I need suggestions as to which piston I should get... This is for a 98 explorer 5.0 engine that is going into a boat. The engine had to be bored .030 over but is otherwise stock; it will end up with a carb. The original engine had the deep dish style pistons but the explorer engine had the valve reliefs...

Not sure if the differences here would make much of a difference in compression or more importantly, the type of fuel needed or performance. I want to run standard octane fuel. If there isnt much of a difference I will go with the first set. Thanks!

 

You can run 87 with the flat top's. That's what I'd run.

 

Sounds like it was a 72-76 motor if it had deep dished pistons. As for the comp ratio with flat tops, that depends on which flat tops you're talking about. The pin height(compression height) varies by.035" among flat tops. Typically the flat tops will raise the comp ratio from the high 7's to 8.5 to 9.5 to 1. With no other change (head chamber volume)

 

The original engine for this marine application was a 74 302. I am rebuilding a 302 from a 98 explorer to replace the original. For this particular engine, it all ready had flat tops. I was wondering if there was any advantage to going with the dish style pistons as it relates to marine use; torque, etc... It appears that this new engine will be ahead of the game on torque and hp so being able to use the cheaper gas is a real plus. The old engine requires 89 octane minimum. Thus flat tops it is.

Being bored 4.030 isnt going to change the compression much is it?

 

The bore increase will yeild about a quarter point. The Explorer had zero deck flat tops with about a 9-9.5 to 1 ratio with it's heads. It will likely run on 87 being in a boat where the engine temps stay pretty cool with the fresh water running thru the engine, so it likely will be fine on 87. In a car or truck they run better on 89, depending on how much timing you set it up with.

 

I just read something that said the lower compression ratio is better for lower octane fuel. Considering this block and heads will be decked, and the bore is 4.030, and the heads are gt40p's with the cam from the exploder, should I go with the dished pistons to offset the compression ratio increase?

Emissions and such are no issue; running good is. Sorry if I am all conflicted on this; just want to do it right. First build.

 

There's no reason to go below about 8.5:1 IMO, measure the parts once they are milled and put the numbers in a compression calculator with specs from commonly available pistons.

 

It really boils down to the operating temps in a boat. If they're as low as I think they will be with a fresh water cooling system, you'll be fine even with the slight increase in the ratio. Or if you want to play it on the safe side, there are lots of 302 flat top sets out there with a 1.585 comp height that will compensate for the slight increase, this in effect is somewhat akin to running a thicker head gasket but without the headaches in doing that. It will drop the piston tops down the hole at TDC by .030-.035", adding about 8 ccs to the compressed volume. The Explorer cam has really no overlap to speak of and as such doesn't bleed off compression like many cams do. This helps the bottom end and slightly increases its octane requirements and fuel economy. I ran a motor like this in my 89 Ranger with a carb and in summer, it would ping slightly with 87 octane, not at all in winter. Running 89 was enough to make the ping go away in summer temps. Great runner with a carb and that cam, smooth powerband from idle to 5500-6000 with a high rise dual plane aluminum intake and a 570-750 VS Holley carb. (ran this combo with three different Holleys (570SA , 1850 600 and a 3310 750)

 

Cool. Thx for that. It does have a thermostat but I will replace with one around 160°. The challenge with measuring is the block is still with the machinist. He suggested going through summit for pistons as they are significantly less than what he can get them for. Just got off the phone with him and he suggested simply going with the valve relief style; no dish. Same thoughts on running cool cause its a boat. So if the compression is a little high and pinging a little higher octane will be the fix?

 

Sometimes you can give someone to much info. He only ask- dish or flat top. He said it came out of a '98 Explorer and going in a boat. So, That motor came with flat top's so replace them with flat top's. If it spark knock's, back the timing off. All of this info every one is throwing out is useless unless you know what it really means, and I'm betting most of you don't.

1. Run the flat tops
2. Marine cams are for boats and cams for cars/trucks are for cars/trucks not boats. There is a difference.
3. If you buy the 273 pistons that you have listed the machinist needs to mic each piston and then finish honing the bores to fit the pistons. Them pistons usually come in different sizes in the same box +/- .002 to .0002 Good-Luck.

 

Thanks for that... Machinist told me the same thing as far as the final hone goes. The cam is getting reground a little but has been confirmed to be a good option for marine use. The new grind makes it just a little more agressive.

 

Well, excuse us for "pretending to know" what you're obviously the resident expert in here. The Explorer cam will do fine in a boat. As for the pistons, all you've contributed here is to tell him to reuse what came with the truck motor. WOW, that was easy !!! How about 'splaining why the Exploder cam won't work in a boat ? Or is that too much to ask ? Or is it you think we're not smart enough to comprehend what you'll tell us ?

 

In general, an automotive cam will work in marine application, for example the big issue is going to be the rpm range that the cam will produce its hp and torque. Which is different between boat and auto.

And then you have-
profile centerlines, centerline separations, adv durations, 050 durations, lift, rate of lift, ramps, profiles, lifter types, etc... all will come into play to tailor a cam for a particular engine application.

They have already admitted that the cam was wrong by saying "The cam is getting reground a little".
And he didn't say if the 302 was replacing 1 motor in a boat with 2. If so what if it is the reverse rotation engine- Then the cam wouldn't work. He didn't tell us that it was the reverse engine and everyone said to run it and then it doesn't work what does he do then? post it here I guess.
I guess what my problem is after going into great detail explaining things to so many different people over the years I have found out that answering their questions is the easiest some time's. So, run the flat top's.

And another thing, if your 302 is an early model the flywheel wont work on the late model 302. I do wish you all the luck.

Baddad- Did we use to work together? I had a co-worker who use to have to "do some splaining" to everyone.

 

I agree that sometimes a quick answer is better however when discussing all this on a forum, for all the reasons you mentioned, a discussion is needed to be thorough.

The explorer cam has a nice smooth power curve with low end torque up through 5500rpm; in the boat it is governed @ 4500rpm to protect the outboard. I failed to keep the lifters from the explorer in sequence so they need replaced; if it were not for this the cam would not be getting reground. Since it is, its costs nothin more to put a cut on it made for a boat. It will work either way. It is not a reverse rotation. I have accounted for the 50oz balance.

Conanski and Baddad have provided council throughout this process; they have the background not included in this thread. Lets all be nice.

 

You had nothing to fear from reusing the Explorer's lifters, they're roller lifters and as such you can mix em up all day long and it'll make no difference as to wearing the cam lobes. No need to regrind the cam cause of it either. Flat tappet cams are a different animal when it comes to the lifters.