The Fordson Tractor Pages Forum

I recently purchased a new water pump for my 1952 petrol FM and it did not fit properly, the casting was too big in one section by 3-4 mm and interfered with the cylinder head. After grinding a bunch off it worked ok, but I had to fabricate a new gasket, plus the bother of taking everything apart again....There were no real identification marks on the new pump to determine the manufacturer, so be sure to check the new pump with the old in detail before installing...

Bob

https://postimg.cc/gallery/MxpX6Sy

Replacement parts are often wrong, but this is the worst water pump I'd seen yet in that sense. Looks like your old pump wasn't original either (but right for later Mk2 engines, possibly an original one for them), as the bypass hole on the original pump would have been circular.
Sandy

The early engines did not have a bypass port in the head, the water pump casting and the gasket were slightly different for these engines. It is important to get the right pump and gasket to match the engine, this is something that the spurious parts suppliers do not appear to have picked up on, with only the one later pump usually listed. For the early engines it is also important to have a thermostat with a "jiggle pin" as this is the only way air can escape from the block and head when re-filling with fresh antifreeze and water.

The jiggle pin should be on all thermostats. All Mk2 and later must use shrouded thermostats (such as AC TF2, opening at 176ºF). Mk1 engines do have a bypass hole, but it's smaller and doesn't need to be sealed by a shrouded thermostat (see Water pump to block pic in Bob's album).
Sandy