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What pistons should I have?

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Mr_Roboto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mr_Roboto Quote  Post ReplyReply Direct Link To This Post Topic: What pistons should I have?
    Posted: Jul/09/2022 at 9:23am
I popped the valve cover off my 82 Eagles's 258 and set a dial indicator up. Based off the valve lift being .384 it has a stock cam in it. I was told it had a Comp when I bought it. I'm going to swap the head bolts to ARPs since one was broken already when I got it therefore the rest are sketchy. 

That said, this engine is supposed to have 9.2:1 compression from what I understand. It's a fresh rebuild done by the PO (the head was clean as a whistle, I'm sure this thing had some sort of a rebuild done.) I'm wondering if he put pistons in it or if it has a set of stockers. Game plan is to get a bore scope to see what's in it. Any idea what this would have come with from the factory, or preferably a picture thereof? Just trying to figure out if this thing is going to be a decent running stocker or a low compression pig. 

As of now I'd argue it's way over carbed, it has a 500 CFM Carter Competition (AFB) with the Competition style Offy intake. I suspect I'd be better off trying to swap on the 4.0 intake and EFI at this point drivability wise. From what I understand the secondary air door is demand based so I'm hoping that will account for it some. I'd still like a smaller carb though at this point, but we're going to make the most of what we have. 
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Trader View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2022 at 3:06pm
Factory 82 pistons were 8.3:1. Factory 83 pistons with 11.5 cc dish were 9.2:1.
This was a mid year change and some 9.2:1 SCR engines made it into 82's. 
The only way to know for sure would be to pull the head and measure a piston cc.
If it was a fresh rebuild, then 11.5 cc cast pistons may have been used.

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Mr_Roboto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mr_Roboto Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2022 at 6:19pm
Since the head shows up 81-85 by the casting, I believe that means I should be able to poke around and tell probably. The engine is date coded 11-81, so I'm going to guess it's going to be the lower SCR unless pistons were put in.  I'm guessing these never used flat tops but rather a very shallow vs deeper dish? Playing with this in a CR calculator I'm looking at 10CC difference roughly between the two compression ratios. I'd love to have delusions he zero decked the block or something but I don't think so.

I'm guessing the 11.5CC casties were the common rebuilder pistons then? The guy who I bought it from was a kid at the time he built it out but his dad was a diesel mechanic so it's likely that there was some adult supervision in the build. The oil pressure's good though, so regardless I'll probably run with it and see what it does. Admittedly it will be kind of a mixed bag between the rebuilder pistons having the smaller dish and that they typically are .010 deeper in the hole, which I'm none too fond of.
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Trader View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2022 at 6:53pm
13 cc and 21 cc pistons seem to be common now for rebuilder cast pistons. 
Picked up 2 sets of forged 232 TWR 11 cc for my 290 build with 304 heads and crankshaft (half the price of cast 290 pistons). The local machine shops cannot keep staff and have been waiting since January for the heads with chev valves.
You will be able to tell SCR by fuel requirements. If you can run on 87 then it's 8.3:1 and 9.2:1 will likely require 89 or 91.
Wouldn't trouble too much on this if it was assembled well. Plenty of power for the Eagle and putting on a 4.0l head would be a better performance add on later if you think it's required.
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Mr_Roboto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mr_Roboto Quote  Post ReplyReply Direct Link To This Post Posted: Aug/05/2022 at 10:59pm
Originally posted by Trader Trader wrote:

13 cc and 21 cc pistons seem to be common now for rebuilder cast pistons. 
Picked up 2 sets of forged 232 TWR 11 cc for my 290 build with 304 heads and crankshaft (half the price of cast 290 pistons). The local machine shops cannot keep staff and have been waiting since January for the heads with chev valves.
You will be able to tell SCR by fuel requirements. If you can run on 87 then it's 8.3:1 and 9.2:1 will likely require 89 or 91.
Wouldn't trouble too much on this if it was assembled well. Plenty of power for the Eagle and putting on a 4.0l head would be a better performance add on later if you think it's required.


I got a 4.0L to modify (preferably a stroker engine eventually) and grab the head from. For the price getting the head and the EFI intake/fuel rail alone were worth it IMO. It's a 94, so it's not the best year but not the worse either. The game plan is to H/C/I swap all at once, but I want to get the head checked out for health first since it seems to have some oil in it. I purchased springs and have a custom cam in mind now. I may try to use a syringe through a spark plug hole to get the piston dish, if I can do that and the head CC I'll at least have a ball park even if I don't know the deck height. If it has the smaller dish then I think the cam will be a bit bigger.
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