1972 340 TRW piston question

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125Gretsch

125Gretsch

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Hello! I could really use some help on my street 340 build.
Its a 72 340 block bored 040 over with rebuilt 68 X heads which CC'd at 67.2cc. New valves with hardened seats.

Block is at the machine shop getting new cam bearings and will need to be honed. The TRW 2316 Pistons looked pretty good except for #8 which had about a 1" scuff mark on the skirt below the oil ring. Oddly enough the cylinder did not look like it had seized.

The shop mic'd the pistons and I have about .003 of clearance before honing. He told me the #8 piston has slightly more clearance and I could reuse the pistons but I may get some piston noise. He's leaning towards new pistons but its on me to decide. He's recommending basically the same piston (speed pro) as a replacement.

Reading about forged piston clearances it seems like I should have good PTW clearance sticking with the pistons I've got.

Any help would be greatly appreciated!
 
125Gretsch said:
Hello! I could really use some help on my street 340 build.
Its a 72 340 block bored 040 over with rebuilt 68 X heads which CC'd at 67.2cc. New valves with hardened seats.
Block is at the machine shop getting new cam bearings and will need to be honed. The TRW 2316 Pistons looked pretty good except for #8 which had about a 1" scuff mark on the skirt below the oil ring. Oddly enough the cylinder did not look like it had seized.

The shop mic'd the pistons and I have about .003 of clearance before honing. He told me the #8 piston has slightly more clearance and I could reuse the pistons but I may get some piston noise. He's leaning towards new pistons but its on me to decide. He's recommending basically the same piston (speed pro) as a replacement.

Reading about forged piston clearances it seems like I should have good PTW clearance sticking with the pistons I've got.

Any help would be greatly appreciated!
Click to expand...
Piston knurler. I was shown how, but having the tool or machining to do it is another feet for newer shops.
As the title says..
The Lost Art of Knurling Pistons Takes Skill – And Guts
 
I run my hypers at .0035, and only hear them at the end of the season, like cold September mornings. I think you'll be just fine with the forged 2316s.
I'd be more concerned with the condition of the ringlands which is where the sealing sometimes goes away on used pistons.
 
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on my 1970 34- idumped the old heavy TRW pistons for a set of Mahle pistons
they are lighter and make no noise put 1200 hard mile on them and that engine never burnt oil between changes and was quiet , no piston noise at all . I have another 1970 340 i'm building this winter , the old TRW pistons are going on a shelf and a new set of Mahle are going in . would not waste my money on the speed pro pistons
 
Speed Pro pistons are great replacement pistons. I have used them many times. I would just buy a new set, but maybe that's just me. After all you are rebuilding the engine. My motto... when in doubt throw it out. They aren't that expensive.
 
I'm using 060 over trw 2316s in my hillbilly 340 rebuild. They are a little heavy but you can beat on them with a hammer if you want to. Lol
My heads are 68cc.

Got a picture of #8?
 
If you can afford new pistons, I'd replace them. If you can't, at least get a new one for #8, weight match it to the rest, and run it.
 
Hypers for street are lightweight, but you will be forced to lighten your crank or maybe a 318 crank will be close. My 367 Revs like a Wankle with those.

But on the street, those 2316s should be fine; at WOT, you only have one shift from zero to 65mph, and you'll probably be spinning most of the way,.... so piston weight is not gonna be a big deal.
 
AJ/FormS said:
Hypers for street are lightweight, but you will be forced to lighten your crank or maybe a 318 crank will be close. My 367 Revs like a Wankle with those.

But on the street, those 2316s should be fine; at WOT, you only have one shift from zero to 65mph, and you'll probably be spinning most of the way,.... so piston weight is not gonna be a big deal.
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Awesome! Thank you all for the responses!

Sounds like a new set of speed pros will probably be the best way to go. I can get the pistons and molly ring set for right about $400. The crank just came back ground 10 off as it was pretty scored up by the rear main.



Ill add a pic #8 when I pick up the old pistons. It seems like the previous owner of the motor re ran them after whatever caused the scuffing on #8 didn't show on the cylinder.

The Mahle's seem like an awesome set but a little out of my budget.
I also found a new set of 40 over silv o lights for $215. Don't know anything about them tho.

Any thoughts on my compression being too high for a street motor in CA? I only measured one piston before I tore it down but it was about .011 positive of the deck. I tried one of the calculators and came up with 10.1:1

Thank you all! Ive had this Dart for about 7 years. Brought it a long way and can't wait to get this 340 finished and in!
 
Be prepared to use high octane fuel if you truly have a 10:1 compression ratio....especially with our "great" Ca oxygenated fuels.
 
Compression ratio is a tool we use to estimate the cranking cylinder pressure, along with the Intake valve closing angle, and the elevation at which the engine will be operated in.
10/1 by itself means very little.
With iron,open chamber heads, the usual target is 160 psi, for WOT operation with best available gas; 91 (R+M)/2 octane or higher.
Wiki says Santa Rosa CA is at 164ft elevation, so you will have to be careful on your cam choice for 10/1Scr
The Wallace calculator suggests an Ica of 65* or more. IIRC, the factory 340 cam is very close to that, and I think it will work nicely; just don't expect any fuel economy with that cam,lol; she loves her gas.....
I have heard that CA gas is tricky to work with.
One thing you can do, is run alloy heads. I have run those at over 185psi with 87E10 gas with full for alloys timing, of 32 to 34 degrees. Others here on FABO have run up to 200psi on best gas. So that gives you a lot of freedom to chose components. But if you do that, I suggest to run flat-tops at zero deck, and closed chamber heads. After that you can do darn near anything. Like run a smaller more modern tighter LSA cam to get both STREET power, AND decent fuel economy.
Running your 340 up at 185 psi will make the bottom end feel like an iron headed BigBlock of yesteryear.
 
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I like the sound of 185 psi and alloy heads but I’m a little married to the X heads for now I guess.

I went with a comp xe268 for my cam. I couldn’t find the Ica on the cam card but doing a little research I believe it was calculated at 70 Ica a couple years ago.

so much knowledge and info on this site it’s awesome!
 
i ran my 340 at 10.4 compression with iron heads needed good gas and took a while to tune . the heads were mildly ported and polihed the chambers . had 190 cyl pressure
good luck with your build
 
125Gretsch said:
Hello! I could really use some help on my street 340 build.
Its a 72 340 block bored 040 over with rebuilt 68 X heads which CC'd at 67.2cc. New valves with hardened seats.

Block is at the machine shop getting new cam bearings and will need to be honed. The TRW 2316 Pistons looked pretty good except for #8 which had about a 1" scuff mark on the skirt below the oil ring. Oddly enough the cylinder did not look like it had seized.

The shop mic'd the pistons and I have about .003 of clearance before honing. He told me the #8 piston has slightly more clearance and I could reuse the pistons but I may get some piston noise. He's leaning towards new pistons but its on me to decide. He's recommending basically the same piston (speed pro) as a replacement.

Reading about forged piston clearances it seems like I should have good PTW clearance sticking with the pistons I've got.

Any help would be greatly appreciated!
Click to expand...

“Send it.”
 
I'm just throwing this in there for you.
1600569919548.jpg
1600572512088.jpg
I don't know if this will work for you but I have these on the Shelf unused.
 
I run Speed Pro’s in my 340. Have no issue with them. Engine made 432 hp on dyno. I’m running a little bigger cam than you are.
 
Dblackmt said:
I run Speed Pro’s in my 340. Have no issue with them. Engine made 432 hp on dyno. I’m running a little bigger cam than you are.
Click to expand...
That’s awesome power! Ordering the speed pro’s tonight. What else did you do to make that output? I’ve got a set of dougs headers, going to run an airgap and leaning towards a street demon 750
 
Running the Comp XE275HL Cam. X heads that have a good valve job. 10.5 compresson. Air Gap intake and 750 cfm edelbrock. TTi headers, 3200 stall and 391 gears. It’s in my 67 dart. Run 93 octane gas.
 
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