They will definitely grip an allen head; I've done it several times. Just seat it with a few hammer blows to help it find grip, and the come right out.

__________________
- K.C.Adams

'77 300D Euro Delivery
OM617 turbo / 4-speed swap
404 Milanbraun Metallic / 134 Dattel MB-Tex

Current status:
* Undergoing body work

My '77 300D progress thread