The 1926 owners manual list the spark plug gap at .030". That is for the type 'B'  AC spark plug. Regardless of the plug you use it should be .030" because the gap is related to the strain on the coil. 1924 was .020", 1925 - early '27 was .030". Late '27 & '28 used a .025 gap per owners manuals.

I use a hotter plug (Champion W89D) in my 1928 AB Canopy Express. The points are set at .018" and the spark plug gap is set to .035".

I've been using these plugs for the last 12 years (3000+ miles) and they perform well.

One thing I've found to be important is setting the timing correctly.

This is how I set the timing for the 1928 engines in my two trucks:

1. Set cylinder one at top dead center (TDC) on the compression stroke (look for the U|C mark in the inspection hole in the flywheel cover)
2. Set the timing lever to full retard position (pulled clockwise towards the driver)
3. Set the position of the distributor so that the points are on the verge of opening for the cylinder-one wire

Then, whenever I start my engine, I always pull the timing lever to full retard so that the spark happens at or just past TDC. This prevents the engine kick back.

A couple of moments after the engine starts, I shove the timing lever to full advance and leave it in that position while driving.

Both of my trucks usually start within the first two revolutions.

Cheers, Dean