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Old 08-13-2019, 11:26 PM
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Chevy 350 Piston ID

Hey guys,

I for thr life of me can not find pistons similar to this one to compare to. These are in a 350 I bought and I am trying to figure out what CC these would be. They look like something a turbo car would run, must be super low compression. 84 block probably out of a truck.
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Old 08-14-2019, 11:05 AM
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They look a set of old Badger pistons. Circa. 1980's Not sure when they quit making them.

But, that's stock bore?
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Old 08-14-2019, 11:40 AM
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I believe they are 40 over pistons. I am what CC / CR these pistons are. Seems like they would be 7.5 or 8:1
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Old 08-14-2019, 11:46 AM
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there is a good chance there is a part number cast into the side of the skirt or bottom of the crown
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Old 08-14-2019, 01:42 PM
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The pre-1967 low compression economy engines had dished pistons. In 1967, the low compression vehicles had economy engines with standard flat top pistons and cylinder heads with larger volume combustion chambers. Those were optional low compression engines that could use cheap (economy) regular grade gasoline.

I had a 1963 Pontiac Catalina 389 Sport Coupe with dished pistons.
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Old 08-14-2019, 02:54 PM
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Quote:
Originally Posted by Twisted46 View Post
Hey guys,
I for thr life of me can not find pistons similar to this one to compare to. These are in a 350 I bought and I am trying to figure out what CC these would be. They look like something a turbo car would run, must be super low compression. 84 block probably out of a truck.
Let's begin with what static compression ratio you want to be at and go from there. Can you post the casting number of the heads? It's cast right into the cast iron in between the valves when you remove the valve cover. Remove both of them and take a look, because sometimes motors are built with two different casting number heads. That will let us look up the combustion chamber volume and start to get a handle on the SCR (static compression ratio).
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Old 08-14-2019, 05:56 PM
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I stared looking at this the other day and am not sure if I'm seeing a dome rising from the floor of the dish or just a dish.



At 40 over it's pretty likely this has got a rebuild on it. If this is post 1972 block that's about as far as most of these blocks can be safely bored.


Engines that went into heavy trucks and industrial use used very deep dish pistons and had compression ratios in the mid 7's with extremely mild cams.


Bogie
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Old 08-14-2019, 06:12 PM
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It came with some 58CC 305HO heads so that combination probably isn't horrible. I want 9.5 to 10.5 BUT I plan to put on a set of nice aluminum heads next year and I don't think these pistons would be worth the money without better CR. If I can get 9ish out of it with thr 305 heads i may just run it for a year and then build a new motor.. 383? 😉
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Old 08-14-2019, 08:13 PM
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Bogie, there is no dome in the dish.
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Old 08-14-2019, 11:22 PM
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When I look at the picture I can't tell if there is a dome or if the coloration of deposits makes it appear as it's lower on the edges of the relief and raised in the center. Then again my 80 year old eyes ain't what they used to be nor is the brain behind them. I have a tee shirt I picked up from J&P Cycles with the caption "I ain't as good as I once was but better than I will be". That goes with the shirt my wife gave me a couple birthdays ago "Sons of Arthritis, Ibuprofen Chapter". Yes I still ride but it's winding down.


Probably the easiest way to figure the volume is to get or make a 4 inch straight edge and with a mechanical caliper extend it's push rod end to get the depth then do the math. If dish slopes to the center then make some concentric circles and measuree the depth at each circle as a bandwidth from circle to circle then calculate each band individually and sum them. For a slope this will be pretty accurate way to measure the volume not exact but pretty damn close.

Bogie
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Old 08-14-2019, 11:40 PM
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You can cc the piston in the bore. It won't give you the dish volume but it will give you the total volume you can use in a compression calculator. A little grease around the piston, a piece of plexiglass and a cc syringe can give you an idea.
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Old 08-17-2019, 11:17 PM
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My problem is that the block is in a car right now and I won't be pulling it for a while yet. I'm on the fence if I should mess with these pistons or not. If I can make decent midrange power I'll use email while I build a racy motor on the side. I just don't want to waste my time and end up losing a drag race to the mail truck 😄
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Old 08-18-2019, 12:42 PM
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Quote:
Originally Posted by Twisted46 View Post
I'm on the fence if I should mess with these pistons or not.
This is the most horrible piston crown design I have ever seen. There, does that help?
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Old 08-18-2019, 03:24 PM
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Quote:
Originally Posted by Twisted46 View Post
My problem is that the block is in a car right now and I won't be pulling it for a while yet. I'm on the fence if I should mess with these pistons or not. If I can make decent midrange power I'll use email while I build a racy motor on the side. I just don't want to waste my time and end up losing a drag race to the mail truck 😄

These are more or less a copy of factory 350 pistons, the following is specific to that engine other displacements vary from this in detail.



The big round dish has been typical Chevy fare for what seems forever. The compression ratio is adjusted by GM with gasket thickness and cylinder head chamber volume that being either 64 or 76 cc's. Only serious factory attempts at "race" engines got flattop pistons as with the original LT-1. The SMOG era brought large chamber heads across the board, even the early 1970's LT-1's got open chamber heads but Chevy held on to forged flattop pistons for a few years before that became a pointless expense regardless of what the advertised engine option said it was. Basically back then all a racer needed to do was backdate the heads to pre-SMOG or buy the GMPP Bowtie head and a "race" cam as the rest of the short block had a forged crank, pink rods, and forged pistons.


Basically with this piston a .019 to .026 thick gasket with 64cc heads you're looking at compression ratios between 9 and 9.5 to 1.


For flame propagation a flattop piston works best with no dome to get in the way of the flame front travel and a large flat surface for maximum squish/quench effects. Since a flattop can go to excessive compression for street fuel the next best choice is either the D dish or step dish designs. These place a dish under the valve pocket for compression ratio control with a flat surface under the chamber step for retaining the squish/quench effects of a flattop piston. Improving squish/quench extracts more energy from the mixture and makes it easier to set on fire by squeezing the mixture into the valve pocket with great force that breaks up fuel globules, stirs the mixture and forces it before the spark plug for easier ignition. At the other end of the burn cycle the close proximity of the flat piston and head surfaces pulls excess heat from the unburnt mixture portion so it doesn't self ignite before the flame front gets there. The combination of these events introduces what's called "mechanical octane" it makes the fuel behave as if it has 4 to 6 higher octane than its rating implies.


If you look at modern heads and I'll use the L31 Vortec as an example sine there are a lot of pictures of its combustion chamber when compared to SMOG heads you see a tighter chamber intended to swirl the incoming charge over the spark plug. The spark plug pushed inboard almost between the valves so the burn radius thus burn time is reduced. A beak projects between the valves to redirect incoming fresh flow away from the exhaust valve during cam overlap. You will note how SMOG chambers are the antithesis of this design as they are aimed solely at minimizing NOx formation by slowing and keeping the burn cool. There are better ways of keeping the NOx down than that.


Bogie
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Old 08-18-2019, 07:24 PM
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I stumbled upon Oldsmobile pistons and they look like a match. Two suppliers have nearly identical specs. They are 24 CCs.
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