How To Repair A TH400 Transmission With A Cracked Bellhousing
Divide & Conquer
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.