I still think the tech was wrong. Pitch is a measurement that involves two planes. Your roof of your house might have a 4 in 12 pitch meaning the roof height changes 4" for every 12" of horizontal length. A 2-1/2" pitch mentioned by itself means nothing since there is no ratio, and I suspect the tech was probably just referring to the total drop of the blade regardless of the length. Really what matters on how well the fan clutch works is the combined resistance of all the factors of the fan blade -- diameter, depth, width of individual blades, and finally the shape of the blades with some being a simple flat surface and others having a considerable curve to them.
Many of us have proven the GM 19-1/2" stock AC fan blade works great with the severe duty fan clutch, and I'm thinking the last one of those I measured had 2-1/4" of total height from front to back. We are limited by the radiator to the front and the power steering pump towards the rear on how deep of fan we can run.
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Mick Batson
1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon in progress.
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