TR2/3/3A Spark Plug Gap?

Why is the TR3 plug gap .032 and the 4/4A gap .025? .....and if mods are added like using the the longer 4A intake manifold and exhaust header, does that change which gap to use or is it just a matter of experimenting?
Also...I think I read somewhere that the motor would perform better if the timing was set at 3/8" before BTDC? Is that 4 degrees BTDC or 8 degrees? OR is there a sweet spot in regard to timing for performance or are there just to many variables to say?:emmersed:

My TR2 and TR3A are/were gapped at .025. The gap is based on the power available from the coil. 3/8” is roughly 4degrees. But final timing should be determined by road testing.

The TR2/3 manual does say .032. I wonder if there was a service bulletin at some point. I've always used .025 also but I don't remember where I got the number. Perhaps from the plug manufacturer's guide.
Tom

Yes, there was a service bulletin, fairly early. I've posted it before (but maybe not on this forum), I'll see if I can find a copy.
.025" is in the owner's manuals.

Here it is, Feb 1956


PS, 3/8 translates to roughly 8 BTDC. Whether you can run that much without knock depends on a great many variables, even things like altitude and climate. Whether more is better is another question with no clear answer; I've seen tests on some engines that show better power without running on the edge of knock.

There's also the chance that you might not hear the knock under some circumstances; like high speed cruise. I never heard this one knock:


So it seems to me that backing off a bit is probably a good idea, overall. Assuming you want the engine to last, anyway; racing would be a different matter.

My book said 4 degree BTDC so I started there. Then I advanced it 2 more degrees and it ran better, so I advanced it 2 more degrees and it runs great.
No noticeable knock. I might try 2 more degrees some day.