This is a common failure we see when the incorrect O-ring is installed on an Allison Transmission filter housing.
What Happens When the Wrong O-Ring Is Installed on an Allison Transmission Filter Housing
On older Allison transmissions, a square-cut O-ring is required when servicing the transmission. The problem is that this square O-ring does not come with the newer, updated Allison filter kits. Those kits only include rounded O-rings, which are designed for newer control modules.
Square-cut O-ring (left) and Rounded O-ring (right)
The old-style filter housing that uses the square O-ring is part number 29507434. The updated filter housing that uses a rounded O-ring is part number 29524447.
When a rounded O-ring is installed in an application that requires a square O-ring, the O-ring becomes over-compressed. That excessive compression transfers force directly into the filter housing ears, which eventually causes them to crack or break, as shown in firstImage.
Why the Updated Filter Housing Causes Problems on Older Units
It’s important to note that you cannot use the updated filter housing (part number 29524447) with an older-style control module.
On newer control modules, the rounded O-ring sits inside a machined groove, which properly supports and contains the seal. Older control modules do not have this groove. Instead, they are machined flat and are designed to seal against the square O-ring used with filter housing part number 29507434.
Because of this difference:
- The rounded O-ring relies on a groove
- The square O-ring sits flush against the control module
Installing a rounded O-ring or the updated filter housing where a square O-ring is required leads directly to housing damage.
Identifying the Correct O-Ring Surface
An image of old style filter housing with square o-ring groove
Here is an image of the new filter housing that requires round o-ring.
On older control modules, you can clearly see a square machined surface where the square O-ring rides. This is a critical detail that often gets overlooked during service.
If the surface is flat and square:
- You must use the square O-ring
- You must use the old-style filter housing (29507434)
- Do not install the updated rounded O-ring or updated housing (29524447)
Filter Housing Recommendation
Older Allison units can be updated to the newer filter and control module design, and this is often recommended when appropriate. However, mixing old-style control modules with new-style filters, O-rings, or housings will lead to failure.
If you are ever in doubt:
- Verify the control module style
- Verify the O-ring shape
- Verify the correct filter housing part number
- Do not assume the new kit applies to older units
If you have questions or need help identifying the correct components, you can always contact Reaction Force before installing parts.