i need a little 340 help. Lets Talk head gaskets

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roscoes-garage said:
i don't require 12.5:1 compression just want to use the setup since my dad ran this combo a long time ago. Yeah Little worried about the intake gaskets with the thicker head gaskets. I'll see if i can put the J heads on to see if they fit with the .040 head gaskets then make the decision.
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I totally understand wanting to run your dads combo. If the J heads work you could possibly mill the Edelbrock chamber to match the J heads. This is just thinking of ways around your issues as I’ve never compared the two chambers.
 
While that is a lot of dome above the deck, that's not what they called a 12 or 12.5 piston. Those are more like 11 or 11.5. The true 12.5 pistons have a massive dome that's so big it has to have a spark plug groove cut into the middle of the dome and those do not. While I'm sure you can get 12:1 or more, those do not have the dome that the true 12:1 pistons had. Here's what the real 12.5 340 pistons look like. There's no mistaking those.
12.1 340 PISTONS.jpg
 
RustyRatRod said:
While that is a lot of dome above the deck, that's not what they called a 12 or 12.5 piston. Those are more like 11 or 11.5. The true 12.5 pistons have a massive dome that's so big it has to have a spark plug groove cut into the middle of the dome and those do not. While I'm sure you can get 12:1 or more, those do not have the dome that the true 12:1 pistons had. Here's what the real 12.5 340 pistons look like. There's no mistaking those.
View attachment 1716093769
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Those are the old TRW, Sealed Power pistons that Direct Connection doubled the price and sold as 11.5-1 pistons.
 
Arias also had a 12.5:1 small block piston. I have a set here. Big dome.

4C542ECF-3DC1-4008-9611-93DF2A4E6002.png


7B275411-DFFA-4590-A2DC-23B2E500B78F.png
 
First picture pistons are 11.8-1 I ran this motor on C14 at the track . Pump gas mixed on the street. second two pictures are pistons 13.95-1. The compression ratio is determined with the combination of parts not just the piston. That 11.8 piston is zero deck height. as is the 13.95 piston.

These numbers were calculated with 4.080 bore and a 4 inch stroke .63 cc head volume , 4.20 dome volume .035 head gaskets, Cubic inch is 418 ratio comes to 13.95-1.

100_0074 (3).JPG


100_0206.JPG


100_0207.JPG


20211006_180709.jpg
 
Yea I (and I believe we) were speaking about advertised CR numbers. All pistons have em and honestly they shouldn’t.
 
TT5.9mag said:
Yea I (and I believe we) were speaking about advertised CR numbers. All pistons have em and honestly they shouldn’t.
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I've always felt the same, but they do. Maybe use the phrase "Can achieve 12.5:1" or some such.
 
roscoes-garage said:
Hello All,

I have a 340 I purchased several years ago as a short block. I was told it was 12.5:1 pistons they are .040 over. I've cleaned it and am ready to complete for my duster.
the issue is I purchased a set of the 340 Edelbrock Heads (60179) with the 65cc combustion chamber i am using .040 MLS head gaskets and the top of the piston are hitting the heads.

The combustion changer depth is 0.050 I also had a set of J heads sitting around (they need to be reworked) they are at 0.100

I know there are a lot of thickness options i have on the MLS head gaskets. Has anyone else run into this issue? the block has never been decked. (I know the full history) is there a combination that worked?

Thanks
Chad
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Seems the pistons were designed/intended for open chamber cast iron heads.
I would put strips of clay on a piston, install the head with gasket you intend to use and turn it over with a wrench. When it stops without forcing, pull the head off. This will show where and about how much to have a machine shop take of the piston crown. Might be possible to set up the pistons in a lathe to cut the offending amount off.
With a 0.040" compressed gasket you probably want the piston 0.005" above the deck for strong quench and swirl.
 
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