I've seen more than 1 set of plugs destroyed due to over torquing them into the head. The only accurate test for glow plugs is a amperage draw test which requires a amp meter capable of handling 30amps.
The plugs should start cold around 21 amps and fall to 15-12 in 15 to 20 seconds.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.