Big Block Power

[apollo18]

Can you use a Jegs or a Summit engine that they sell for cars in a 18' jet boat? If you cant can you please tell me why. (new to boating and looking for a new engine) Thanks for your help.

[SmokinLowriderSS]

2 answers, Yes, and Maybe.
The main thing is that boat engines get run hard which causes pistons to heat & expand quickly, they rev fast, wind down fast, and yet run in a relatively cool engine block (definitely compared to a 195* equalized car cooling system after a 30 minuite drive). My coolant leaves the block at 140* cruizing or WFO. The cylinder bores do not expand as much as they do in a fully heated car engine.
The piston to bore, ring gap, and lower end bearing clearances should all be (in my opinion) somewhere aproximating the middle of the factory spec range. If the bearings are too tight, you can scuff one, tear it up, and all the rest down from it (as well as crank & rods). If a piston ring closes gap to 0, you can loose the top of a piston as well as cylinder scoring. If a piston heat-expands to fill the gap to the cylinder, it seizes, you know the rest.
I think one could be a good buy, but I would then open it up to try to check all those clearances first, just to insure you do not get one that got assembled "too tight". Then might as well have tripple-checked all those important torques down there anyhow.
Personally, I'd buy an engine to re-build or a block & a rotating assy, and part up from there. That way you know what it goes together with.