I THINK I KILLED THE 340

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spd691

spd691

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Here is the deal. I bought a 73 Dart Sport 340 car. It has a 69 340 in it with X heads. I had a bent valve and other issues that needed addressing so I took it out. I decided to "freshen it up". I took it out and apart. Found bent intake valve and heads needed rebuilding. I decided I would put a set of closed chambered aluminums on and set the quench to .040 with gaskets and if needed crown machining. The deck didn't look so good, very grainy, so I sent it out for decking. Crank was .020 under on the mains and .030 under on the rods and looked nice. Home polished it miked it, good to go, pistons are .030 over and look great even the timing chain was tight. I had one odd ball rod so I bought a set of "H" beams in stock length. I was told the block was twisted and a special fixture was needed to deck it properly. I told him its the only block I have so make it right. When I picked it up I asked how much was removed. His reply was which end? I had a sinking feeling... Fast forward to today. I was doing my mock up to see where I stood to get the quench I was after. I measured the piston to deck before I disassembled it. It was +.018.

Now the problem. After carefully measuring the deck and triple checking it and inline with the wrist pin, checking the rock, I think I'm screwed. I am way out of the hole now (see pics). With a .040 gasket I would need to remove .040 off the top of the pistons. Seems like to much. Then I may have a problem with the valve pockets. I already own the heads so... Any ideas you motor heads could throw my way would be REALLY appreciated. I don't want to source a another block. But the car must live on. (I soda blast the top of one of the pistons)

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Did you actually bolt on a cylinder head with clay and check the clearances with the valve train and a used head gasket?
 
So machine shop has not a clue what the actual deck height is? When I had my 340 block machined they stamped 9.60 on the pad by distributor. 9.60 is the spec deck height. Whats the deck height of your block now? If they needed to deck below spec, you should have got a phone call hey we don't wanna machine it below 9.60 cause thats under spec height?
 
4eighteener said:
Yes, I see several options for .070 and .080 thick head gaskets on summit's website. That is probably the most cost effective fix.
Click to expand...
That would also help the intake manifold fit between the heads if the block is decked below spec. If they are MLS need to make sure surfaces have been machined smooth enough to seal with that type of gasket....
 
I have run the Q on my 367 as tight as .028 without problems, but have been running .032 since 2004. And
With alloy heads, she has run up to 195psi, still on 87 gas.
So, here are my guesses:

1) Or
you could just mill some shallow bowls in your alloy heads, one above each chamber.
Adding .032Q, and the gasket .039, then subtracting .034pop-up, I get .037chambers. You could share that with the piston crowns, depending on your piston to valve clearance.

2) Or
just put the X-heads back on.
As I recall, the chambers on those are around .100 deep; to which you add the .039gasket, and then subtract the .034 pop up, and I get .104 quench which in terms of Detonation resistance, is good to go.

3) Or
IIRC, there is available, an .052 composition gasket. Subtracting .034 from that, I get a crown-cut of just .014 to get a Q of .032.

4) By selecting the right cam,
you may not have to deepen the eyebrows, or if you do then not by much, and maybe only the intakes. You can buy a tool for that and do it in your garage.

5) In any case,
your probably gonna end up with a lotta SCR, which, if using the iron heads, is gonna take a late closing intake event, to get the pressure down to a safe level.

6) So then
I'm favoring the alloy heads on this one. If you haven't already bought some, Edelbrock makes 'em just for the 340, with 65cc chambers, which by the math makes the depth of the chambers about .020; tho I haven't measured them..
So then, with the thick gasket, you have;
.052 + .020, less the .034 pop-up; I get a Q of .038, dang near perfect.

7) piston to valve clearance is gonna have to be measured regardless.

8) but above all, check the distance from the top ring to the deck. If the compression ring gets to be too close to the action, yur gonna have trouble, there is no water up there. So then,

9) you may need some shorter rods..
 
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Syleng1 said:
Did you actually bolt on a cylinder head with clay and check the clearances with the valve train and a used head gasket?
Click to expand...
I have to figure out how I'm going to bolt the head on without piston interference first.
 
340sFastback said:
So machine shop has not a clue what the actual deck height is? When I had my 340 block machined they stamped 9.60 on the pad by distributor. 9.60 is the spec deck height. Whats the deck height of your block now? If they needed to deck below spec, you should have got a phone call hey we don't wanna machine it below 9.60 cause thats under spec height?
Click to expand...
Don't know the number. I won't be using him anymore.
 
Take it apart and change pistons or have the domes machined.
 
340sFastback said:
That would also help the intake manifold fit between the heads if the block is decked below spec. If they are MLS need to make sure surfaces have been machined smooth enough to seal with that type of gasket....
Click to expand...
Yup, both are smooth as glass. Never used MLS gaskets. I read about a lot of leaks. I might be over thinking though.
 
spd691 said:
I have to figure out how I'm going to bolt the head on without piston interference first.
Click to expand...
Double-up the old head gaskets, check one bank at a time. My Bud w/a 10sec. Buick GN runs Cometics, plenty of boost, so..., & I put a stock thickness Cometic on Pop's '87 ShelbyZ TurboII 5yrs ago, not one issue.
Just make sure to get the bore closest to Your actual 'top of chamfer' diameter +.020".
 
The '71 340 in my car has been rebuilt but is still standard bore (with a little wear) and I assume standard deck height. I found my pistons are above deck .025-.033" but pretty consistent on each bank. I wanted to put a set of small chamber aluminum heads on but going from J-heads to aluminum would require a .065-.070" Cometic gasket, or machining the tops of the pistons or opening the heads up. The Cometic gasket is the EASY button, no doubt. My build plan has changed and I'll be going .030" over but I found a set of L-2316 +.030 pretty cheap which should end up at about the same place above deck if I understand correctly, so the Cometic is still likely going to be the solution. I may mock it up and trim the pistons back to flat deck and then have it all re-balanced, but i can do the piston machining myself to save some cost and avoid custom pistons while maintaining the compression I hope to achieve. Either way, I would have no hesitation running the thicker Cometic as long as the deck surface was correct for the design.
 
Before I did anything, I'd take it apart if it's together and take the measurements with clay as outlined. You don't really know what you have unless you measure. I've had cases where I "thought" this and I "thought" that and I was dead wrong. Then I measured. lol
 
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