Since the mid '50s Harley has used a pair of back to back Timken taper roller bearings as the sprocket shaft (drive side) crankshaft main bearing. This bearing set up has always proven to be immensely strong and reliable. It takes the side thrust of the crank and carries most of the load that the engine produces.
Because of the combined width of this pair of bearings it also supports the crankshaft very well and helps keep the flywheels in line. When the Twin Cam A was introduced in 1999 for the FL and FX models it utilized the Timken bearing on the drive side and a new, large diameter, roller bearing on the pinion (cam) side.
Harley's change to a regular roller bearing for the '03 model year was not the best engineering decision they've made! We modify the later Twin cam cases to accept this earlier bearing by using a tool steel insert. This gives the crankshaft much greater support.
The Twin Cam B motor for the Softails, introduced in 2000, also used the Timkens, but had a smaller outside diameter pinion bearing to give more room for the chain drive to the balance shafts.
In 2003 this later bearing was used on both the pinion and sprocket shafts on both the A and B motors in order to reduce assembly costs. (The Timken bearings have to be carefully set to ensure the correct end float). Due to various issues this new roller bearing had four further upgrades between 2000 and 2006. It was replaced with an -07 part number version.
Whilst there have been reported failures with this type of roller bearing, (it does not take side loads very well) we haven't seen any yet. However, another pattern has become apparent, and that is crankshaft run out. The earlier Timken bearing engines have far less instances of the flywheels shifting in use. When the wheels shift excessively the oil pump and cam support plate fail.
All of our larger capacity performance upgrades which require the engine to be disassembled are converted back to the Timken bearings. We also recommend that this conversion be done any time there is cause to strip any of the '03 up Twin Cams. This is useful extra insurance.
We carry out this conversion by press fitting a tool steel insert into the left hand case, and machining the case for a bearing oil feed, oil return, and 6 retaining screws. The original crankshaft inner bearing race is removed with a hydraulic puller, then we install the Timken bearings and set the crankshaft end float using the same proven method and tooling as the earlier engines.
Read more in our Timken Bearing Conversion Technical Article