YFM80 Moto 4 80 rear diff lube
 

I go a old one of these 1987 moto 4 80. Rebuilt the top end and have it running good.
The rear diff seems good but it is one of those lubed for life ones with no dipstick.

I'm wondering if after all these years it would be a good idea to tap a grease zerk into it and pump some fresh grease into there?
Even the yamaha manual does not say anything about maintaining it. The tear down and rebuild procedure does not show adding lube to the pinion and crown area either.
There must be grease in there??

I have a complete spare diff for this thing but hate to kill the original just because I never did something i should have
Ken

I would not put a grease fitting in it, Yamaha was pretty good about putting them where they need to be, and that would be another place to introduce water into the housing, plus I don't think a rearend uses that kind of grease, all the ones I have seen use 90 weight gear oil.

I haven't worked on a Moto 4 80 so I'm not sure how they are, but all the other shaft-driven Yamahas that I have messed with have a drain plug on the bottom of the housing and a fill/inspection plug somewhere along the middle of the housing. To fill them, there is no dipstick, you just take the plug out and add the oil into the fill hole until the oil is level with the bottom of the fill/inspection hole.

Thanks for the reply.
These are different then any other diff I have seen. There is no fill or drain plug, the only way to get in there is to tear the diff apart. Yamaha went to a diff with oil when they updated the Moto 4 80 to a Badger 80 in the early 90's.
They do have a vent line out the top that runs on the frame to keep water out.
If I blow into this line the diff is not sealed as the pressure escapes somewhere. So no point in putting 80/90 gear lube in except to see where it runs out.
No idea if this rear end should be sealed and mine only leaks pressure out because it is old and has a bad seal or if that's just how they are.

I was thinking if i tapped a grease fitting in I would tap it 1/8" pipe thread and screw a plug in after I greased it. Still debating if that is a good idea.
The Yamaha factory manual on these is pretty vague on the diff part. it shows putting grease in certain areas when assembling the rear end, but not in the crown and pinion area.
It must have some there though, imo.
Being this is 25 years old, the thought is it might be good to refresh it?

Ken

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Found that on the web.

Sorry I'm not much help, but this might sound crazy, is there any way possible that the engine oil actually lubes the rearend? I think you should be concerned about it, my buddy ran his on his Grizzly 660 completely dry for a while and ended up destroying the $800 housing.

The engine oil can't get to the diff, I have one of these for a parts bike and pulled the rear diff out of it, the drive shaft is not sealed, only covered. Hard to believe there is no oil, but I have heard of others pumping grease into gear boxes with seals in to poor of condition to hold oil, with good results.

Can't see what it could hurt to try it other then greasing up the brakes if it is overfilled.
I just thought after 25 years someone would have heard what to do with these.
I have heard they have some problems with the rear ends, could be duo to the sealed for life nature of it, but who really knows for sure

Mine seems perfect for now, as does the spare I have. No real idea how many hours are on these bikes

HEY beyond up north i was just wondering if u would sell your spare rear end mine went out on my moto 4 from lack of maintenance.. just wondering thanks

beyondupnorth I would leave it go if everything is alright its probely just like a lawn mower rear they have grease in them when u put them togather and sealed I mea i could see if it was making noise or something I would leave it

I kinda hate to part with it because my boys are still young. I just did the top end and a bunch of other work on that little bike and it has to last 5 years or so if possible.

I have a spare trans gear set and a rear diff from another one that the kid sunk and bent the con rod. Bike sat out for 10 years after that, so plastic was all junk, bought it cheap.
I'd say watch craigs, or do a searchallcraigs.com and select the region near you. One will pop up. There are some on ebay, but not cheap. If your in the US you should be able to find one not too far away or a whole bike for cheap as parts.

Did your run out of grease, have a bearing fail or just strip out the gears?

Almost debating pulling the spare one apart and checking seals and bearings, I tried a bit once , but man those axel nuts are tight, and I did not have time to mess with it more.
Ken

i had a bearing fail ........ iv been looking on craigs list but no luck so far

Was it still nice and greasy in there or not?