About 10 or so years ago I was down at Lightning Performance in Easton, IL (grew up 10 miles from there) with a friend who was looking to build a 540 for his Checkmate. After Don gave me the grand tour of the place we sat and talked to Keith Eickert about building a 540 off a standard deck block and he flat out said no. When pressing him on why you had to go tall deck as there are many 9.8" 540's out there he barked back, "Not with my name on them there isn't". Take that for what it's worth when building larger inch N/A marine engines. I was really surprised at 540 cu in he insisted on a tall deck or he wouldn't build it. Maybe he just didn't want to p!ss around with such a menial build and figured insisting on a new block would would discourage my buddy from the build. It worked. :rolleyes:

IMO I have been down this road a couple times and regretted it haha. Leave the 502MPI as it is, buy a 2nd block and build it on the side until it is completed, chances are you wont get it built on one winter if you take your MPI apart, and you will end up boatless for a season. Been there, done that, rushed to save a season and got bit by shortcuts. Just get a nice big-inch shortblock and finish the rest, with cost of some of whats on market, isn't much price difference than you will have with machining costs and time of dicking with old block.

In the end, the only thing you will use on the new build is the block of the original motor. Yes they are expensive, but worth getting another. Leave the original motor on the stand when new one goes in as a backup or if you sell the boat.

Smitty,

I get that. My question is what is the ideal CH? I know too short is bad, due to ring wear and potential for catastrophic failure, and I know too long is bad, due to excessive reciprocating mass. I just don't know what that hi/lo is, and the interwebs is all over the place.

Thanks. Brad.
(937)545-8991

[QUOTE=Brad Christy;4862124]Smitty,

I get that. My question is what is the ideal CH? I know too short is bad, due to ring wear and potential for catastrophic failure, and I know too long is bad, due to excessive reciprocating mass. I just don't know what that hi/lo is, and the interwebs is all over the place.

Thanks. Brad.
(937)545-8991[/QUOTE
theres 2 smittys commenting here, lol.
I wont build ANY endurance engine with less than a 1.270/1.280 ch which is what you need to do a 4.250 stroke on a 9.8. And then, your dealing with a pretty short skirt length. That short skirt length comes at expense of ring seal I'm rebuilding 2 540s right now from.a guy from.Virginia that have excessive bore clearance, taper and piston wear.
I've seen guys claim.400, 600, 900 hr rebuild intervals with 9.8 deck 4.250, 4.375 combos but I have NEVER seen it personally. I have built plenty of 1.270 ch 540 variations, come 200 hours I've seen ring seal issues, on blower engines at 100 hours. sure, if it spent its whole life idling or running 2800 or something I guess I could believe the outrageous hour claims.
On other end of ch spectrum, I don't see a need for pistons with all kinds of extra mass with significant ch, like mentioned earlier, Id go 6.800 vs 6.700 if ch was getting into the 1.500+ range on a tall deck

https://cimg0.ibsrv.net/gimg/www.off...29c1d3d57c.jpg
https://cimg6.ibsrv.net/gimg/www.off...3eaf8dea05.jpg
The 540s I just mentioned had these pistons in them, very little skirt, very lightweight, had no purpose of being in a 5400 rpm low compression 540. I DID not build this or pick them out. They were already so loose on his last refreshen someone coated them to try to "fix" it. Well they did a poor job and the coating came off and went thru the bearings and torched the cranks, 77 hrs since that refresh is my understanding. Cylinders have .0015 taper, pistons are at .008 from a coated spot to tightest spot in bore BEFORE honing, more like .010 ,.011 to a loose spot where coatings missing. I am going next oversize on these 2, getting rid of these. Could have it honed round again which would put it at .012 THEN have bad coating stripped off then pistons re coated a ton, but not worth it. I would on a drag engine that has a 5 or 10 hr lifespan between rebuilds, Smitty

Smitty,

Got it. So, looking at the stack-up, does one want to stop just short of the deck height and machine it to the stack-up, or go past it and use head gaskets to make up the difference?

Building my fantasy engine, this is what I've got so far:

Dart BigM 10.2
Crank - Molnar #454-4375DC8F-6385 (4.375" stoke)
Rods - Molnar # CH6760VTB8-A (6.7" C-C)
Pistons - JE, Chevrolet, 4.530 in. Bore, Kit (jepistons.com) (1.27" CH)

This stack-up stops .0425" short of the 10.2" deck height. Or..... Molnar # CH6760VTB8-A rods (6.76" C-C) puts you .0175" over the 10.2" deck height. I know that we can machine the deck height and use appropriate head gaskets to zero in on the exact gap we're looking for. Which is the preferred approach?

I'm hoping this is valuable info for the OP as much as it is for me, so it's not TOO much of a derailment..... :party-smiley-004:

Thanks. Brad.
(937)545-8991

Almost anything is possible with any combination, but most shelf pistons are based off a set rod length, stroke, and deck height.
Then they derive the compression height.
Check the PDF printed catalogs, way more informative.

IMO, at 4.375 stroke, make a sacrifice on the rod length and go 4.5 stroke. 580CI at your bore. :cheer:

https://cimg3.ibsrv.net/gimg/www.off...535f0b8b82.png

Cheech,

That chart pertains to standard deck blocks. My hopes would be to use a tall deck and use longer rods to minimize rod angle. I was talked off the proverbial ledge on the idea of a 632 for this very reason. I'm just looking for where the conventional wisdom is on where to aim for the stackup. Do we aim high, over the deck height, and use head gaskets to establish final head gap, or do we aim low and machine the decks? I'm sure some of both will be in the final recipe, as I'm sure the 10.2" is not a super precise dimension with a new block.

Thanks. Brad.
(937)545-8991

I'm aware that was a short deck block.
I highlighted the piston number YOU linked in your previous post.
A piston doesn't know what deck height block it is in. The builder does!

Reread this: Take half of stroke listed at that part # you picked out 2.125" + rod length 6.385" + CD 1.270= 9.780"
Just under the 9.8" nominal for short deck BBC. All laid out in those PDF catalogs.
Here if this fits you better here's "tall deck listings".
Just go for broke. 4.375" crank same $ as 4.5" stroke, you're above the 1.270 CD Smitty likes, rod is a menial .165" shorter than a 6.7.
And keep your Procharger regardless!!
https://cimg0.ibsrv.net/gimg/www.off...34c1d6d047.png

I like where this thread has gone! Keep it coming fellas....I'm not currently building engines trying to meet these goals just mainly opened it up for discussion...

Should we clear up some of the acronyms for us stupid people? CH CD etc?

Is the general consensus that you can't get tq and hp up areound 700 with a GM 502 mag block then? In a 540 or 548 configuration? I read through everything and it seems like the hp is "easy" but tq will suffer...did I read that right?

Thanks for all the replies! Turning into a very informative and fun thread I think!