LawnBoy S21ZSN Transmission Lubricant

My S21ZSN would not pull itself, but was kinda hard to push or pull backwards. When I lifted the rear the wheels would turn, but as soon as I set it down they would stop. I read the other thread about this, which gave me the right start on the problem. I took the bottom of the drive case off. Contrary to a previous post, it can be done without removing the shaft, wheels, etc. but the rear-most bolts are very difficult to get a wrench on (7/16"). I found the inside of the case filled with caked, hardened, 27-year-old grease. I scraped it all out and cleaned up the 'innerds'. The question now is: What lubricant should I use?

- The stuff I got out reminds me of wheel bearing grease
- The dealer I bought it from this week (new) did not know. After talking it over with him, he suggested 90W gear lube.
- There's a tiny drain plug on the bottom of the case. Clearly, grease would never drain from this plug. EVER.
- If you remove the engagement arm you can also remove the O-ring and adapter for the arm. At that point, you can squirt in lubricant around the shaft.
- Looking at the worm drive, drive return spring, etc. I don't see any reason why they would need a heavy grease for lubrication.
- The plug looks to be the same threads as a zerk. If the gunk inside is indeed grease, maybe the factory used a temporary zerk to fill it and then replaced it with the plug (just a guess).

So what do you guys think? I am tempted to use maybe a heavier motor oil like 40 Wt, or the suggested 90 W gear lube - something that can be drained out in the future. I do not want to mess around taking this thing apart again if I can possibly help it. Thanks in advance for your advice!

I ended up using Mobile1 Synthetic gear lube. At first, this worked great. After about 5 minutes, it stopped pulling again, but the mower pushes and pulls backwards fairly easily. I suspect the wavy washers that are supposed to grab the shaft are too well lubricated to do so. I dunno. Very disappointing Saturday. At this point I'm ready to take off all of the self-propelled stuff and turn it into a straight push mower. :frown:

I tried synthetic gear lube in the rear end of my Toyota Tundra (limited slip differential) and it acted WAY weird, making noises and slipping. Ended up replacing with conventional rear end fluid and the problem went away.

Snapper uses 00 grease in their transmissions available at Tractor Supply Company.

Jeff

jp1961 said:

I tried synthetic gear lube in the rear end of my Toyota Tundra (limited slip differential) and it acted WAY weird, making noises and slipping. Ended up replacing with conventional rear end fluid and the problem went away.

Snapper uses 00 grease in their transmissions available at Tractor Supply Company.

Jeff

Click to expand...

They make a limited slip additive that you must put in. If you don't it will destroy the rear end.

Yep...old grease out....new grease in. Use 2.5 ounces of 00
Lee

I have read on here that you can drill a hole in the bottom of the trans case and flush it out with you know that stuff in the red can then thread the hole to install a grease zerk and pump it with 00 grease

BlazNT: Didn't find anything on Google for this additive. What do I use to find this additive?

unclelee: So you'd say dump the gear lube out and refill with something like Stens 770-123 "00" Grease? I found something on Amazon that looks promising:

Stens-770-123-00-Grease

Or just add the kind of friction additive like they put in limited-slip differentials? If so, any particular's on this?

I'll give you guy's advice a try before I yank off drive parts, but fact is I'd probably be happier with a straight up push mower. I have a '92 680529 (looks just like the S21ZSN, but not self-propelled) which I've been happy with for years, other than issues with junk aftermarket wheels. Pushed it around about 3 hours this week with a smile..but I tell you what, that NOS S21ZSN has a LOT more power, even though the engine is the same. I guess that's the difference between a tired, 24 year-old F engine and a brand new 27 year-old F engine.

beg: I read that thread, too. There is a plug near the bottom of the case which may be the same threads as a zerk. How do you put 00 in with a zerk? The 00 I've seen pictures of are squeeze-looking bottles.

Some zerks may be self threading. I ran across this with auto steering rod ends. Although I could not ID the threads with a gauge they looked like small pipe thread, tapered. The hole, as I remember it, admitted the end of the threaded zerk and from there you carefully keep vertical alignment while screwing in, hopefully with a tiny hex socket (only with straight zerk?).

This is what I did my google search with "limited slip rear end additive" Any automotive store will have it in stock. Maby not this brand.
amsoil.com/shop/by-product/gear-lube/slip-lock-gear-oil-additive/

I found that stuff. I thought you were referring to an additive specific to LawnBoy drives.

By the way, today it wouldn't move on its own at all when cold, but it did drag some. So I used my old, trusty S21ZPM to mow (not self-propelled). If I didn't have that one to fall back on while I fuss with the S21ZSN, the ZSN would already be "de-propelled".