That's what I originally thought I needed to use with the MZ "R" head, but the piston hit the ground strap and closed the gap. The NGK BPR7EIX was just that little bit too long.
The E refers to the length of the threaded section.
I think they should have made the head thick enough for the E length plug because a bit extra thickness there would make it easier to unscrew my compression tester.
Anyway as it is, with the BPR6HIX there's some threads inside the head that aren't covered by the plug, so they're bound to hold onto sooty deposits which I hear can glow and be a cause of pre-ignition or whatever it's called.
It's like they made the head just in between the spark plug sizes so that it is impossible to get a plug that's exactly right.
So choose carefully. Before buying a new plug with the E length, check the clearance under the ground strap with your H length plug. Use some soft putty like Play Dough maybe or make some dough? Idk.
As you can probably tell I forgot what the measurements are that the H and the E refer to. I think H is 10mm and E is 14mm long, but I'm not completely sure so you best Google a chart of NGK plug codes.
Anyway I don't think a too short plug would prevent starting. A too long plug would, if the gap is squashed closed by the piston.
If you have difficulty turning over the engine then maybe that's just the high cranking pressure?
If the engine will not turn over even when the plug is out, then maybe you don't have a thick enough head (or base?) gasket and the piston isn't clearing the head at all.