Technical information

Special Main Shafts

I have developed a pair of special engine mainshafts, initially in response to some badly worn shafts in a pair of my flywheels.

These shafts differ from standard in that instead of being 22mm diameter when they become parallel, they are 25 mm in diameter where they fit into the flywheels and protude at that diameter for the width of the bearing. In addition, they have a flange that prevents them from being pulled out of the flywheel.

This arrangement has several advantages :

The shaft is stiffer.

It means that when a new shaft is fitted to a flywheel, enough material has to be removed from the bore to give a good fit, absolutely square to the flywheel.

Enough material is removed from the flywheel to eradicate the hole form the snapped-off bolt on the periphery of the shaft.

The shaft cannot be pulled (I appreciate that this is not generally a problem with cammies)

A readily availble bearing can be fitted - NF205ETN - this bearing is relatively cheap, available and has an increased duty cycle over the originals. It is 25 mm inside diameter, parallel bore, 50.8 mm outside diameter and 15 mm wide. Though the assembly is 2 mm narrower, the rollers are the same width. A 2 mm shim washer is fitted between the bearing inner and the flywheel.

Naturally, the outside diameter needs reducing to 50 mm but this is a simple job and the oversize OD presents the oportunity to make a sound repair for crankcases where the bearing has come loose in its housing and oversize bearings are required.

After making an initial batch for me, Max Nightingale of Alpha Bearings will supply and fit these special mainshafts. the shafts are about £60.00 each. He is prepared to supply the bearings and grind the outers to size.

It should go without saying that I am using these shafts successfully in my Mkviii.

Rob Drury   November 2006

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