 750. It does appear to be the "beefier" FD. It does have the y-2 cast in it.

Right on, Nate, good information!

I can understand a seller/breaker missing the year but when they're also selling the side covers or badges from the same bike with "750" on them it's probably a '750.

[natemoen]

I will open it up and see what's inside at some point but I am in no hurry to put it on.

[cywelchjr]

Right on, Nate, good information!

I can understand a seller/breaker missing the year but when they're also selling the side covers or badges from the same bike with "750" on them it's probably a '750.

But that doesn't mean that it's the FD that came on the bike from the factory. It's always possible that the FD was replaced for some reason before the breaker got the bike (like due to poor care of the previous unit or something like that). It's amazing how many PO's took poor care of these things, and just changed the engine oil and assumed things were good with doing that.

[3Phase]

But that doesn't mean that it's the FD that came on the bike from the factory. It's always possible that the FD was replaced for some reason before the breaker got the bike (like due to poor care of the previous unit or something like that). It's amazing how many PO's took poor care of these things, and just changed the engine oil and assumed things were good with doing that.

Yes, I suppose that some hammered '750 final drives might have been replaced with '850 drives but there seem to be a lot more '750s around than '850s.

When I was looking for another final drive there were only eight or nine '850 drives listed. All of them had pictures showing the Type II pinion assembly and the heavier ring gear. Most of them also had the 850 side covers/badges from the bikes listed along with other '850 parts.

There were over a dozen '750 drives listed with pictures showing the Type II pinion assembly and the heaver ring gear. There were only five or six '750 drives listed with pictures showing the Type I or Type II pinion assembly and the dished ring gear.

Again, most of them had the '750 side covers/emblems from the bikes listed along with other '750 parts. It is possible that one or two of them had their final drives replaced at some time with an '850 drive but it is highly unlikely that the rest of them were actually misidentified '850s that had been made into 'sleeper' bikes by putting '750 badges on them.

[cywelchjr]

Yes, I suppose that some hammered '750 final drives might have been replaced with '850 drives but there seem to be a lot more '750s around than '850s.

When I was looking for another final drive there were only eight or nine '850 drives listed. All of them had pictures showing the Type II pinion assembly and the heavier ring gear. Most of them also had the 850 side covers/badges from the bikes listed along with other '850 parts.

There were over a dozen '750 drives listed with pictures showing the Type II pinion assembly and the heaver ring gear. There were only five or six '750 drives listed with pictures showing the Type I or Type II pinion assembly and the dished ring gear.

Again, most of them had the '750 side covers/emblems from the bikes listed along with other '750 parts. It is possible that one or two of them had their final drives replaced at some time with an '850 drive but it is highly unlikely that the rest of them were actually misidentified '850s that had been made into 'sleeper' bikes by putting '750 badges on them.

So, the dished type II 750 FD that you had the misfortune to get may have been a transition type unit as they were moving toward the unit they knew they were going to need for the 850 (they may have even had problems with 750's with the weaker FD) based on things they had learned with the 1100? That might make more sense. Do we know anything about timing as far as when the first type II's came out in relation to the 1100? Did they come out before the 1100 did and then improvements based on the 1100 design were incorporated into the design without designating another type since the 1100 was also considered just a beefier type II?

[3Phase]

Some of the folks over at the Yamaha Triples web site might know but I have no idea when the first Type II pinion assembly was added to a '750 final drive.

I keep seeing 1979 mentioned but Yamaha had already begun using a Type II pinion assembly for the 1978 model year XS1100, so, maybe some XS750s got Type II pinion assemblies in 1977/'78?

Whenever they made the change it makes sense that Yamaha would start saving money and man hours as soon as possible: use the new pinion assemblies and run out the existing stock of dished ring gears while designing a stronger ring gear with an improved side cover casting.

All of the '750 and '850 final drives appear to use the same casting and part number for the final drive housing with no stamped or cast revision numbers that I can find. If that's true then you could probably use any of the '750/'850 bearing, ring and pinion sets in just the one casting.

[3Phase]

Back to eBay

I just got back from a test run and I drained the final drive oil when I got home.

After only 300 miles the oil has sparkly stuff in it and there was a huge blob of black sludge and gray powder on the magnetic drain plug. The oil is already starting to turn dark and stink from heat.

So the bottom line is I screwed up and the drive didn't last.

[cywelchjr]

I just got back from a test run and I drained the final drive oil when I got home.

After only 300 miles the oil has sparkly stuff in it and there was a huge blob of black sludge and gray powder on the magnetic drain plug. The oil is already starting to turn dark and stink from heat.

So the bottom line is I screwed up and the drive didn't last.

Well, you did say the drive was toast right off the bat. Should have stuck with your first instincts. Just the look of those gears would have scared me away from trying to use the final drive. Of course you made me think of the fact that I'm due for the first oil change on my final drive, so I really should do that today and pay close attention to what things look like as I do it.

[crazy steve]

Scott, did you replace the pinion bearings when you 're-set-up' the FD, or just inspect? The manual assumes new bearings (and specifically tells you not to reuse them) will be installed, and the preload figures take that into account.

I know from hard-won experience that preload is critical to bearing life, and different numbers are needed depending on the parts. I've seen figures on automotive rears that had one set for used bearings/used seal, different for used bearings/new seal, and yet again different numbers for all-new parts. The used parts always got a lower preload setting (sometimes as little as 1/3 as much) and using the 'new' numbers on used parts almost always resulted in quick failure.

I see this issue as the major problem with this swap if you disturb the pinion nut. I personally think that the best course of action would be to check the FD 'externally' as per the service manual (not the FD manual), and if it's within spec, then check to see what the actual preload is. Record and use this number when re-tightening the pinion nut, not the number in the FD manual. I'd also be very hesitant to change any other clearances on a 'worn-in' gear set that externally checks good, as you're changing the wear pattern and IMO that can only promote more/faster wear....

Just my .02.....

I suppose somebody will come along and say they've done this swap using a rock and a stick for tools and haven't had any problems, but the fact remains that the factory gives very specific instructions for setting these up; I doubt if it was just to entertain us...