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Here is the modification for the idler gear. To use the 305 timing cover with the gear setup you have to shorten the length of the idler gear nose or snout as it sticks out too far. The 478 timing cover has a large dimple cast into it to house the snout. This idler gear is pretty over-dimensioned. it gets fed with a continuous supply of oil from the main oil galley. I figured a little less bearing surface wouldn't hurt a thing. The idler gear assembly was still as tight as the day it left the factory.

After measuring the depth of the 478 cover I chose to shorten the snout to fit the 305 cover. The depth of the cover is about 36mm (1.42"). The stock idler gear is about 56.5mm (2.23") including the bolts to retain the front plate.

I went ahead and indicated the idler gear shaft, which was then shortened by about 9/16". I also polished the surface. I tapped the old oil supply hole and screwed in a set screw is retained with LocTite. A new oil hole is drilled opposite the old hole to supply the oil. I then made a tool to press the bronze bushing into the idler gear 9/32" (half the distance) deep. This lined up the oil groove in the bushing with the new oil hole. The existing 5/16 UNC holes were tapped deeper and a new front plate was made, with uses countersunk fasteners to reduce the depth. It is coated on the inside with LocTite gasket sealer to cap off the oil galley. These bolts are also secured with Loctite. I tried to reuse the oil front plate but it was too hard to machine.

The idler gear was then indicated and shortened in the lathe by the same amount, keeping the stock axial play in the setup. The total length of the assembly from the engine to the front plate is now a little less than 1.42". There's always a little variation in the casting which means I have to grind a little clearance into the timing cover, but that is no issue since it is very little material. I did not want to shorter the idler gear any further because I would intersect the old oil galley that I now capped off with a set screw.

Machining was surprisingly easy. The idler gear mount is made from a good grade of cast iron, and the bearing surface is slightly hardened. The gear is made from cast steel which also machined beautifully.