Cracked Magnesium Hubs

What to look for:

By Al Buehner

Photos by Lee Buffenmyer & Al Buehner

Originally printed in issue #69 of Still….Keeping Track

Alloy front hubs were installed on the Penton motorcycles from 1968 thru 1972. In the later part of 1972 and starting with the 1973 model year bikes, the alloy hubs were replaced with magnesium hubs.

From time to time I will get a call from someone looking for a new hub to replace one with a crack in it. The purpose of this article is to show you where to look for cracking in the hubs, be it on your bike when you are doing your maintenance checks or when shopping for a used hub or wheel assembly, or looking at a motorcycle to purchase.

The most common request I get is for a replacement for the black magnesium front hub where the cracking is on the inside around the steel sleeve where the bearings go. This was a problem that KTM became aware of because a replacement front hub was produced (part no. 51-09-010-400) which has a thicker magnesium casting around the bearing sleeve.

Photo showing a crack along the outside facing of a hub
Photo showing a severe crack on a front hub from the brake lining to the outside edge
Photo showing a crack running along the outside ribbing of a front hub
Photo of a rear conical hub showing a crack at one of the sprocket mounting bolt holes