Joined: Jan 2010
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From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
For the alloy crankbolt covers, what you want to do is get penetrant working around the edge and also inside of the covers.
After some penetrating time, get a wooden dowel about 3/4" diameter and use this as a drift with a hammer. Impact the area around the outer periphery of the cap, moving around in circles. The cap should then unscrew with only modest torque on a good 5mm Allen wrench.
You could also use a thick leather toe strap as a buffer, allowing direct impacts using a small-headed, lighter hammer. The impact momentarily unloads the threads and breaks any corrosive bond as the cap flexes slightly.