I am currently in the process of building a 509. While looking at the JE Pistons Website, they call for .002 to .004 extra piston to cylinder clearance for marine engines. When I mentioned this to the vendor that I bought SRP pistons from, they said that the .002 to .004 extra was only for engines that are sea water cooled and if you are using a closed cooling system (as I plan to do) then the extra clearance is not necessary because an engine with a closed cooling system has more control over engine temps and can be run as hot as a car engine. Can anybody shed some additional light on this?

Thanks

Raw water cooling has no ability to consistently control cooling system temps. Closed cooling does- just as an auto cooling system. If the engine coolant stays in the 130 to 150 range, the block never warms up and the cylinders don't completely expand. This can cause what's called "cold seize" where the piston surface scuffs and sometimes the areas at the top of the piston in the ring area can gall. The bore most likely will score as well and the piston rings can stick from the galling.

This is also a common occurrence from running an engine hard prior to bringing it up to operating temperature. The piston dome - being aluminum and of much lower mass- expands faster than the bore does.

What you said is correct . If you plan on closed cooling run the piston manufactures piston to wall clearance . raw water cooled motors require additional clearance. Tom

Thanks, that makes a lot of sense. I have Dennis Moore's Marine Big Block book and he didn't really breach the topic of differences in tolerances between Sea Water Cooled engines and Closed Cooled engines.

Always follow the piston manufacturer's clearance spec. Different materials and manufacturing techniques greatly affect expansion.

You still may want to run a little more clearance. Because the engine is under a load most of the time, as compared to a car, the pistons run at a hotter temperature. Since the pistons are hotter, they expand more. If you are running JE forged pistons, they also expand more than cast pistons. Because the engine will be "closed cooled" you don't need that much more clearance, but I'd still go with a little more. JE forged pistons have "centered" wrist pins, so be prepared for a little piston noise until the motor warms up.

I have to agree. running an extra .002 isn't gonna hurt a thing. reguardless of type cooling.
With the likely hood JE is calling for .005, going to .007 is actually ideal in my book.

DAVE