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  #1  
Old 05-22-2007, 08:26 AM
bandit67 bandit67 is offline
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Default 318 Are my pistons too deep in the hole...............

Hi Guys, got a 72 stocker I am trying to rebuild to get me going. Punched the block 30 over and installed new cast pistons. Since I have full floating rods, I stuck old and new pistons on same pin and both pistons seemed to be the same height. But after installing them , they measure about .075 down below deck. This seems to low to me , but , I cannot find any specs in my books. Did notice the new pistons have four valve releif notches where as the orginals were flat tops. I did not think to measre them before disassemble..........anyone know what the deck height should be......John
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Old 05-22-2007, 04:00 PM
70Ted 70Ted is offline
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Not if low compression was the goal.
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Old 05-22-2007, 04:45 PM
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pishta pishta is offline
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Ill look it up tonight.
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Old 05-23-2007, 05:21 PM
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rumblefish360 rumblefish360 is offline
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Deck height is basicly 9.6. But that doesn't matter, you'll need to measure since there are factory differences even in the same block.

Click here for 318 pistons; http://kb-silvolite.com/forged.php?a...0&RodLen=6.123

Oooops, those are stroker 390 pistons. But theres the KB link. Click around some.
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Old 05-23-2007, 10:12 PM
it's all dodge it's all dodge is offline
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the Mopar Performance circle track book SAYS 1.74 comp height on a '72 318, so thats about what you had, the stocks would be .081" in the hole if they where 1.74. 1.821 would be a zero deck if the deck height was a real 9.599, your low low 8s for compression, but hey, they once sold me a set a 318 pistons that were 1.72 comp. height or about .100 in the hole, that was in Colorado Springs, altitude above 6000 feet, need all the compression you can get there, I took them back, only thing better they could get me was 1.75s, so, I went out and bought some custom Ross pistons with Chevy dia pins, more freaking machining to put them on my rods, then I gave up on it, got a deal on a 340
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Old 05-24-2007, 02:58 AM
DartGT66 DartGT66 is offline
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All the factory 318's had abotu the same compression height pistons, and therefore about the same compression ratio. Your pistions are pretty typical, the ones I've seen have been about .080-.090" below the deck at TDC. The factory open chamber heads are typically about 68-70cc's, add .040" thick typical head gaskets and you are right at about 7.5:1! I milleed my cylinder heads .090", and achieved a whopping 8.7:1 CR. Without changing pistons, there is no easy way to bring the CR of a 318 up to reasonable figures for street performance.
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Old 05-24-2007, 08:38 AM
bandit67 bandit67 is offline
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Default 318 pistons...........

Thanks , Guys, for the input. I called my machinest back and we relooked at the pistons used and came up with the same results. Seems like the 1.74 comp height is the standard from the sixties into the eightys for the 318. No wonder these motors are not known for anything special. The link to KB Rumblefish sent shows their stock piston on the hyperutectic side to be at 1.81 comp. height which would be much better I beleive. I had the heads cut some and am planning the use the Mopar thin head gasket which comes in around .30. This would yield somewhere around 7.9 I guess.........gezzzzzz.....not worth the effort . John
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Old 05-24-2007, 09:46 AM
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Quote:
Originally Posted by pishta View Post
Ill look it up tonight.
block deck height is 9.94 for al LA motors. compression height for stock pistons 318 is 1.759 67-69 (.036 in the hole) 1.750 (.060) 70-71 and 1.739 (.056) 72-86 according to Taylor/Hofer
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Old 05-24-2007, 09:55 AM
345Dart 345Dart is offline
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Quote:
Originally Posted by pishta View Post
block deck height is 9.94 for al LA motors. compression height for stock pistons 318 is 1.759 67-69 (.036 in the hole) 1.750 (.060) 70-71 and 1.739 (.056) 72-86 according to Taylor/Hofer
me thinks you hit the wrong keys - block deck height for SBs is 9.600" I believe.
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Old 05-24-2007, 08:48 PM
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Quote:
Originally Posted by 345Dart View Post
me thinks you hit the wrong keys - block deck height for SBs is 9.600" I believe.
That is what they listed. But they are wrong..From the crank centerline to the head gasket surface. Lets add it up:
1.655 crank throw (3.31 / 1/2)
6.123 rod length
1.759 piston compression height
.036 piston to deck (in the hole)
That is 9.573 they are clearly misprinted! Thanks!
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Old 05-24-2007, 11:40 PM
it's all dodge it's all dodge is offline
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9.6 " deck height, according to my M.P. circle track book. here are some pistons heights they list for 318: '67-1.75, '68-9 is 1.77, '70 to '84-1.74, 1.76 for 85 to 86, for what it's worth, not sure if they really changed that much, or why that much. my book was wrote after 87 or so, bought in 91 0r 92, heck, the cylinder heads only list to '83 for 318, and '80 for 360 You can say they coulda done better and listed all the stuff! maybe the current oval book is better lotsa $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ $$
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Old 05-25-2007, 01:58 AM
DartGT66 DartGT66 is offline
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When I milled the heads .090" to achieve even decent CR, I had to fix several other things. First of course, milling the intake faces of the heads so, that the intake would fit. This ended up with very narrow valve cover gasket rails; I could live with that tightening the vc bolts very lightly. There is a chance, that the narrow rail cuts through the gasket, if you overtighten. Next was the end rails fo the block, the intake was hanging on them. I hand trimmed the end rails and the intake to fit. And there was still the lifters; there was now .090" more preload than before. I fixed that by grinding the stock pushrods shorter. This was a "no buck" project, and the results were actually pretty satisfying. Even with the still only 8.7:1 cr, the engine produced some power and was very crisp. I overcammed it a little for the combo, because I got that particular cam dirt cheap, and that propably hurt the real world performance a bit. The cam was a mopar performance 268/272 degree, and I installed it five degrees advanced. Anyway, I installed the 318 in to a '80 Aspen sedan, 904 with stock converter and 2.95 gear ratio without sure grip. Other than that, the engine had home ported stock valve 318 heads (70's castings), performer intake, 625 carter carb, cheapo headers to 2 1/4" duals with turbo mufflers going in to a single 3" with one long straight through muffler to keep it quiet. There was also some "no buck" tricks, splash shields below the intake, heat channels for intake blocked, modified ignition curve in the stock distributor, reversed pistons etc. When installed in the car, I had a total of 2000$ in the car, and that includes every penny put in to parts or outisde work, so that includes the tranny rebuild, exhaust etc. The only time I have ever kept book about the expenses. The car was originally a /6, and I did performance and mileage comparation between them. The mileage got about 20% worse, but the performance increase was well worth it. I run it at the strip only once, had some bogging when the secondary trap door opened, but still run 15.1 in the 1/4. Didin't get the trap speed, but half way speed was 77 mph, and half way et 9.71. Considering the car wasn't very light, and the converter, gear ratio and the lack of sure grip, I think it run pretty well.
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Old 12-17-2018, 02:44 AM
Ironhyde Ironhyde is offline
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Default 318 pistons too deep

I hope that the OP DID NOT end up milling his heads or his block. EVeryone gets so caught up in compression ratios that they forget that port flow in the heads dominates the quest for power gains.

Were this my 318, i'd saunter over to Aero Head and buy a set of their RHS style LAX heads. Big valves, big ports, small chambers. Whatever you mount these to, you will notice gains. Small chambers bump compression a bit. 1000 bucks for a pair makes them cheap, too.
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