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Call or Text (352) 722-5295Allison parts by job on the Sierra HD
Think in jobs rather than aisles. A leak job means cooler lines, pan gaskets and reseal kits. A service job means Dexron-VI, the TF920 sump filter or the TF950 spin-on, which Allison also sells as 29539579, on the 50,000 mile normal or 25,000 mile severe schedule. A capacity job means a deep pan with the correct filter and additional cooling, on top of the auxiliary external heavy-duty transmission oil cooler the HD Sierra already came with. A strength job means a converter, flex plate, valve body or clutch and shaft parts. The parts categories themselves are short: converters, flex plates, valve body, cooler lines, filters and PTO covers.
Where these transmissions give trouble
Cooler lines first, because they leak at the factory crimp where hose joins hardline and have done it on trucks with less than 25,000 miles, with heat from the nearby exhaust making it worse; you will see red ATF on the frame and crossmember and feel flare shifts as the level drops. Then the converter, which is the part that notices added power: cracked stator, ballooned front cover, and metal in the fluid afterward. Then age, in the form of valve body valves seizing in worn bores, pump gears losing efficiency and worn babbitt bushings, felt as soft shifts, quick downshifts under load and a delay into gear. Slip in the C3 gears and limp mode in third generally arrive with power or heavy towing behind them.
Stock replacement vs. upgrade
For a stock Sierra, replace with equivalent or updated OE pieces: updated cooler lines or braided lines with flared unions, correct fluid and filters, a good pan gasket. For a truck that tows or makes more than stock power, the upgrade list is specific: TCM calibration before any hardware, then a billet-cover multi-disc converter, then valve body work. Internally the parts that hold up are a billet C2 hub, a billet P2 planetary, a PTO cover that feeds oil to the C3 clutches, hardened pump gears, bronze bushings and genuine Allison solenoids. The stock unit itself runs out around 430 to 450 rwhp and 850 to 900 lb-ft.
What we carry
PPE supplies the broadest selection: billet input, intermediate and output shafts, a billet C-2 clutch hub, C5 clutch kits, Stage 5 and Stage 6G transmission kits, valve body recalibration and pressure calibration kits, deep and low-profile aluminum pans with matched filters, pan gaskets, bar-and-plate and performance coolers, a transfer case brace and gasket, and a performance transmission mount. BD Diesel brings six-speed valve bodies plus cooler and filter kits, with transmission hard parts from ATS Diesel Performance and Fleece Performance Engineering. Seals and service parts come from Merchant Automotive and Allison Transmission, cooling from Mishimoto pans, universal coolers and a fan-equipped cooler plus XDP's direct-fit cooler and flex plate, deep pans from Mag-Hytec, fluids and additives from Hot Shot's Secret, and fasteners and small seals from Dorman and ACDelco.
Fitment notes
Match the filter to the sump, deep with deep and standard with standard. Six-speed valve bodies and shift kits for 2006-2010 are cataloged separately from 2011-and-up, and the shift kit only ever covered 2006-2008. The 2011 Sierra HD is a different truck underneath, so treat 2011 listings as a separate platform. Verify by VIN before ordering, or let us check it for you.
Frequently asked questions
Will a 2011 valve body or shift kit work on my 2010 Sierra?
No. Parts for the 2006-2010 six-speed are listed separately from 2011-and-newer, and the shift kit was only produced for 2006-2008, so a 2009 or 2010 truck goes the valve body route. The 2011 HD trucks changed to the LML with a new frame, suspension and driveline, which is why the catalogs break there.
How do I stop the ATF leak at the cooler lines?
Replace the lines rather than trying to seal the crimp, since the failure is at the factory hose-to-hardline joint. Updated OE lines or braided aftermarket lines with flared unions are the common fix. Check the fluid level before driving it, because a low Allison will run hot and flare its shifts.
What is worth upgrading before I add power?
TCM calibration comes first, because limp mode locks the transmission in third on excessive slip, heat or load, and an ECM-only calibration gets there fast. After that, a billet-cover multi-disc converter, then the valve body. The stock unit is good for about 430 to 450 rwhp and 850 to 900 lb-ft, so if you are staying under that, strong maintenance and cooling do more for you than hard parts.
Not sure what fits? Call the shop at the number in the header, or use the fitment checker on any product page. We install everything we sell.