A few more bits of information. The shimed main bearings continued to be used up to 1H6236, at which point the block was changed to use non shimmed bearings. Cat offered non shimmed replacement rod bearings, if one did not want to replace rods, they were same as later rod bearings but included steel spacers to take up the space originally used by shims.
Also Cat offered replacement blocks for the gear drive fan engines 1H1 to 1H2408, these blocks used the later non shimed main bearings, the only way to identify, short of taking engine apart, is to check the casting number. I have a replacement block and the serial number tag was installed on the new block, so one would not immediatly realize it was not original block.
OM, I am not sure it would be practical to convert a gear fan drive to belt, both timing gears and timing cover are different.

Hi Brian,
Thanks for the info.
May not be practical to convert but if the parts could be interchanged (eg. same bolting arrangement and fit) might be doable.....don't know and not likely I'm going to find out:)

Back to being able to use a later crank, whats the chances of converting (machining) the long nose snout to accept the gear drive configuration?

Looks like it might be possible to modify a later crank. The gear drive gear is retained by the jaw that one put crank in to turn engine over. The jaw is treaded into a hole in the end of the crank. The belt drive crank the gear is retained by a nut treaded onto nose of crank. But it looks like at least from parts book pictures that one could cut the long nose off a later crank and drill and thread a hole in the end to accept the jaw. The only possible problem would be if there enough space on the remaining end of the crank for the gear to engage on shaft without it being out on the externally threaded portion of the crank. The gear drive gear appears to extend further forward, due to provision to couple front PTO drive, than the later belt drive gear.
I have a gear drive crank, but no belt drive crank to compare to. If I remember, the next time I go parts searching I will get dimensions of a belt drive nose.
It would be nice if one could modify, as finding usable gear drive cranks is difficult.

Thanks again for the info,
That is exactly what I was thinking........but have no D8 stuff to compare dimensions. I suppose it may be possible to get prints from Cat but direct physical comparison would probably be easier. There has to be later cranks laying around out there somewhere.