Filling a modified Tuff Torq K46 / T40 transaxle while ON the mower (Oil Fill Mod Series Video 3)

FINALLY we can change the oil on a K46 / T40 transaxle without having to take it off of the lawn mower! This video shows mounting a modified transaxle and external expansion tank. We use a cheap marine lower unit pump to fill the trans with oil, using a clever trick to prevent the oil from spilling out of the drain plug hole while we unscrew the pump and replace the drain plug.
Tuff Torq K46 Oil Fill Mod Series (⭐ = you are here)
1. • Tuff Torq K46 / T40J T...
2. • Modifying the Tuff Tor...
3. ⭐ • Filling a modified Tuf...
3.1. • Relocating a modified ...
Materials & Tool Links (some are affiliate links)
Oil hose hole + oil filling
- 1-1/4" hole saw: amzn.to/3L2pVVr
- 1-1/4" grommet: amzn.to/3GKMg7m
- Marine lower unit oil pump: amzn.to/3obOWod
- Mobil 1 5w50: amzn.to/40h9EjU (I'm likely going to switch to Tuff Torq's oil next time: amzn.to/3OIKzKP)
Modified K46 parts (refer to video 2 for more details)
- Tuff Torq Drain Plugs: ebay.us/oonhtw
- Hydrogear 71938 expansion tank: amzn.to/3A5ph3n
- Threaded M18x1.5 to 1/2" hose barb: amzn.to/3ohKS63
- 1/2" oil/gas line: amzn.to/3oiK017
- Stainless hose clamps: amzn.to/3MNwHjh
Misc parts / Tools
- Cutting fluid: amzn.to/41aLo4o
- Half round file: amzn.to/3GMXgkQ
- Adjustable file handle: amzn.to/3mDpo2Y
- Undercoat spray paint: amzn.to/3GOb5zk
- Klein 1/4" spinner drive handle: amzn.to/40gH4iy
- Really nice hacksaw: amzn.to/43N7WtI
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The MostlyBuilds channel is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. MostlyBuilds also participates in a similar affiliate program for ebay.com.

[@MostlyBuilds]

Update: I checked the oil level in the expansion tank before and after mowing. The level in the tank went up about a quarter inch when the oil was hot. This was with the outdoor temperature at about 65 degrees F + mowing for an hour. FYI @billmolzon7723 since you asked.

[@MegaBomber1965]

Awesome Mod…!! Top notch camera work and voice over!

[@MostlyBuilds]

Thank you!

[@jaycowen11]

Just an FYI, it's easier to drill a 1/4" hole first (the same size as the hole saw pilot) or just loosen the set screw on the pilot drill & pull it out a little further. Neat mod. I've rebuilt a couple of these & actually replaced mine with a K57, world of difference. Nice to see the mods, great tool to help others! Mobil 1 worked well for me. The worst of this, is John Deere is well aware of this issue & does nothing about it.

[@MostlyBuilds]

Ah thanks for the tip about the hole saw. I’ll file that one away for next time. FWIW I’m impressed with how simple the k46 is. It seems pretty well designed for the price point (hats off to the Tuff Torq engineers). What bugs me is that John Deere just says it’s sealed + non-serviceable. They could easily just say THEY don’t service them, but if you want to, here are some resources. Most people prob learn the full scope of things when theirs breaks, versus at 50 hours when they could take some preventative measures. Anyway, what are some of the things you like about the K57?

[@jkepps]

Nice Job!

[@philliphall5198]

Very nice and well done 👍 Just need a list of parts now and I will do it also 😊😊😊

[@MostlyBuilds]

I listed the parts used in this video in the description. The second video in the series that shows modifying the transaxle lists the parts for that in the description on that video as well: kzbin.info/www/bejne/fJrJo5uXZbalo68 Consider subscribing if you are interested in future updates. I posted one the other day for moving the tank location because it was too close to the exhaust on my leaf vacuum: kzbin.info/www/bejne/aaashaCkqptgosU

[@user-yd3en3vt4c]

I am in the middle of this mod. Wanted your thoughts and opinion on not using the expansion tank. The original expansion plug is only held in by the compression of the rubber. That original plug happens to fit on the end of the 1/2" tubing you used just as tight as it did on the tans axle originally. I am considering just using about 12 - 16" of the tubing with no tank. Just use the original vent plug on the end of a piece of clear tubing that I can see the oil level through. I plan on draining and filling the unit from the bottom anyway. The expansion tank I assume is for both a vent (which is built into cap) and also expansion of any fluids. I think that amount of tubing would allow for enough expansion. I could allows go back with a tank if it didn't work out. I could drill a hole somewhere and jut put enough tubing to go through opening and then insert original vent plug.

[@MostlyBuilds]

I think that would work as long as you use enough tubing. I posted a comment to this video a few months back to let people know that I’m seeing about a quarter inch of expansion in the tank after mowing. If you can find a clear tube that is oil resistant and won’t melt from heat, that would make it easier to monitor the level (maybe just temporarily while you dial it in). At your own risk of course 😀 I’m really curious to know how it works out. If you have time, please reply back with your results! How did threading the vent hole go? Someone else said they had trouble with that and it leaked oil because of poor alignment.

[@tomgia1964]

Cool , how did you get the drain plug in there? Did you drill it and thread it?

[@MostlyBuilds]

Yup I drilled two drain plug holes and tapped them. You need two because there are two reservoirs in the transmission. Here is the video where I made the modifications: kzbin.info/www/bejne/fJrJo5uXZbalo68 All the videos in my series for modifying the transmission are linked in the description of each video.

[@cracker141]

You're using the Mobil 1 5W-50 in place of the Tuff Torq oil? Did it work alright?

[@MostlyBuilds]

Yeah that’s what I’m using (I might switch to the tuff torq oil since that’s what they recommend). I’ve only used it this mowing season. No problems at all. The mower is more torquey than it was before (in a good way), but I’m not sure if that’s from the transaxle rebuild, the oil, or both. I can easily spin the tires if I stomp on the gas pedal. Short-term, everything seems good. Long-term, no data yet. I will probably change the oil in the trans next spring and see what it looks like. If I do that I’ll be sure to film it so everyone can see how the mod is working out. Consider subscribing if you’re interested.

[@billmolzon7723]

A friendly heads up - nice detailed video. Good copy and good video. After seeing your video last night, I looked at another K46 videos and read this reply “You overfilled it. The oil level is supposed to be 22-25mm (7/8") from the top edge.” When pouring oil into the transaxle, the fill tube has two openings - one directs oil to the pump side and the other directs oil to the gear side. If the recommended amount of oil is put into the transaxle, what prevents the oil from one compartment from flowing out of its opening into the opening for the other compartment when the tractor is on a step hill? I don’t think there’s a check valve for either of the two compartments. It’s my understanding the oil level is supposed to be 3/4” from the top of the fill tube to allow room for the oil to expand when it gets hot while using the tractor. Initially, I liked the idea of the overflow tank. It’s similar to another JD K46 video I saw. That owner installed a small motorcycle radiator just inside the front of the engine hood. He installed hoses between the transaxle and the radiator to help cool the transaxle fluid (similar to small radiators in trucks for cooling transmission fluid). Have seen videos with people saying the hot oil makes the transaxle slip or not work at all. The transaxle has two compartments - one for the pump and the other for the gears. It’s my understanding the oil in the two compartments do not mix. A removable filter is in the compartment with the gears. There also are magnets in the gear compartment to catch small metal fragments from the gears. It appears the hot oil from both compartments will expand into the overflow tank. Some of that oil might be from the gear compartment. That gear oil will mix with the pump compartment oil in the overflow tank. That mixed oil probably will flow back into the both compartments of the transaxle after the oil cools. If you or any of the people reading this reply have ideas about the info in this post, I’d like to hear from you. Thanks.

[@MostlyBuilds]

Hi Bill. Thanks for the kind words. I thought about the oil mixing between reservoirs. I'm not an expert, and it's possible that my mod could result in a failure. I have a warning in each video of the series about this mod being potentially dangerous for the transmission or the operator etc. Here is my thought process: You are correct that the fill tube is open to both reservoirs with nothing to stop the oil from migrating from either side (other than the fill level). There is no check valve or anything like that. There are magnets in both reservoirs and a magnet inside the fill tube just under the cap. There are older K46 transaxles out there without magnets on the pump side. I believe the bracket that holds two magnets against the pump's center case is an improvement that Tuff Torq added some time ago (you can buy this separately if yours doesn't have one). There is also the filter that you mention (it's on the pump side, not the gear side) -- it is more of a screen really and not a high quality filter, unless if you have one that has a charge pump, which has what looks like a proper filter (I *think* any variants with a charge pump are called a K57). I suspect that under normal operation, oil will cross between reservoirs. Either from expansion or driving on hills etc. How much oil transfers between is an open question. I also observed that Tuff Torq makes a K46 variant that has an external expansion tank. I don't know if there are any differences internally, but it looks like it has a single fill hole, so I imagine it is open to both sides like the K46 without the tank. To be super clear, I don't know if there are any design differences with this to mitigate migration of fluid. Here is a reference for the K46 with an external tank: tufftorq.ordertree.com/media/promo/TuffTorq/K46%20-%20External%20Expansion%20Tank%20Oil%20Level.pdf Based on all that I felt like doing this mod was worth the risk of potentially having an issue with oil migration possibly transferring metal particles between reservoirs. If that happens, hopefully the magnets will do their jobs. I'm also going to change the oil every year, so to me, the benefits outweigh the risk. If my transaxle blows up I'll post a followup video (feel free to subscribe if you want to see more 🙂). Let me know if you have any other thoughts.

[@billmolzon7723]

Thanks for the quick and detailed reply! Thanks for the clarification about the two oil reservoirs being connected. The info and link about the K46 with an external expansion tank is another nice nugget. I’ve spent time on the Tuff Torq website and didn’t stumble upon the expansion tank drawing. Did you know about the expansion tank before you did the mod? The diagram for the K46 with an expansion tank looks like the tube from the bottom of the tank goes into the transaxle’s space in the area of the vent cap. This suggests the hot oil from both reservoirs enters the tank and is mixed in the tank. After the mixed oil cools, it re-enters the transaxle. This week I found another KZbin video showing how to change the oil for a K46 and how to remove the internal filter. The JD riding mower still had problems moving. The owner sent the transaxle’s center base and related parts to a guy who rebuilds the center base, the rotor motor and the rotor pump. My impression is rebuilding includes smoothing surfaces that have worn - the wear apparently reduces oil pressure and causes moving problems. The repair guy has an eBay page: Rebuilt Services: Tuff Torq Transmission Transaxle K46/T40. UT: John Deere K46 Transmission Removal with Rebuild-Part 3 Series Finale Also found another UT video with a guy who uses a face honing machine to remove scratches on some of the parts (I think the open bottom of the rotor motor and the rotor pump). Again - I think removing the scratches increases oil pressure. The video doesn’t show the honing machine. UT: Repair the hydrostatic transmission in a Tuff Torq K46 (Lawnmower Stiga Park) Thanks to your video, I now know there’s a thread-on hand pump that fits a quart of oil. Please consider putting a mark on the side of the expansion tank to show the cool oil level before using the mower and the level soon after using the tractor to mow. I’d like to learn how much hot oil goes into the tank. Finally, why did you decide to use the pump to pump oil into one of the drain plug holes instead of putting oil in the expansion tank? I’m going to repair the K46 in a 2014 John Deere x300.

[@billmolzon7723]

kzbin.info/www/bejne/fmetopiObaygeNk

[@billmolzon7723]

kzbin.info/www/bejne/gZLaYaqma62Wi68

[@MostlyBuilds]

Yeah I knew about the K46 variant with the external tank before I started on the mod. Originally I was hoping that I could just buy the Tuff Torq tank, but it looks like top of the case is a bit different on that variant. It looks like there is a bit of a neck that sticks out (farther than the vent hole on mine). I agree with your analysis of the oil expansion behavior in the K46 that comes with a tank. The only thing I would add is that I don't know if there are any significant differences going on inside the case. Not sure if you saw the other 2 videos in my series about this transaxle. I sent out my pump parts to be rebuilt by a guy on ebay. All the videos in my series are listed in the video description if you want to check those out. There seem to be several people on ebay selling their rebuild services. Some of them are suspiciously cheap, and I wonder how good of a job they do. I used this guy from Canada (tthg-services) that another person on youtube used successfully: www.ebay.com/itm/284338450460 Trying to clean up the surfaces of the pump parts yourself might work out. I thought about doing it myself, but I decided to have someone else do it who has more experience. I was more interested in focusing on the mod. I would have been unhappy if I did all this work and it failed because I did something wrong resurfacing the pump parts. After my first mowing session this year I'm going to suck out any extra oil in the tank that is above the fill line at the bottom. After that I'll take a look at the level difference between cold/hot. I used the pump because I figured it would do a better job at preventing air pockets from forming inside the trans. I've always assumed this is why they pump gear oil into lower units on boats (maybe there is more to it?). I didn't like the thought of pouring oil into the tank and the air having to escape through the same hole that the oil flowing through. Filling from the bottom just felt safer to me. Good luck with the x300! Would love to hear how things work out.

[@stoneyswolf]

Why couldn't you mount that tank under the body? It looks janky as F where it's now.

[@MostlyBuilds]

There isn’t enough room on my machine. The gas tank is above the transaxle and you have to clear the fan/pulley on top of the trans. I agree it doesn’t look great where it is currently mounted. It does have one benefit of being very easy monitor and top off though. If anyone can figure out a better mounting location, I’d love to see it.

[@darrylthurston4336]

John Deere's position on these is no serviceability. My position is. This is the last John Deere I will ever own.

[@MostlyBuilds]

Pretty much all the competing mowers at this price point are going to be the same situation. It’s not just a John Deere thing 😀