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R-Postbit

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Newb with a 91 Chevy 2500

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  • #31
    One more question, the pistons I thought were flat top are actually dished, and the block has quite a ridge at the top, so I think I may be best off to have it bored .020 or .030 just to be safe, I'll check it tonight with my telescoping gage, but the ridge is definitely more than .010" tall, so looks like I may be in the market for oversized pistons after all, if I stick with the flat top plan, should I go with cast or shell out the extra few bucks for hypereutectic? I saw one thing about hypereutectic being prone to cracking if they encounter any detonation, but I thought hypereutectics were a tiny bit stronger than cast...? Also, the flat top pistons I'm finding all say 10:1 with 64cc heads, if my milled heads end up around 70-72cc what kinda compression will I wind up with?

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    • #32
      Well that is interesting. Using my telescoping gage and micrometer I get 4.001" to 4.002" under the ridge, but the ridge is 3.972"-3.978" so I guess I may go with renting a ridge reamer and honing and sticking with stock bore after all, saves me some money! But I DO want some more compression than stock so I'll still be buying flat tops. But the question remains, go for hypers or cast?

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      • #33
        I would go with flat-top hypereutectic - that is what I use. Go with the piston manufacture ring gap recommendations, they are larger gap than what the cast or forged pistons use. The horror stories you hear are based on using to tight of ring gap and the pistons end up breaking because the rings expand too much. The factory replaced using cast pistons in the big blocks with the Gen V that came out in 1991 and in the small blocks in 1996 with the Vortec engines, they went to hypereutectic pistons and how many factory engines have you heard of with piston issues?

        As for compression, your Mexico heads have the larger Open Chamber and you will be around 9:1 compression with the flat-top and those heads. The TBI used the dish piston and closed chamber heads to achieve 9:1 compression - you are basically looking at the same compression with a better flowing head.
        1990 GMC Jimmy (not the pretend kind) in progress - Gen VI 454 TBI/TH400/NP241 Dana 60/14 Bolt

        When I die, I want to go out peacefully in my sleep like my grandfather, not screaming my head off like the passengers in his car!

        If you are a "Supertramp", wouldn't you give more than a little bit?

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        • #34
          Good deal. So even if I mill off a sizeable amount, I'm starting at 9.0:1 so whatever I take off will be increasing from there. That's kinda what I was thinking as far as cast vs hyper, I just couldn't remember what years they switched. Thanks again for all your help!

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          • #35
            For the old open chamber smog heads, every .007" milled reduces the chamber by 1cc - mill as little as possible to make flat, if you need more use a shim head gasket. If you take too much off the head surface they will warp easily, by using a shim gasket (.016" compressed) rather than a composite gasket (.039-.041 compressed) you can increase the compression without making the head weak.
            1990 GMC Jimmy (not the pretend kind) in progress - Gen VI 454 TBI/TH400/NP241 Dana 60/14 Bolt

            When I die, I want to go out peacefully in my sleep like my grandfather, not screaming my head off like the passengers in his car!

            If you are a "Supertramp", wouldn't you give more than a little bit?

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            • #36
              Oh good to know. So if it's flat already then probably just go straight with a shim gasket then, no milling. What compression would that put me at?

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              • #37
                On a 350 you gain about 4/10th so you would go from about 9:1 to 9.4:1 - about all the higher you want to go with pump gas and cast iron heads.
                1990 GMC Jimmy (not the pretend kind) in progress - Gen VI 454 TBI/TH400/NP241 Dana 60/14 Bolt

                When I die, I want to go out peacefully in my sleep like my grandfather, not screaming my head off like the passengers in his car!

                If you are a "Supertramp", wouldn't you give more than a little bit?

                Comment


                • #38
                  Perfect! That's right about where I want to be then.

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