The built automatic transmission is a mainstay in the diesel world. At some point, even the toughest factory transmission needs to be reinforced in order to continue the never-ending quest for additional power. But, as this article illustrates, having a built automatic in your truck doesn’t mean it’s exempt from ever requiring a once-over, if not a full-on rebuild, from time to time. Although having to rebuild an “already-built” transmission is something a lot of folks don’t like to hear, in the end it’s practicably inevitable. While a performance transmission can be prepped to handle big horsepower and gobs of torque (and do so extremely well), it can’t handle the job forever—especially if you’ve added power since the last time your transmission builder was in there.

After noticing the unmistakable smell of burnt transmission fluid, a quick check of the dipstick confirmed our suspicions that something was amiss inside our E4OD. Once we pulled the drain plug, it was obvious something metallic was breaking down inside. Following a quick call to John Wood Automotive, one of the premier E4OD/4R100 builders in the transmission world, we had an appointment scheduled to get our slushbox into his highly capable hands.

Such was the case with our E4OD: a transmission built once upon a time at John Wood Automotive. When the old-school four-speed was first put together back in 2011, it was intended for a 400-rwhp OBS Ford. Now, having added 300/200 hybrid injectors from Unlimited Diesel Performance, an Irate Diesel competition electric fuel system, and a Fleece Performance Engineering Billet S468, the same truck sits at nearly 600 rwhp. After more than eight years of trouble-free use and 70,000 hard-earned miles, the transmission had never skipped a beat. However, before pursuing further horsepower goals it was time to: 1.) Find out why the fluid was burnt, and 2.) See how well the transmission’s internals had held up over the years—especially with the near-50-percent increase in horsepower it had seen.

So, with the E4OD pulled and strapped to a pallet at Flynn’s Shop in Alexander, Ill., it made its way to John Wood’s facility in Holtville, Calif., while we caught a plane to join Wood for the tear down. Upon disassembly we discovered that, while nothing had failed catastrophically, there were some warning signs that things were about to go south. Follow along to see what components needed attention, what parts checked out just fine, and which billet upgrades Wood included in our new Street Performance transmission build.

Built by John Wood back in early 2011, the four-speed was initially spec’d to live behind a 400- to 450-rwhp truck. But believe it or not, it had spent the past six years flawlessly dealing with nearly 600 rwhp, 1,100 lb-ft of torque, dozens of boosted, four-wheel drive launches, and being hot-lapped at the drag strip. Throughout this time, it never skipped a beat. Had it not been for the scent of burnt ATF, there would’ve been no reason for us to even think about the transmission’s health. It was shifting and performing perfectly fine before we pulled it.
Upon arriving at John Wood’s Holtville, Calif., transmission shop, he got started by pulling the cast-aluminum Goerend Transmission pan (which the folks at Goerend were kind enough to ship a replacement Duraprene gasket for). Wood then removed the valve body before turning his attention to pulling everything out of the transmission case.
Based on the type of metal flakes he discovered in the pan, Wood deduced that the torque converter was beginning to come apart. The first thing he noticed on the converter (and one of the only things there is to notice until you cut it open) was that the turbine splines for the input shaft were close to stripping out. Wood attributed this weak link to a lack of heat treating of the splines, something all of his converters receive now.
As for the primary cause for the metal debris both in the pan and on the drain plug, Wood explained that the converter clutch or stator might’ve been in the early stages of failure. Back before anyone was making billet stators for Ford converters, Wood used OEM Ford stators in his converters (reason being, you couldn’t buy brand new stators at the time). Long story short, it was anyone’s guess as to how many total miles the stator in our converter actually had on it.
On the lathe and with the old converter cut open, Wood was able to definitively say the converter was the source of the problem. In addition to more signs that the converter had seen immense heat and that the turbine splines were about to strip out, Wood discovered that more than 0.030-inches of material was missing from the converter clutch and that the piston was about to make contact with the front cover. For all intents and purposes, the converter had already failed.
Here you can see that the transmission’s input shaft had been wearing into the splines of the overdrive planetary. Made from 300M billet steel, this input shaft is a strong piece, but its design lacked the OEM-style rolled edge at the end of the shaft’s splines. To solve this problem, Wood not only planned to replace our 300M unit with a shaft made from stronger, Maraging 300 steel, but also one that featured a radius’d edge at the end of the splines.
In this photo, Wood demonstrates just how far the transmission’s input shaft was protruding into the overdrive planetary. Along with the aforementioned Maraging 300 steel input shaft upgrade, Wood let us know that he would be replacing this cryogenically treated cast-steel planetary with a billet one during the rebuild.
After a quick inspection of all overdrive clutches revealed they were practically brand-new, Wood showed us these. Proprietary planetary washers allow him to adjust end play to his tight standards, especially when planetary gearsets or other hard parts are changed. The washers he uses are also heat-treated to resist wear.
According to Wood, the E4OD/4R100’s left the factory with as much as 0.070-inches worth of play in the center support. That’s a lot, and definitely not up to Wood’s standards. Re-engineering the center support, he tightens up the tolerances by machining both sides in order to accommodate snap-rings, which serves to keep everything centered in a straight line within the transmission
To both prep the Low/Reverse assembly for added power and ultimate longevity, Wood addresses several key things. First, with the factory-installed bushings on the Low/Reverse planetary known to break down over time, he replaces the washer with a bearing.
As for the Low/Reverse hub itself, Wood adds a reinforcement ring to the top and bottom of the assembly (shown on the right and left in this photo). The factory Reverse hub is notorious for flexing and even blowing apart under high torque loads (and is an especially common failure in high-powered trucks with big tires). The combination of two reinforcement rings, each TIG-welded to the hub, guarantees this infamous failure never occurs in any of Wood’s transmissions.
Wood’s Low-Reverse hub reinforcement rings are made from Inconel, the same type of material that’s used to make turbine wheels and valves. Inconel is an alloy that’s well-suited for use in extreme environments, and it won’t deform or distort when subjected to excessive pressure or heat.
From the factory, the coast clutch drum’s retaining snap-ring is known to eventually work itself loose and wreak all kinds of havoc within the transmission. This is because the OEM snap-ring is only secured in place via a tiny metal dimple. To permanently solve this problem, Wood machines a groove completely around the coast clutch drum so that the drum itself holds the snap-ring in place. Of course, Wood machines the coast clutch drum to accept a larger snap-ring than stock, for optimum insurance, while he’s at it.
The only signs of wear on the clutches were these faint black marks on the overdrive units. Not bad considering these clutches are downwind of the biggest torque numbers the engine can produce (1,146 lb-ft on its last chassis dyno pull).
To our astonishment, after more than eight years and 70,000 miles of abuse, nearly every clutch disc looked brand new. It’s proof that Wood’s use of select BorgWarner and Alto clutches is intended for maximum longevity just as much as it is about harnessing big horsepower.
When it came time to inspect the transmission pump, a fair amount of damage was discovered on the stator side, as well as on the pump gears. According to Wood, these marks are signs of pump seizure, and he also told us that the front of our transmission had seen between 350 to 400 degrees at some point (most likely from hot-lapping the truck at the drag strip).
Starting with the customer’s good used core or a re-manufactured version, Wood resurfaces every transmission pump on his CNC lathe, whereby 0.040-inches is taken off each side. This is one of the most vital processes in the build, as the transmission pump (along with the case itself) can become considerably warped over time or when it has been exposed to long periods of heat. Once resurfaced, the pump receives a fresh boost valve sleeve as well (for converter lockup)

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