These were the biggest engines that could built at the time.
Thay had 3 13/16" bores and 4 3/4" strokes. They pulled like
crazy on the low end and would keep right on pulling until the
John was always of the opinion that bigger was better. Having
reached the practical limit to bore size with the stock H-D
heads and crakcase he built his own cone case with a higher
deck and a much larger cylinder spiget and offset lifter blocks
(exactly as Murch and S&S would later do.) The same with the
heads. He basically made a larger version of the shovelhead
with the intake and exhaust ports opposite each other. This
way it was easy to mount two carbs and to make do with a single
casting for both heads.
The problem with the Harman engine was that making it larger
amplified all of the problems inherent in the shovelhead. It
had an even larger hemispherical combustion chamber so the 4
1/4" pistons needed high domes which made them very heavy. The
large surface area of the zero % silicon pistons soaked up a
lot of heat so they had very poor thermal dimensional stability.
When combined with the heavy cast iron cylinders (which were
slow to heat up and slow to cool) it was impossible to maintain
a reasonable piston to wall clearance. The heads used stock
shovelhead rocker boxes. The short levers on the stock shovelhead
rocker arms made it impossible to get enough lift to use the
engines very large valves efficently.
A few more comments on the pistons. Some were made by ForgeTrue,
some by Vinolia and some by Aries. All of them slapped like
crazy when they were cold and stuck hard when they got hot.
Most weighed over 900g each with pins and rings or almost double
the weight of a stock EVO piston assembly. When combined with
the heavy Corrillo rods that John specified the vibration was
absolutely unbelievable. It was scary in fact; you just knew
the engine was going to come apart any second. Since bearing
loads increase at the square of RPM, the stress on the lower
end was the ultimite limiting factor with these engines.
Regards, Alan Sputhe