572 vs 598
 

So, I’m having the motors for my Cafe gone through this winter and we’ve been planning on rebuilding them to the same 4.5/4.5, 10.2, single 1050 configuration they are currently, but... The other day I was skimming through a thread on here and the OP was planning a 572 build, someone asked “why dont you go 598 if your going 572?” This got me thinking, maybe the positives and negotives are obvious but for the sake of good conversation and talking about motors, what is the good/bad/ugly in this comparison? So the Dart big M can go max bore of 4.625 so theres not a ton of meat left if something goes wrong, right? A variable in my build is we’re going to a set of Iron Eagles and putting my Big Browdies on the shelf for now probably. Unless they end up being mint; doubtful.. Is there some decent power to be made up by going bigger cubes since we’ll obviously be dropping the compression ratio for the heads? I’ll be running 91. Anyway I was bored and floating around out here off Long Island so I thought I’d ask..

curious to read the answers!

Ya right... are your rods 6.7 currently

One thing is the cylinder rigidity could be less with increased bore (thinner wall), ring seal is compromised.
Marine raw water cooled engines corrode so I like more 'meat' in the wall.
Head gasket has more deck area between the pistons.
Of course if not putting too much strain on the deal, then 'there's no replacement for displacement' LOL.

I went down this road with short deck some time back. You'll get old school and modern school answers. If you currently have 4.5 cranks and 6.7 rods with 4.5 bore you might just consider opening up to 4.560 for refresh and go 580 and leave some oops space. 15-20 ci or so isn't gonna make or brake it.

What current combo?

Yeah, I’m pretty sure the Eagle rods in there are 6.7s.
I’m not at home so I cant look at part numbers.
The corrosion arguement seems valid, thanks for pointing that out.

4.5-4.5-6.7 or do you mean all the rest of the components?

You wont have a problem with a big M at 4.600 in a NA engine the walls are still thicker than a 454. Like the iron heads on that combo.

Aren't those Siamese anyway?

I was thinking you still have .300 btwn cylinders if I'm not mistaking. Safely go 4.600 but some don't like the gasket.

yes

I stayed at 4.530 but made a drunk decision of going SC.

I hear ya, yeah I didnt think I’d have any sort of a problem from the 598 at 4.600, it was more of apprehension of rebuilds down the road and only having .025 to work with. Dart claims 4.625 max I believe. I kinda thought it would be cool if I could make up in cubes what I would loose in going from aluminum heads 10:1 to iron heads 9.8:1... It’d be real nice to keep it around 750/side..

cool but I dont think I’ve got the pockets for that..

I doubt you'd lose anything. Aluminum itself doesnt flow better than cast iron. The 2/10ths of compression isnt gonna matter. I'd focus on the heads condition and flow. A simple nice valve job, blend etc could be worth as much power as you'd gain with an extra 26 cubic inches if the current valve job sucks. Just had a friends Merlin iron heads over here. Good god , those things are F'cking awful in stock form. The transition from the valve seat into the chamber is horrid. The lips from the throat to the seat is horrid. No backcut on the intake valves, and had a chitty valve job on them. Tons of airflow potential with just a little TLC .

If I had a 572, with good pistons and bores, I'd leave it. If I had to buy a set of pistons, I'd bore it to 4.600 with that Dart block. I wouldnt sweat the head gasket material between the cylinders, especially with iron heads. Theres tons of 4.600 bore engines out there doing fine.

BBC bore spacing is 4.84 so you will end up with .240 between cylinders at 4.600
This is where a 5" bore spacing would pay off. I would love to build nice marine motors at 700"

Thanks Mild, understood. But, for example say my 4.5 bore just needs a hone and no skim; theoretically they have 50ish hrs on them. Why go all the way to 4.600 from 4.500 if were buying new pistons, which we are (current slugs arent the quality I want).. ? Or if a skim is necessary why not go to the next je piston stock size to make a 588? I’m still learning, glad you chimed in; I like reading your posts and threads. Thanks.

Merlin Superblock, 11.625, 4.9 bore spacing...

It really does sound like you’d end up at .300 @ 4.625, no?

What model Iron Eagles do you have? 345's?

One of the benefits of a large bore, besides more cubes, is it helps cylinder head airflow. If you have a head with a 2.30 valve and a chamber designed around a 4.250 bore, taking advantage of unshrouding the valve by pulling back the chamber some is a nice improvement in airflow.

We havent bought the heads yet. I assume 345s yes.
Theres still a very small chance my heads will be decent; gotta wait till disassembly..

4.9 super block will have more in the middle not a standard spacing.
Standard bore spacing at 4.625 you will have .215 between cylinders.
They say .300 min that is on the major thrust on the cylinder wall not between the cylinders.

Oh, I gotcha, thanks.

No problem if you have any other questions let me know.. Btw if your brodix heads need welding send them back to brodix they do them cheaper than anyone.

Yeah will do, thanks! I’m sure I’ll have more questions. Yeah, one head was already sent back to Brodix once when the previous owner dropped a valve; I dont have the documentation but from what I was led to believe it was spray welded and cleaned up in 1 chamber area..

i only like to bore them enough to get a round hole,that way you have plenty of material for more over bores down the road.

That sounds like a good motto; so do you aim for a finish bore whatever it may be that you can utilize non custom pistons or do you matter of factly just take enough off to get it round then see what you've got?

that depends on the build but usually bore to the next size shelf piston,if building something custom that requires a specked piston i have gone as little as .005 over.

I read the same thing on two different sites for BBC dart big m blocks. GM blocks different.

Agreed