The manual makes it seem like you have to be careful with these engines. The camshafts changed hardness, and if you replace it, you need to replace the tappets with harder ones. [I'm not sure why] The gears were widened and made of stronger material. they also changed from three screws to a single center screw holding the main cam gear in place. A lot to be careful with if you go to salvage parts.
Mk 1 engine : before Serial Number 1,425,097 3 bolt Timing gear, thin gear, all manifold ports in a row, crankshaft front end diameter 2.500 inch. Oil pan has a clean-out hole. When servicing, the screws holding the mounting plate to the block should be replaced with the later high-tensile screws. [indicating the originals could not hold tightness]
Mk II, and Power Major engine: between 1,425,098 and 1,518,653 : Crankcase breather added.
at S/N 1,445,056, horseshoe thrust washer introduced, later changed at s/n 1,511,488. (both will work) Power Major following from 1,518,654: at S/N 1,599,502, single center screw to hold cam gears. At same time, gears widened and stronger material used. Width of cam gear = .928/.938 ,about 1/8 inch thicker. These are not supposed to be used except in sets. This may be because of worn teeth in the older gear, so you'd have poor contact to spread the loads.
For 3-screw cam gear mounting use gasket E28-CN-9. For single-screw cam gear mounting, use gasket E105-CN-9. Note, the thrust face on the single screw mount is in the front gear cover.
When you get to the front mounting cover, they get very coy. They don't say how the thickness changed, but they have 4 different mounting plates that either come with the camshaft kit, or can be ordered as replacements. E1ADDN-6030-A (latest type). E1ADDN-6030-B, for older engines with newer camshaft gears. E1ADKN-6030-C for older engines with original cam gears. E1ADKN-6030-D, for MkII engines with new camshaft gearing.
So you'd better know what you have before you order or swap parts for these engines. Depending on the sleeve and piston, there are 4 different head gaskets.
Tractor companies don't like to make changes, but apparently they try to fix problems without too many major changes. It is lost to history how many owners of these tractors had to take them in for "servicing"
because of these minor problems led to expensive failures.
You can apply a bit of engineering archeology in trying to figure out where and how the shortcomings arose. For example, marginal or inadequate fasteners, marginal lubrication,
marginal, or even inept metallurgy and heat treat.
Yet these tractors seem to have a warm reputation among Ford collectors. They eventually got it right.