Primary drive backlash

[urbantangleweed]

Spent today building up an engine that's been hanging around for some time, all is generally good except the clutch is marked 60 and the primary gear is 91 giving a backlash of 151, the manual states 154-156 as being required. Thing is, can I go with 151 without problems or do I need to source a primary gear of 94-96 to make it right?

[4l04ever]

I think Yamaha thought it mattered, otherwise they would not have measured and numbered them.

If you can find a pair with suitable numbers, then that is the best way to go. If not, you may have to just try what you have...

[dusty350]

There are a few threads on this issue in recent times - worth a search on the forum. But general consensus was to try and get to the magical 155 +/- 1. But, you may find running with what you've got could be ok - these gears are getting on for 40 years old and will have worn somewhat over time, so the original tolerances have probably changed. Downside is baggy gears my be a bit nosier than a matched set. I had the same issue on an engine I had. I wanted the 155 match so luckily found a good 94 primary through the forum

Dusty

[Tobyjugs]

We have a saying which is

It's better to hear the gears than smell them.

[urbantangleweed]

Turning the engine over slowly with the clutch basket, there is some binding at the same point every rotation, it's not much, but would obviously be better if it wasn't there. I've posted for a 94, 95 or 96 primary in the wanted section and if I can get one it will be interesting to see if the tight spot goes.

[Tobyjugs]

I am 100% sure your tight spot is not due to the the lash numbers.
I've done a few measuring experiments to try and understand this. Unfortunately I could not find a pattern and the last matching set (155) I have measured were the tightest with an average clearance of 0.08mm. I have deliberately mismatched numbers and never got anything smaller than 0.10 mm.

Check the deflection on your crankshaft, input and output shafts.

[4l04ever]

I have seen mismatched gears that did have tight spots and was resolved by changing one or both gears, but it could have been a faulty/bent/damaged gear too. It is worth checking run out on the end of the shafts and crank as Tobyjugs mentioned above.

[jon]

Jul 29, 2019 10:02:04 GMT 1 Tobyjugs said:

I am 100% sure your tight spot is not due to the the lash numbers.
I've done a few measuring experiments to try and understand this. Unfortunately I could not find a pattern and the last matching set (155) I have measured were the tightest with an average clearance of 0.08mm. I have deliberately mismatched numbers and never got anything smaller than 0.10 mm.

Check the deflection on your crankshaft, input and output shafts.

I’d love a definitive answer to this to use as a sticky.

As mentioned 30+ years of wear will have taken its toll. That’s even to say it’s 30 years of matched wear. It’s quite possible an engine has been built with mismatched gears, run, then rebuilt with others?

I wonder on average how many times our engines have been rebuilt, well and badly?

Also what exactly do the numbers mean? The bigger gear has a higher number and suggests thats related?

Is it total clearance on all teeth under or over a target? In micron? That would make sense of a combined target of 155.

Also does a smaller combined number mean tighter or looser gears? I suspect a smaller combined number is looser based on the above assumption?

If so with wear, higher number combinations would be more factory spec these days, and lower looser.

Maybe Tobyjugs the gears you measured had very little wear? 250 engines will also probably wear less.

More questions than answers I’m afraid.

Interesting I think Yamaha san now sell these with no backlash numbers? Maybe they are centre tolerance? Or maybe it does really matter?

Jon

[urbantangleweed]

All interesting points. Regarding run-out, that would make sense but I don't think I could measure the tiny differences without some intricate measuring devices? This is a complete mongrel of an engine largely put together with what I'd got lying about, the cases are RZ250, the crank is the best of two second-hand spares I had, the gearbox came from somewhere, the cylinders are American market 350's, the head came from someone on eBay, the clutch from eBay too but someone in Germany, the list goes on. Part of my intrigue is to see how it turns out given it's got different bits from all over the place in it.

[jon]

That shouldn’t matter as Yamaha had/has? Very good tolerances,

However if they felt the need to mark these particular parts as selective assembly, I would err on the side of caution.

Jon

[4l04ever]

I think the number relates to how much over a particular reference size the gear teeth are, so a smaller number should be smaller teeth on the gear, which would need larger teeth on a matching gear to get the perfect meshing between them. This theory would match to why you have to add the two numbers up.

Another thought is it could be a tolerance measurement where a larger tolerance on one gear would need a smaller tolerance on the matching gear???

[urbantangleweed]

I've got a 95 primary gear on the way, be interesting to see if there's any discernible difference as this will give me the magic 155 rather than the 151 I currently have.

[urbantangleweed]

Received the 95 primary gear to make the magic 155, thing is, I've got the barrels, pistons and head on now and that very slight tightspot is indiscernible so I can't tell if it makes any difference from the 91. Typical! Anyway, it can't be that bad, but I'll probably go with the 155 set up just so it's as pops Yamaha intended.

[Tobyjugs]

I don't understand the rule and have never found a clearance too small, but I still go by the 155 rule.
You've got the magic numbers so it's a good idea to use them.

[Deleted]

So how do we find the correct numbered gears we need

It surely can't be pot luck when a new primary is ordered from a supplier, we'd all end up with heaps of them until we got the right one

as mentioned is there a section where we can put up what backlash numbers on spare gears we have to swap them around

a bit like the key sticky