Champion REM40E massive electrode .016 - .021 thousands of an inch. Use new copper crush washers on plugs & anti-cease on threads; not first 2 threads.
TEMPEST Spark Plugs. 5/8" x 24. UREM38S Fine wire Iridium electrode gap at .016-.021 thousands.
That's what I use. They do much better at resisting lead deposits. They were designed as an alternative to the 40's that offered better resistance to lead deposits.
In our piston Belll 47 helicopters Tempest get 500 hrs or a little better Champion close to 300hrs.
600hrs a yr for the last 4 yrs its been consistent between the two.
As others have said, use the UREM37BY plugs if you use 100LL fuel AND have 5/8" spark plug wires. If your ignition harness is the 3/4" all weather plug wires, then UHEM37BY would be necessary but I am thinking that is not an option or not available.