Mixing bentonite and regular oils or greases is not a good idea. Bentonite is a clay based grease and when mixed with “oil” it turns very runny and will start working it’s way out of every crack, joint or seal.
Regular oils and greases are “soap” based, Bentonite is clay based. When mixed, they essentially “attack” and liquify each other, effectively destroying the lubricating and protective qualities of each lubricant.
Trivia point: Bentonite grease gets it’s name from the specific type of clay used to make the product - Bentonite clay.
You’re better off to either go with 80w90 gear oil (or 85/140 is fine, doesnt matter as long as its a gear oil) OR Bentonite. But not both. If you do switch over to something other than Bentonite, make sure you get ALL of it out of the case.
I would not use an ATF in these gearboxes. These are straight cut gears and the shearing forces are very high on the gear faces. ATF is spec’d in some modern automotive manual transmissions, but that’s more for fuel efficiency than protection. The loading in a modern manual gearbox is very different from these old relics. You need the robustness of at least a gear oil in these boxes. Something designed to stand up to the high gear face loading you will see in both the speed gearing and the differential loading. ATF is not going to like the internal environment of these transaxles and it will end up offering less protection. Not to mention, ATF is meant to flow, not hang on surfaces. Gear oil and greases like Bentonite are designed with “tackifiers” to aid them in clinging to the gear faces as they spin and be less likely to be “flung” from where they need to be. That also means they “cling” when stopped as well. Lastly, when you park it, ATF is going to flow off the internal surfaces and sit in the bottom, like it would in an automatic transmission pan. ATF is meant to flow back to the sump where the pump can draw it back up and operate the pressurized circuits in an automative transmission. Gear oil/Bentonite will cling to the surfaces and maintain corrosion protection on the bare steel surfaces while the transaxle cools and condenses water out of the air.
I have a preference for the Bentonite in the 820, because that is what is spec’d by the manufacturer. Some guys use 80w90, for various reasons: cost, availablity, opinion that it works better, etc. To each thier own, but for my money: it’s Bentonite baby!
But it’s not cheap, nor is it easy to find. I got lucky and my local shop had a full sized bottle on the shelf that had been hanging around for years. They let me have it for more than half off the price, just to get rid of it.
Bentonite is also pretty hard stuff to work with. It’s impervious to water, so it’s damned near impossible to clean it up. Thats what makes it so hard to work with: hard to clean or remove. It’s also what makes it so good for the 820: it’s super tenacious goo and super hard to wash away.
In my experience, Varsol works best to cut it and wash it away from the transaxle parts. Citrius HD hand cleaner (with pumice) works best to get it off your skin, along with a stiff fingernail brush or one of those plastic scrubbies they use for surgical scubbing up. If you do mess with the Bentonite, wear clothes you can throw away (or not care about) and either cover the floor (and the bench, and the walls, and the ceiling, and...) or do it outside somewhere. This stuff ruins everything as it sticks to anything and spreads everywhere. The same properties that makes bentonite great inside a transaxle are also the same ones that make it a major PITA when it’s NOT in the transaxle....lol!
I use my 820’d Murray for the same uses you describe in down to -20c with none of the issues you are having. My transaxle is still running the Bentonite grease from 1991 without issue, although it is time for a thorough clean out and refill. The Bentonite is still in decent shape, it’s just time to refresh it.
Shift keys that are worn on thier tips (triangle part in the pics below) will catch and drag inside the gears, causing hard shifting or potentially, a stuck gear. Depending on how (or how badly) they are worn, it may even jump in and out of gear when driving. Mine was doing that in 5th gear before I installed the new keys.
The only thing that engages the gears in the transaxle is that tiny triangle shapped wedge, so any damage has very negative effects on the transaxle performance/behavior.
Be aware there are at least two part numbers for shift keys: 792180A and 792123A. As mentioned, they do not interchange:
The three keys are the 123A, the lone different one is the 180A.
I was able to order them from my local small engine shop if I wanted, but it was more affordable to find them on ebay and order them that way. The local shop was expensive for keys (almost 25 cdn for ONE key) and it was questionable if they could even get them anymore. Perrless 820 stuff is getting pretty old, so it’s getting harder to find parts for them these days. My ebay keys were “new old stock”. Meaning: they were lying around somewhere from long ago.
I would recommend actually pulling the keys out before ordering. I have found conflicting info regarding what versions of 820’s had which keys. My 820 is an 820-018 and I have found both style keys listed for it, depending on where you look up the parts.
A quick tip to make installing your keys easier is fo put them on the shaft, zip tie them in place and then compress the ends to slip the shift collar over the ends. Push it all forward until the tips just go inside the first gear and the shift collar is on the shaft. Then cut the zip tie off and finish assembling the gearbox. Because there are 4 keys and they are spring loaded, it is near impossible to hold them in place on the shaft while getting the shift collar on. The zip tie is your third set of hands.
Also be aware that the transaxle bolts are steel and the case is aluminum. The threads often strip out if you live in an area where corrosion is a problem. Mine was really bad, but I also live in Atlantic Canada where the lifespan of unprotected steel can be measured in weeks
If they strip out, they can be repaired with a heli-coil insert. A heli-coil insert replaces the weak aluminum threads with a steel insert so they will never strip out again. It’s an easy process: drill out the stripped hole, tap threads in the new hole, locktite red thread locker on the insert, screw it into the new hole, give it a minute or so for the locktite to set up and then it’s ready to go. Because you are no longer running a steel bolt in and out of aluminum threads, the holes will never strip on you again.
Some have replaced stripped out bolt holes with a longer bolt with a nut on the other side. That works if the hole goes through the case, not so well if one of the many “blind” holes in the case strip out.
My transaxle is now heli-coiled in every bolt bole. I got tired of them randomly stripping out and when I only had a few left that hadn’t been repaired, I just went ahead and replaced them all.
When replacing the case bolts, make sure you use a good quality in/lb torque wrench. Run the bolts in lightly by hand, then torque to spec (180-218 in/lbs). You want to use the right torque setting for several reasons: not warping the case, seating bearings and shafts properly and most importantly on something this old - not stripping out the aluminum threads in the case.
Here’s a link to download the Peerless transaxle service manual: http://www.wfmfiles.com/download/Tecumseh-Peerless_Motion_Drive_System_-_Transmissions_&_Differentials(691218).pdf
That will tell you everything to know under the “820” chapter.
The 820 is a bit unique in the peerless line. It’s one of the few aluminum cased Peerless’s that is rated for ground engaging impliments. Tough stuff for a “lawn tractor”.
The Murray Garden tractors are kind of a “foot in two worlds” Garden tractor. They’re not as heavy duty as a “real” garden tractor, but they’re tougher than the typical “ride on” tractor.