454 rebuild with blower... Pistons and clearances.
 

My 454 B&M 250 project has really started to get moving, and in the process I've come across some stumbling blocks. I found that four of my pistons showed evidence of detonation; definitely one for sure (#3... Slight erosion around the intake area at the crown extending to the top ring) with the other three having blemishes in the crown's face. Needless to say, these won't work with the blower. My bearings all came out looking good, and my crank should require polishing to put it back together so that's good!

My question centers around pistons: what to use and how should I have the block bored for proper clearance? My plan is to run less compression than the original LS6 pistons provided. I figure a flat-top with a zero deck, .040 gasket, and a 120cc chamber will give me right about an 8:1 compression ratio, maybe a touch over would be good for supporting 9-10 pounds of boost with aluminum heads.

The block will go to the machine shop this week to have it cleaned, sonic checked, decks trued, and finally bored and honed. I'm hoping that I can go with a .060 over piston to take advantage of the extra cubic inches as well as unshroud the intake valves of my heads. If I was to use a JE 139478 piston, ould it be safe to say bore it to 4.31 (if it checks clear), and then finish hone the bores to 4.315 to give the proper piston to wall clearance (assuming .003-spec'd piston clearance plus supercharged and cold water application). Thinking that ring end gaps should be set at .022 on the top ring and .025 on the second (plasma-moly rings). I guess I'd need to have the pistons in-hand first.

Looking for some advice from the experts here. Thanks!

Tom piston manufacturers have different piston to wall clearances. And if the Pistons are coated that changes things also. As far as ring gap you really need to go by the manufacturers suggestion on that one and even more importantly the block needs to be honed a certain way depending on which style ring you are using. This is crucial to ring seal. .060 overbore is not going to unshroud the valve that doesn't start happening till about .250 I personally wouldn't put 8:1 in a 454 with a blower it's gonna be a complete turd down low. And lastly if you want to bore your block .060 it needs to be sonic tested to be done right.

the Merc 525SC only had about 7-1 and they ran well...8-1 will be fine with the blower

Closed cooling or open? Thermostat?

Merc 525sc couldn't get out of its own way.

How would you build this?

I figured lowering the compression and turning up the boost (which would start low... figured it would start to build boost around 3K) would make up for the .75 lower ratio.

Raw water cooling. Not sure on the thermostat. Our water temps in the summer are generally in the 70's to mid 80's at the most. I'd probably run it with a restrictor but open and flowing all the time.

I seem to remember the guys from b&m to prefer around 7.5 to 1 and overdriven blowers with the 250 , on a street car would make for a glorious tire frier and suprisingly reasonable inlet tempatures.if you have aluminum heads get the block deck nice and flat and go mls gaskets and closed cooling should be able to make a reliable 550 or so hp with right cam and decent exhaust im building same thing now with flat tops and large ovals 122cc have some good 250 blower tech specs and extra parts also .pm me if you like ill share more about what ive learned about the ole original 250

This is how I would do it not that I'm an expert
9:1 so it's not lazy, using the speed pro pistons with raw water cooling and no thermostat, .006-.007 piston to wall clearance. I had an issue, with .004 scuffing pistons with iron heads and a 174 blower.