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Yukon 5.13 gears in Factory M210 Front Drive Unit: It Fits!!


I Started This Gangsta Shit
So Cal Broncos OG
I've been gathering up parts to do a drivetrain overhaul on my 2-door Base non-Sasquatch, which currently has an M190 front end, open diff of course, and 4.46 gears due to the manual transmission.. So I bought a used M210 FDU and M220 rear axle assembly that had come out of a Badlands Sasquatch (thanks @KermitBronco !), so they already had the electric lockers. The FDU still had the factory 4.70 gears but the rear end had 4.88, so they were mismatched anyhow and I was going to have to re-gear at least one end to match them up. But part of this upgrade also entails 37" tires (currently running 35's) so I preferred to re-gear both axles to an even deeper gear.

The conventional wisdom on the Internet has been that any gears lower than the factory 4.70 simply would not physically fit into the factory M210 gear case. As is often the case on the Internet, this was first said, and then repeated, by people who carry some authority on the subject, in a YouTube video. So voila, instant fact. But I'm always one to question authority :)

I'm pretty good friends with the guys up at Yukon due to my industry connections so I called up there and asked if they knew anything about it. They said of course, they actually have gears specifically designed for the Bronco M210, all the way down to 5.38. I chose to go with 5.13's (I'll explain why later), so I ordered R&P's and install kits for front and rear.

Ford Bronco Yukon 5.13 Gears in Factory M210 Housing: It Fits! 382599789_10229715584370617_6537263925449717597_n

Ford Bronco Yukon 5.13 Gears in Factory M210 Housing: It Fits! 381388355_10229715584570622_456392232240308065_n

So yesterday I had a chance to load everything up and haul it over to my friend Mike Duncan's shop (4XDoctor) in Riverside to set these up and independently verify, for myself and for the rest of the Internet, that yes indeed you can re-gear the factory Bronco M210 to lower than 4.70. Now, Mike is mostly a Jeep guy doing straight axle gears out of Jeeps and Early Broncos, but he'd done a few IFS front drive units so he was somewhat familiar with them. I'm not going to say the Bronco M210 FDU didn't throw him a few curveballs. But he figured it out and got it all set up. The final result is that the gears fit fine, the setup went fine, all of the bearings, shims, crush sleeve and seals that came in the Yukon install kit were correct for the application (not simply re-purposed from the Jeep JL M210 as some have found with other install kits). Basically, once we figured out how everything comes apart and goes back together, everything went smoothly and fit just right.

Ford Bronco Yukon 5.13 Gears in Factory M210 Housing: It Fits! IMG_6128-1200w

Ford Bronco Yukon 5.13 Gears in Factory M210 Housing: It Fits! IMG_6138-1200w

Ford Bronco Yukon 5.13 Gears in Factory M210 Housing: It Fits! IMG_6164-1200w

Ford Bronco Yukon 5.13 Gears in Factory M210 Housing: It Fits! IMG_6166-1200w

Ford Bronco Yukon 5.13 Gears in Factory M210 Housing: It Fits! IMG_6181-1200w

So the FDU is now buttoned up and back sitting on the bench in my own shop ready to be installed. It got late on us so I left the rear end with Mike to do at his leisure and I'll pick it up later this week. But there was no question about the M220 so I didn't see the need to hang around for that. I won't actually get these installed into the Bronco for at least a month though, as I'm waiting on some other stuff to come in so I can do everything at once. So I can't give you any seat of the pants or performance results yet but I can only verify that yes the Yukon gears do fit and everything in their install kit fits and works as intended.

So, about that Internet myth. I believe there are two things at work there. First, it was a claim made by the guys who have a vested interest in selling the very expensive complete "Ultimate" FDU. So there was an incentive to simply say their product was the only way you could get deeper gears into a 6G Bronco. Second, I believe the "deeper gears won't fit" claim was based on an early state of affairs, in which the only M210 aftermarket gears were specifically made for the Jeep JL version of the M210, and perhaps those did indeed not physically fit, at least back at the time that claim was made.

But the Nitro 5.29's have been out for a while, and those were just verified for fitment on the Bronco6G board a couple of days ago. I don't know how long these Yukon's have been on the market. But it looks like we now have a couple of options for deeper gears in the stock M210 FDU. So we can put that Internet myth to bed.

As an aside, Ford themselves now offer 5.13 and 5.38 gears pre-loaded in complete FDU assemblies through Ford Performance, in what appear to be factory FDU housings (not a re-branded Dana Ultimate housing). And of course Dana makes those too. But as of the last time I checked, neither Ford nor Dana offer those Ring & Pinion sets by themselves without having to buy the complete FDU.

Lastly, some have asked why I chose 5.13 gears, why not go deeper to 5.29 or 5.38. I came up with it after playing with Grimmjeeper's excellent gear calculator. I remember when I first got my Base-non-Squatch with MT, and it was on the stock 30" pizza cutters. To me it felt like it was too torquey for comfortable street driving. It felt like 32" or maybe 33" tires would have been perfect for that drivetrain. When I went to 35's it felt slightly under-geared. The stock final drive "crawl ratio" for my Bronco was 79.92. Going to 5.38s would have put me at a crawl ratio of 96.41 on 37" tires, which is slightly lower than the stock Sasquatch crawl ratio of 94.75, while highway RPMs at 75mph would be 2367. In the stock congifuattion on 30" tires, its was turning similar rpms at highway speeds (2420 at 75mph) and I always felt like it was winding out a bit too high for me. With 5.13's on 37's I'll still get a very respectable crawl ratio of 91.93 and turn 2257 rpm at 75mph on the highway. So it seems like a decent compromise for a Bronco that sees quite a bit of both freeways and off-road.

Who knows, maybe I'll regret not going with the 5.38s. We shall see!
UPDATE: We just published an episode on the 5.13 gear install for our Horsin' Around YouTube channel. Please check it out :)

So, back in the day when I was young and had hair and had one of those OTHER kind of 4x4s, we talked a lot about matching gear ratio and tire size. Also how to adjust the speedo when we changed those things. Sounds like we are heading back to the future here. :) BTW, I'm just waiting for my 35's to wear out, and then I want to make the move to 37's. But the cost of doing that right is starting to really climb! Wheels, tires, a lift, and now gears. Not to mention dealing with the tie rod situation, and maybe the steering box! Lucia's probably going to kill me in my sleep! LOL
Another piece of the puzzle showed up today. Let's hope they're nicer on the inside than they are on the outside :)


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