It seems that the TT engines use a much "tighter" plug thread than many others.

I've found quite a few different glowplug brands that won't thread past a turn or two by hand into the head of my TT engines (Glo-devil for instance), but the OS#8 always goes in all the way.

The same plugs thread all the way by hand into my GMS and other engines without any problems at all.

I suspect the TT heads have slightly undersized threads (perhaps hoping to achieve a better gas-seal as a result) and some plug manufacturers produce slightly overseized threads (for the same reason). Unfortunately, when you get the combination of a head that's threaded too small and a plug that's threaded too large -- problems occur.

I just stick to those plugs that actually fit my TT engines and use the others in my other engines.