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How To Repair A TH400 Transmission With A Cracked Bellhousing

Posted in How To: Transmission Drivetrain on May 22, 2018
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As robust as the GM TH400 transmission is, years of aggressive use will still take their toll. High torque, low gearing, and off-road romping can eventually manifest in the form of a cracked bellhousing.

Dating back to the 1960s, the TH400 was ahead of its time in many ways, but also ahead of the modern technology called FEA (finite element analysis). FEA allows engineers to better understand the stresses a part will endure, where unnecessary material can be removed, and (even more important) where material should be added. The bellhousing on the TH400 is a bit on the thin side and prone to failure.

While a cracked housing may put an end to your day on the trail, it certainly will not put an end to your transmission case. Repairing a crack can be accomplished one of two ways: by TIG welding the crack, or in more severe cases, such as ours, by cutting off the original bellhousing and installing an aftermarket unit.

In the world of drag racing, cracked factory bellhousings are a well-documented problem and aftermarket solutions are plentiful. When our TH400 bellhousing opened up, Grand Canyon–style, we ordered up a J.W. Transmission Ultra Bell from Summit Racing Equipment, broke out the cutoff wheel, and promptly made a mess on the garage floor. Even if your transmission is not broken, an Ultrabell is a great upgrade in strength and is available for a variety of engine bolt patterns, allowing you to mate a TH400 to a non-GM application.

Chopping up the factory TH400 case sounds intimidating, but it’s not. Retrofitting an aftermarket bellhousing takes under an hour. And for $328.05 it’s a heckuva lot cheaper than a new transmission.
Get a look at that crack! What likely started out as a fractional fissure has since opened up into a canyonesque crack. This transmission case has seen more than its share of abuse, and it shows. In addition to the crack, one of the bolt ears is sheared.
Safety first! Cutting off the old bellhousing requires a lot of time with a cutoff wheel, so ear/eye protection is important. An ungodly amount of fine aluminum dust is produced as well, so a mask is highly advisable.
Our first cut was to remove the sliver of cracked bellhousing. Mostly it was an act of revenge, but removing it provided a window to better see the area being cut. We left the cut shallow on this pass, leaving a wide margin for error. Make sure to cover the transmission’s input shaft with a towel or plastic bag to keep shavings out.
J.W. provides rough measurements for where to cut the case. For a TH400, the cut comes in at 17 3/4 inches from the back of the transmission case (with the transfer case removed). Make a mark with a Sharpie every few inches, then connect them to form the cut line.
Follow the cut line with the cutoff wheel of the angle grinder. When getting close to the oil pan rail (arrow), switch to a reciprocating saw. This allows a slower cut and more control. Overcutting the pan rail will ruin the case.
Next, switch to a flap wheel and chase your original cut. The idea is to get the cut closer to the line you drew. Take some time, removing sharp edges and contouring the case as much as possible.
In order for the new bellhousing to fit, you will need to grind the case until it barely overhangs the oil pan. Again, don’t cut too far.
These six bolts hold the transmission pump to the case. They are being repurposed to also hold the new bellhousing to the transmission. Remove them with a 9/16-inch socket, but keep a shop towel handy because the bottom two bolts will puke fluid.
The kit comes with longer, 5/16-inch, Grade 8 bolts that replace the original pump bolts. The O-rings install on the back of the bellhousing and seal against the face of the transmission pump.
An easy trick to get the O-rings to stay in place while you install the bellhousing is to place a dab of petroleum jelly in their respective bores.
The bellhousing slips right onto the front of the transmission pump cover and locates via the bolts. There are actually eight bolts included with the kit, but our case was only tapped for six. Six is more than sufficient for our application, but the case can be tapped for all eight.
A dab of red Loctite on the threads and Teflon sealer on the washer will keep the bolts snug and leak-free. When all the bolts are installed fingert-ight, torque them to 20 lb-ft in a circular pattern.
Not that we plan on collecting timeslips, but the SFI rating on the case is good insurance against bodily harm should anything ever go wrong.
Here is our completed case ready to be reinstalled in the engine and recoupled to the transfer case. On second thought, a little degreasing and a quick rattle-can paint job might be in order first.

Sources

Summit Racing
Akron, OH
800-230-3030
http://www.summitracing.com
J.W. Performance Transmissions Inc.
Rockledge, FL
321-632-6205
racewithjw.com

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