Building the Bulletproof Transmission, Secrets of the Pros

For turbocharged motors, and more heavily modified engines more must be done, involving going inside the transmission.  The first issue is the weak case.  The transmission case cracks in the bellhousing side in the ribbed area near the axle hole.  The weak area is fixed by cutting a small piece of aluminum to fill in the ribbed area bridging the axle hole and tig welding it in place. The welding should be done in skipped fashion to keep warping at a minimum.  The bellhousing should be lightly surfaced after welding to prevent leaks.

The Nissan Sentra SE-R tends to break the transmission case on the bellhousing near the axle hole.  To prevent this a piece of aluminum is cut and tig welded to this area.  After welding the case, it is resurfaced.  Some have filled this area with JB weld with good results as well.

The next series of tricks greatly help a Nissan FWD transmission but they can be done to help strengthen any transmission and are a good idea no mater what sort of car you drive. These steps need to be done in the following order for them to work correctly. Deburring the gears is the first step.  Deburring is a light polishing of the gear tooth tops and sides.  The polishing removes stress risers, sharp areas of stress concentration where cracks can form.  The idea is not to round off the gear teeth but to break the sharp machined edges and give them a light polish. This also reduces the chances of the teeth digging into each other and causing damage that can cause the teeth to fail.  A bench grinder with a scotchbrite red or green wheel works really well for this task.

A scotchbrite wheel is used to polish the tops and side edges of the gear teeth.  This gets rid of stress risers on the teeth and helps prevent the teeth from digging into each other causing failure.

After deburring, the gears should be treated to help improve fatigue strength.  The gears should first be cryogenically treated to stress relieve them and convert austenite (a softer phase of steel crystalline structure) within the steel of the gears to the harder crystalline structure martensite. Cryogenic treatment involves cooling the parts to the temperature of liquid nitrogen then warming them to close to the tempering temperature which is around 350 degrees several times over about 72 hours.  Cryogenic treatment generally can improve the fatigue strength of steel gears by over 100%.  Cryo treatment is relatively inexpensive, usually the gears and shafts of a transmission cost around $100 to process.

After cryogenic treatment the gears should be shotpeened by a special two step process intended for hardened steel. Shotpeening is the bombardment of the surface of the steel with hard steel shot at high velocities.  This refines the grain of the surface of the metal to a depth of around 2-3 thousands of an inch and imparts a compressive stress on the surface. This creates a skin on the surface of the part in which cracks have a hard time developing.  Shotpeening can improve the fatigue strength of a steel part over 100%. The stock gears are nitrided, a process that leaves a hard wear resistant skin on the surface of the gear.  Because of the hard nitrided skin, the gears must be hit hard with hardened steel shot of R65 hardness.  The shotpeening machine must use an almen intensity of .025 or higher.  This is usually the maximum velocity of most shotpeening machines.

Shotpeening leaves a pebbly texture on the gear teeth.  In addition to strengthening the parts, the texture helps retain a protective film of oil on the gear face.

A two step shotpeening process should be used.  First the gears and shafts must be hit hard with a large 330 size shot.  This really helps establish a compressed layer on the tooth faces and tops.  Then the gears and shafts should be blasted again with smaller 220 shot.  This gets into the roots of the gear teeth and the base of the shaft splines.  Shotpeening is also relatively inexpensive; the two step process mentioned here usually costs under $100.