Due to the recessed exhaust valve and generally flat chamber surface, the easiest way to significantly reduce chamber volume (more than a head or block mill) is to add material to the piston dome directly below the exhaust recess.
IMHO the many different 18 bolt head castings may not have the exhaust pockets exactly matching as to location, shape, and depth. From the photos and drawings I've seen, the bump is an irregular rectangle, but some completely fill the recess (but not to full depth), some are taller, some have a "flame notch" to address HRL's excellent point of flame travel interference.
What I don't know (don't laugh!):
Are these bumps made larger than needed, and supposed to be hand-fitted to the actual chamber?
A bump 1/2" tall (example only) will begin to enter the recess 37 degrees BTDC, where does the recess volume (mixture on the compression stroke, mixed gas on the exhaust stroke) exit?
My guess: the exhaust valve will be partially open during the overlap period near TDC with any hot cam, especially with late exhaust closing. For highest CR change should the bump almost touch the valve?
