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Old 10-21-2015, 02:44 PM
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AMC360 Build w/343 Pistons

This post is about a simple rebuild on a late model low compression 360 to get the most bang for the buck. The idea is to use off the shelf 343 flat top pistons to boost the compression ratio along with an inexpensive cam, intake and carb. The goal is to maximize the compression ratio but to keep it below the magical number of 10.0:1 to be able to run on regular old pump gas.
The first thing I did was go through some measurements and calculations to figure out what the compression ratio will be. The engine I am working with is a 1978 (and latter) 360 with 8.3:1 compression and late model bridge-rocker heads. I measured the heads along with a couple of earlier model heads to get the data.
According to the information I can find and have backed up with measurements, here is the list of 360/401 cylinder heads per years and their respective combustion chamber sizes.
51cc - 1970 & early 1971 #3196291C
58cc – 1971 through 1974 #3212993, #3213948, #3220502
60cc – 1975 through 1977 #3220503
63cc – 1978 and up.
Here are some photos of the combusction chambers.




I checked the cc's on the heads that I am going to use which checked out at 62/63cc.
The next thing I did was measure the dish and valve reliefs in the pistons.
The replacement dished 360 piston measured at 23cc and the valve reliefs in the 343 piston measured 6cc total. The compression height (centerline of pin to top of piston) is 0.001" difference so, it is a wash.


Once I measured everything, I plugged it all in to Diamond Pistons's Compression Ratio Calculator and liked the bottom line!



Last edited by SC/397 : 05-22-2017 at 06:32 AM.
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Old 10-21-2015, 03:01 PM
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I enjoy getting the engine building technical info from your threads.
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  #3  
Old 10-21-2015, 10:54 PM
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Quote:
Originally Posted by Strode
I enjoy getting the engine building technical info from your threads.
x2
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Old 10-22-2015, 06:28 AM
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Quote:
Originally Posted by Strode
I enjoy getting the engine building technical info from your threads.

Thanks! I try.
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Old 10-22-2015, 06:00 PM
BA_051 BA_051 is offline
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X3 on the tech info...

Originally I was thinking of using AMC 343 4v pistons in my 360... But changed my mind and stroked it to a 406 instead. Would have been about $1000 cheaper.

Were the 343's bolt in? Did you have to clearance anything?
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Old 10-22-2015, 06:23 PM
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Interesting stuff. What are you using for head gaskets?
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Old 10-22-2015, 07:16 PM
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Are you using stock 707 con rods?
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Old 10-22-2015, 08:13 PM
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Originally Posted by DarkMonohue
Interesting stuff. What are you using for head gaskets?
I am using head gaskets from a Engine-Pro kit mainly because I have a couple of sets. A few years ago I bought a couple sets of every gasket set made for AMC engines which is basically 3. Engine-Pro, Victor, and Fel-Pro. By the way, Fel-Pro is the best and most complete. This is a picture of the Engine-Pro gasket. It is a little thicker than the Fel-Pro but the inside diameter is smaller. This gasket will not work for a 401 like the fel-Pro does.
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Old 10-22-2015, 08:19 PM
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Quote:
Originally Posted by Carnuck
Are you using stock 707 con rods?
Yep! Reconditioned of coarse. This is just a basic rebuild With a cheap low-end torque cam and a Performer Intake. Bumping the compression ratio with 343 pistons is a bonus. It doesn't add costs to a normal rebuild.
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  #10  
Old 10-22-2015, 11:11 PM
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Quote:
Originally Posted by SC/397
I am using head gaskets from a Engine-Pro kit ... It is a little thicker than the Fel-Pro but the inside diameter is smaller. This gasket will not work for a 401 like the fel-Pro does.
Thanks. Wow, .058 is really thick. Nobody makes a thinner gasket with a bore appropriate for a 343/360 piston?

I guess you could deck the block down to get the piston .015"-.020" above the deck at TDC...

EDIT: I did find something. Looks like Cometic has MLS head gaskets with a 4.140 bore and thicknesses down to .027".
http://www.shopengineparts.com/parts...l=360+V8+70-91
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Last edited by DarkMonohue : 10-22-2015 at 11:22 PM.
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  #11  
Old 10-23-2015, 05:58 AM
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Yea, Cometic and Budget Build normally don't go together very well.
I went through the numbers using a Fel-Pro gasket which crushes to about .043", works out about the same compression ratio because the I.D. is 4.3" vs the 4.13" on the Engine Pro gasket. This engine will never know the .015" difference in quench. It would obviously be much worse with the dished replacement piston.

Last edited by SC/397 : 10-23-2015 at 06:20 AM.
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Old 10-23-2015, 06:06 AM
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How far below deck are the Pistons?
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Old 10-23-2015, 06:45 AM
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.02" it says on the chart.

HA! I answered a technical question from Ristow!

Winning!
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  #14  
Old 10-23-2015, 06:48 AM
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Quote:
Originally Posted by Ristow
How far below deck are the Pistons?

It is the same as a normal rebuild at .020" but with flat-top pistons.
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Old 10-23-2015, 11:32 AM
Ristow Ristow is offline
 
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i thought the 343 had a lower compression height?
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Quote:
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I LOVE how Ristow has stolen my comment about him ... "Quoted" it ... and made himself famous for being an ***hole to people. Hahahahahahahahahha!


→ Where the kids hang out...

fsjbuilder.org come for the mindless chat,stay for the hand drawn emoticons.

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Old 10-23-2015, 11:33 AM
Ristow Ristow is offline
 
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oh,lol....none of those charts above showed up on my iphone.
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Quote:
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... like the little 'you know what's' that you are.




Quote:
Originally Posted by Fasts79Chief
I LOVE how Ristow has stolen my comment about him ... "Quoted" it ... and made himself famous for being an ***hole to people. Hahahahahahahahahha!


→ Where the kids hang out...

fsjbuilder.org come for the mindless chat,stay for the hand drawn emoticons.

It's like you're unraveling a big cable-knit sweater that someone keeps knitting...and knitting...and knitting...and knitting...
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  #17  
Old 10-23-2015, 01:17 PM
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Quote:
Originally Posted by Ristow
i thought the 343 had a lower compression height?

They do by 0.001".
I measured the 360 dished pistion at 1.586" and the 343 piston at 1.585".
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Old 10-23-2015, 06:17 PM
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That is interesting. And that was an OEM 360 piston or aftermarket replacement? I've seen OEM described at 1.601 (IIRC) and 28cc dish. Thanks for giving us some real hands-on measurements to counter the wealth of internet rumor.
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Last edited by DarkMonohue : 10-23-2015 at 10:19 PM.
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  #19  
Old 10-24-2015, 10:44 AM
79 Wolfpack 79 Wolfpack is offline
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We're did you get the pistons?
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Old 10-24-2015, 11:29 AM
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343 has a shorter stroke and shorter block deck than 360 which is why I asked about the rods. I can't recall what they used in the 343 originally but I don't think it was 707s. I considered using 290 pistons in my 304 to run it on LPG.
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