Technical Articles

Jim’s Twin Cam Crankcase Upgrade

FOR SOME unknown reason, at the end of 2002, Harley-Davidson dropped the bullet-proof Timken left-hand main bearing assembly which they had been using since 1955, and replaced it with a weaker straight roller main bearing.

If you’re building a hotrod Harley using a late-model Tim Cam motor, you should considering converting back to the old setup.

Jim’s Machine produces an elaborate kit to convert 2003 and up Twin Cam crankcases to the earlier setup.

Jim’s Machine Twin Cam Crankcase Bearing Conversion Kit.

With this kit the weak, left-hand straight roller main bearing H-D #24604-00D is upgraded to the far more robust Timken bearing assembly H-D #9028. The new bearing assembly rides in a precision machined chromemoly steel insert, far stronger than the aluminum surrounding the stock Harley bearing.

This conversion is highly recommended when increasing the capacity or horsepower of Twin Cam engines.

The left-hand crankcase with the main bearing removed, washed and ready to begin the procedure. Note: cast into the crankcase the H-D 24612-03 number indicating the 2003 and up engine, in this case a Twin Cam B.

Jim’s has given this conversion a lot of thought and managed to design the insert as a predominantly press together setup. Most of the components supplied with the kit are jigs for precise alignment and support during the pressing stage. The only machining involved is for the retaining screws and oil passages and there are jigs supplied for this also.

The first step is to install the Bottom Locator Plate #959-3 into the existing bearing boss. This Plate is engraved with all the necessary information and when installed correctly forms the base and pilot for pressing in the insert.

Bottom Locator Plate in place torqued into the left crankcase with the Allen screws supplied in the kit.

Insert being cleaned with lacquer thinners.

The Insert being fitted to the Press Plate #959-2. An 1/8” dowel pin ensures that the insert is correctly indexed.

The Press Plate with the Insert in place and Press Plug installed is loaded and ready to go.

Just before the Insert is pressed in, with everything ready and the press set to the correct travel, the large diameter of the insert is very carefully coated with press fit lube; the small diameter with Loctite.

In it goes. With the Insert completely pressed home the Press Plate now forms the drilling jig for the retaining screws and the oil supply and drain.

A close-up view of the Press Plate shows the incredibly high standard of the components. The three slots machined into the plate ‘oil feed’, ‘mounting holes’ and ‘oil drain’ are used for setting the drill depth for the various operations; all necessary pieces are included in the kit.

With the Press Plate still in place the holes can be drilled for the retaining screws. At the same time, the oil feed and drain holes can be drilled to their exact depth to intersect with the respective passages.

After tapping the threads for the retaining screws, the left-case is now ready to be washed and have an H-D #9028 bearing installed. This assembly comes with the correct preload set at the factory. All further assembly is now routine Twin Cam workshop procedure.

This completes the conversion leaving the case modified to a very high standard. The H-D #9028 bearings can be removed and replaced whenever engines rebuilds are carried out without any damage to the aluminum crankcase.

Redgrave Motorcycles stocks this kit for workshops or individuals who would like to undertake this conversion; they will also carry out the work on your crankcases if required. For any further information regarding this kit or conversion feel free to give Redgraves a call; they would be happy to answer any of your questions.

Tech article by Richard Nicholls @ Redgrave Motorcycles: 02-9484-9900.

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