Welcome to SCJB and jet boats.
Yes an engine should be built specifically for a jet boat. Can you use a motor out of an old truck? Yes but if you run it like we like to run jet boats, it probably won't last to long.
Jet boat motors have bigger clearances (piston to wall, crank bearing clearances, ring gap). There are some other things, but I'm not a mechanic, just know some stuff from reading the forums for a long time.
Best bet is to contact an engine builder that KNOWS jet boat engines. Bostick is the main guy here at SCJB.
I found this thread that has some info about building a jet boat motor:
https://www.socaljetboats.com/engine-mechanical-electrical/let's-stir-the-pot/msg256034/#msg256034There have been a lot of discussions about the subject, so it's time to get familiar with the search feature.
You also want to make aure that you have a thermostat set up that won't allow too much cold water through the motor thus never letting the motor reach proper operating temperature (160-190°ish).
Sent from my SM-G900V using SoCal Jet Boats mobile app
Most jet boats don't have t-stats and run an open water cooling system, hence the reason for some of the bigger tolerances.
They do make a t-stat kit but I think the general consensus prefers not to run them.
