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Thu Aug 14, 2025 11:04 am

Hi all

I've rebuilt my 25hp fuel pump and understand that the pump should be installed with engine positioned at TDC ie fuel pump rod in fully extended position. Why TDC? ... is this to ensure the correct positioning of the rod on the cam as opposed to mounting the pump at BDC?

Thank you
CJ

Thu Aug 14, 2025 3:18 pm

i think that is way for home re-builders to ensure the diaphragm is in the right preload place before you lighten the screws ....thats for folks without the proper preload tools .... at least thats how ya do the later pumps .

at TDC the rod is at its proper height for tightening the screws

there is an article here explaining some of that .
https://shamwerks.com/rebuilding-36hp-fuel-pump

Thu Aug 14, 2025 6:52 pm

hello CJ,
the factory workshop manual makes no mention of engine being at TDC when installing the pump. it does note the engine to be at "working temperature" when tightening the two mounting nuts

the factory tool 328b presses the rocker arm to the point the link pulls the diaphragm down to a flat position prior to tightening the cover. without this, the spring would have the diaphragm in a permanent, stretched convex situation.

best wishes,
Darryl

Fri Aug 15, 2025 5:18 am

Thank you both - I previously made a tool and preloaded the pump before screwing the two pieces together.

Thanks for clarification on the instructions given in the manual Darryl. I heard some people mention previously that it was important to install the pump with the engine at TDC but perhaps - as you both say - they were referring to preloading the pump on the engine.

All the best
CJ