Tuff Torq K46 Oil Fill Mod Series

The hydro pump in my K46 (T40J) transaxle failed, leaving my John Deere D130 dead in its tracks! Follow my journey as I rebuild the pump and modify the transaxle to support oil changes without having to remove the trans from the mower.

Disclaimer: Pretty much everything that is covered in this series would likely void your mower or transaxle warranty. The procedures and modifications shown could also be dangerous. I’m not an expert; this information is being provided as entertainment only.

Background

If you have a riding mower with a K46 / T40J transaxle, you might not know it, but you own a budget machine. This came as somewhat of a surprise to me because 8 years ago I walked into my local John Deere dealer thinking I’m getting a Deere, I’m good to go.

When I got home with my new D130, the younger, less mechanically minded version of myself was going through the operator manual and came across a line that said something to the effect of “this transaxle is a sealed, non-serviceable unit.” I thought to myself, “That sounds strange, but oh well—it’s a Deere, so I should be good, right?”

Fast forward to today, and my trusty D130 has started feeling really sluggish on hills—especially after running for a while. Initially I went after some of the quick hitters: replacing the drive belt and idler pulleys. Unfortunately, these only confirmed my worst fear—the real issue was the transmission.

This was my introduction to the world of the Tuff Torq K46, where I discovered that my transaxle was a slightly weaker variant called the T40J. Major riding mower manufacturers often contract Tuff Torq to supply customized versions of the K46, tailored to their specifications. These specifications can include features like drain plugs, external expansion tanks, and synthetic oil. However, most manufacturers choose to forgo these upgrades.

Even with the potential upgrades, it’s important to note that the K46 is still only semi-serviceable. The internal filter(Amazon) and magnets(Amazon) can only be cleaned or replaced by opening the transmission case.

That said, there are some positives to share about this situation—but I’ll save those for the “Final Thoughts” section at the end. Now, let’s get to the build!

Part 1: Pump Rebuild + Modifications

Let’s Crack It Open

I took the transaxle apart to see what could be causing the loss of power. Inside, the center case and cylinder blocks showed visible wear in the form of circular scratches. These were likely caused by metal particles that worked their way into the pump, scoring the surfaces. While the scratches weren’t super deep, they were significant enough for a fingernail to catch on. I suspected these surface irregularities were letting oil slip by, causing power loss—because every bit of that oil is needed to drive the pump motor.

At this my options were:

• Rebuild the hydro pump parts: $170 (includes tax and shipping)
• Replace the hydro pump parts: ~$450-$500
• Replace the transaxle ~$700-$800
• New mower???

I sent photos of my pump parts to a company called TTHG Services(eBay), that offers K46 / T40J pump rebuild services(eBay) on Ebay. The owner, Jim, got back to me right away and let me know that my pump looked like a good candidate to be rebuilt.

I decided to roll the dice and rebuild the hydro pump because, in comparison to the cost of replacement, it felt like a reasonable gamble. Replacing the hydro pump with new parts is so expensive that you’d probably be better off just buying a new transaxle. And if you’re going to spend close to $1K on a new transaxle, why not invest in a whole new mower instead? Realistically, the big mower brands just want to sell you a new machine.

I packaged up my pump parts and sent them off to be rebuilt. The round-trip time to get everything back was about 20 days (most of that time was waiting for USPS). In the meantime, I began planning my next move—seeing if there was a way to improve the serviceability of the K46.

Part 2: Modifying The K46 Transaxle

Let The Mods Begin

Even though you need to crack open the transaxle case on the K46 to replace the filter(Amazon) and magnets(Amazon), I think it would still benefit from changing the oil at a regular interval (annually?). I’d really like to be able to do this without having to take the transaxle off the machine and flip it up side down to drain.

I drilled and tapped the two M8 drain plug holes in the bottom pan of the transaxle. Nothing earth shattering here, others on the internet have done this as well.

Once upon a time, Tuff Torq used to publish a PDF for adding drain plugs, but they don’t appear to promote that anymore. Probably for the same reason new car manuals tell you not to drink the transmission fluid… it tastes so good though right?

I’ve seen people add drain plugs to their K46s, but I haven’t come across anyone who’s figured out how to fill the trans without removing it from the machine. Without an easy way to refill the case with oil, I’m not sure what value drilling drain plugs really adds.

My solution for filling the transaxle is to convert the vent cap into an expansion tank. This way, I can pump oil into the transaxle through one of the two drain plug holes until it starts overflowing into the expansion tank.

The Tuff Torq M8 drain plugs(eBay) are conveniently the same size as the fitting on pumps(Amazon) that are used for filling the lower unit on outboard motors. We will dive deeper into that a bit later.

Drilling and tapping the drain plugs in the bottom pan was pretty easy, but doing the same for the vent hole posed more of a challenge. I used a 16.5mm drill bit(Amazon) and an M18x1.5 tap set(Amazon)—it’s a pretty large hole to work with. To make sure everything was precisely lined up, I used my drill press and took my time to square everything up before drilling. It’s important to ensure this hole is perfectly square with the case. The seal comes from the compression of the crush washer between the threaded barb(Amazon) and the modified vent hole, not the threads themselves. Someone reached out to share their experience attempting this mod, but they ran into trouble when they drilled and tapped the hole by hand. Their hole was slightly off, preventing it from sealing properly.

I picked up a vented expansion tank(Amazon) made by HydroGear and connected it to a short length of hose(Amazon), which is attached to the newly installed barb(Amazon). Everything fits well, and thankfully, the hose clears the fan blades. However, we’ll need to keep an eye on it to ensure it remains clear once the assembly is mounted on the machine.

Parts & Tools

If you want to check out the parts and tools that I used to complete this mod, expand the following “Links” section:

Part 3: Filling The Transaxle ON The Mower

Will It Work?

With limited space under the mower, I made some minor modifications to mount the expansion tank on the rear body. I attached a square aluminum tube to the plastic fenders as a mounting point. While I considered mounting the tank to the mower’s green body for a cleaner look, I decided against drilling into the pristine paint for now. I designed and added some angled 3D-printed spacers to position the tank properly, making it easier to check the oil level.

Routing the hose(Amazon) for the expansion tank(Amazon) presented some challenges. Initially, I considered using an existing hole in the frame, but it caused the hose to kink and placed it in a vulnerable spot. Instead, I drilled a new hole higher up towards the back for better protection and flexibility. After carefully filing the edges and applying some undercoat to prevent rust, I added a rubber grommet(Amazon) for a smooth finish before routing the hose.

While putting the transaxle back on I had to be careful to guide the expansion tank hose through my newly cut hole in the back of the mower.

Filling the transmission with oil while it was on the mower went really smoothly. I used a hand pump(Amazon) designed for outboard motors to pump in Mobil 1 5w50(Amazon), adding slightly more than Tuff Torq’s recommended 2.3 quarts to account for the expansion tank.

After putting everything back together and filling the trans with oil, I ran through the air purging procedure and everything seemed fine! It was super cool to see this whole project finally come together.

Parts & Tools

If you want to check out the parts and tools that I used to mount and fill the modified transaxle, expand the following “Links” section:

Part 3.1: Expansion Tank Relocation

Uh oh! An Unforeseen Problem

Initially, I mounted the tank(Amazon) on the left side to avoid interference with my mower’s bagger attachment, but I overlooked a potential conflict with my Cyclone Rake leaf vacuum. Its exhaust port directed hot air directly onto the tank, causing the plastic to deform from the heat.

To fix this, I decided to relocate the tank to a safer position. Fortunately, since the tank was mounted on an aluminum square tube attached to the fenders, I avoided repeated drilling into the mower’s body. I cut a piece of 1/8″ aluminum sheet to fit between the bagger support bars, bending it to create a stable and level mounting surface for the tank. Without a press brake, I improvised by clamping the sheet to a work table and applying pressure—jumping on it worked surprisingly well!

After mounting the aluminum sheet and extending the hose with a barb connector for easier maintenance, I secured the tank in its new position, topped it off with oil, and tested everything. The relocation was a success! The tank is now safely out of my Cyclone Rake’s exhaust’s path, and doesn’t get in the way of my bagger attachment.

Part 4: One Year Update

Time Flies When You’re Having Mowing Fun

It’s hard to believe a year has already gone by. Last time you heard from me, I modified the transaxle on my John Deere D130 riding mower to make oil changes easier—no more removing the transaxle from the machine! It’s been working great, so I thought it was time to drain the oil and take a closer look.

Draining the Oil
After running the mower for just a few minutes to warm up the transmission, I pulled the plugs to let the oil drain. For those unfamiliar with this setup, the transaxle has two reservoirs: one for the gearing and another for the hydro pump. They’re connected at the top but require separate drains at the bottom to get all the oil out.

Surprisingly, even though the oil in the expansion tank(Amazon) looked pretty clean, once I drained it, I noticed fine metal particles floating around. The swirly clouds of metal floating around in the oil definitely caught me off guard—I thought after just one year the oil would be cleaner.

New Oil, New Problems
For the oil change, I used Mobil 1 5w50(Amazon), as I still had some on hand from last year. I pumped the new oil in with a marine engine lower unit pump(Amazon). However, things didn’t go smoothly after pumping in the fresh oil. My cheap torque wrench from Harbor Freight stripped out one of the drain plugs(eBay), and what was supposed to be a quick and easy job turned into a bit of a mess.

I decided to use this mistake as an opportunity to inspect the inside of the trans. It pulled it off the machine and cracked it open on my workbench. After removing the bottom pan, I noticed some metal flakes stuck to the filter screen(Amazon). The magnets(Amazon) were slightly dirty, and there were more metal particles in the bottom of the pan.

I enlarged the original 8mm drain plug holes by drilling them out, then re-tapped them with a 10mm tap(Amazon) to accommodate new 10mm drain plugs(Amazon). While I had this thing open, I decided to take the pump apart and see how things were holding up after having been rebuilt a year ago.

I immediately noticed some subtle circular wear marks on the pump surfaces. It was a bit disappointing, but not entirely unexpected considering the metal particles in the oil and filter. Rather than sending the parts out for resurfacing again, I decided to tackle the fix myself. Using 3000-grit sandpaper(Amazon), I carefully wet-sanded the center case and cylinder blocks to smooth out the marks.

Unexpected Setbacks
While reassembling the transaxle, I accidentally sheared the bypass pin(eBay) when bolting down the center case. This causes the transmission to stay engaged even when the bypass rod is pulled out. After opening the transaxle again, I found the bypass pin was sheared in half. Total bonehead mistake. I ordered a new pin and was back in business after reassembling the pump assembly for a second time.

But the fun didn’t stop there. The new 10mm drain plugs(Amazon) meant my old 8mm hand pump(Amazon) no longer fit, so I fabricated an adapter out of a flange bolt(Amazon) to make it work. This was actually my favorite part of this one year update—it’s always satisfying to create something from scratch!

After putting the mower back together, I ran into another issue: the mower wouldn’t start and kept blowing a fuse. Like really dude!? After some poking around, I found that the backup safety switch wires had been damaged by the drive belt—I must have routed things incorrectly when I put the trans back on. I cleaned up the shredded wires, used heat shrink butt connectors(Amazon) to reattach them, and wrapped everything in electrical tape(Amazon).

Back on Track
After purging the air from the transmission and checking for leaks, I was finally able to get the mower back together and ready to go. The transaxle seems to be running fine, and I’ll probably change the oil again next season to see how it looks. If you are interested in future updates, head on over to my YouTube and hit that subscribe button and notification bell.

Parts & Tools

If you want to check out the parts and tools that I used in this one year update, expand the following “Links” section:

Final Thoughts

The K46 is an interesting beast. It is truly a bummer that it lacks the basic features needed for serviceability. Even with my oil drain/fill mods, the trans is still only partially serviceable because of the internal filter and magnets.

On the flip side, the K46 likely contributes to the availability of a whole category of affordable riding mowers. Mechanically the K46 has a beautifully simple design, all the internal parts are metal, and it’s relatively light weight. You won’t plow a field with it, but you should be able to get many years of use out of it on flat terrain. Maybe a little bit more with my fancy mods 🙂

Hope you enjoyed this series. Follow me on YouTube if you are interested in future updates. I’ll update this post, but generally I’ll update YouTube first. Catch ya in the next one!