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1970 390 piston choices |
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smills61074
AMC Nut Joined: Feb/05/2011 Location: savanna, il Status: Offline Points: 270 |
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Posted: Mar/09/2012 at 8:40am |
I have a 70 390 that I want to put in my 68 AMX. I have questions about using cast, hypertetic, or forged pistons. Another AMCer sent me a link discussing forged versus cast pistons. The article is very specific about the forged pistons not expanding equally when they are warmed up, which in turn does not make the rings seal as well. I know a forged piston is heavier. I would assume that there is some difference in balacncing, due to the extra weight. After reading the article. The hypertetic pistons make the most sense. Are the Keith Black pistons a good choice? Here is the link about the pistons. Any help would be appreciated.
http://www.motorcycleproject.com/motorcycle/text/cows-pistons. - www.motorcycleproject.com/motorcycle/text/cows-pistons.html
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390spirit
AMC Addicted Joined: Jun/28/2008 Location: U.S.A. Status: Offline Points: 1849 |
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Buy forged and save your self a lot of hassles, lots to choose from out there, I put Diamonds in mine,you can also contact Bulltear, 74bubblefender on here.
This is just my opinion |
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whizkidder
Supporter of TheAMCForum Joined: Mar/03/2008 Location: North Georgia Status: Offline Points: 2972 |
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I'm with 390 -- go with forged. Even for a mild street motor, forged slugs in a properly prepared block will be strong enough to forgive a multitude of other sins -- poor gas, too much timing, etc. If you maintain the stock compression ratio on your '70 390, and 291c's, you'll be on the edge of ping/detonation with today's gas. My 2 cents.
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Ron Frost
marne1ancient @ gmail.com 910 nine two two 0563 "There is no limit to what a man can do, so long as he does not care a straw who gets credit for it. Charles Montague |
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smills61074
AMC Nut Joined: Feb/05/2011 Location: savanna, il Status: Offline Points: 270 |
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I had a 69 AMX with cast pistons years ago. I had issues with broken ring lands. At the time, only TRW forged pistons were available in standard bore. If the internet was available, factory cast pistons were probably available. I do remember. The TRW's were supposedly expensive at the time. That was $200 in 1980. If you factor in inflation, $600 is probably about the same. Any suggestions on my 70 390 build would be appreciated. I can sell the crank and go with a 401 crank and basically 401 pistons as well. This is a great website with a lot of information. The head porting will probably be my next stumbling block. But, I will post another thread concerning that.
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67RogueX-Code
AMC Addicted Joined: Jan/21/2009 Location: Sylvania, Ohio Status: Offline Points: 1302 |
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You may want to check with SC397, he can probably give you help you out with this.
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Bob Wilcox
67RogueX-Code |
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amx39068
Supporter of TheAMCForum Joined: Feb/21/2008 Location: Arizona Status: Offline Points: 11576 |
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Bulltear, Nick Alfona, Wiseco and Diamond can get you anything you want?
I've gotten 70 390 pistons from both Bulltear and Nick Alfano (Diamond)and both were excellent quality. I just had to return a set of Keith Black pistons for a mild build to Summit and they are sending me out another set due to the first set having two damaged pistons. You can't go wrong with either Bulltear or Alfano's pistons. |
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Dan Curtis-Owner and CEO AZ AMC Restorations; Facebook: https://www.facebook.com/amcmusclecars/ & Curtis Real Estate Development
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69 ambassador 390
Supporter of TheAMCForum Joined: Nov/22/2009 Location: Peoria, AZ Status: Offline Points: 3539 |
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And forged pistons are not always heavier. They are denser and stronger. The extra strength means not as much metal is needed. So not always heavier. The old TRW slugs were heavy. And no piston expands equally, cast or forged. The skirts 90 degrees to the pin contain less metel than the pin axis does. A correctly designed piston is not round. It is designed so that at operating temperature the expansion has causes it to be round. The heavier areas expand more. Cast is less dense than forged so less expansion and tighter clearances cold. Hypers are just brittle harder cast pistons. This is why it is very, very important to run the correct clearances and at the correct temps. Too hot and the piston expands to the same size as the bore or bigger. Thats not good. If your lucky, the skirt collapses and gives extra room. What usually happens is not good. Too cold and the piston never fully expands and is loose in the bore. This leads to poor sealing and slap, which can cause skirt collapse. This is even true in street engines. Remember the Chrysler 2.2/2.5? Always follow the directions from the piston maker on clearances. no two makes are alike.
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Steve Brown
Algonac, Mi. 69 Ambassador sst 390 84 Grand Wagoneer 69 Cougar XR7 65 Fairlaine 500XL 79 F-350 Super Camper Special |
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440sixpack
AMC Nut Joined: Feb/23/2012 Location: Oregon Status: Offline Points: 481 |
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I'm not sure the KB hyperutectics will be in the compression range you want but if they are and you want to run them by all means do they're great pistons .
Most guys problems with hyperutectics is they think they pour the nitrous to them or they don't file the top ring gap to specs. I've run them in my mopar big blocks for years turning 7500 rpm and HP that would tie an AMC V8 in knots and NEVER a problem one. I know many others with the same experience, hypers have gotten a bad rap for all the wrong reasons.
I'm going with Bulltear I think because I want about 10.0 and I'm using 58cc heads so the KB's don't quite get me there. if I was going to use my original 51cc heads then I'm a little too high so I'd have to cut the KB's and there again it's just easier to give Bulltear a call and get exactly what I want fron the start.
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SC397
AMC Addicted Joined: Apr/30/2009 Location: Michigan Status: Offline Points: 5480 |
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I will never use Hyperutect pistons again. They can't handle any detonation.
I use the cast high compression '71 401 pistons in the '70 390's. I will have to go through the numbers but I think it lowers the compression ratio down to like 9.5:1 which of coarse makes it less sensitive to detonation. |
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17tamx
AMC Addicted Joined: Dec/23/2008 Location: Peoria, AZ Status: Offline Points: 1170 |
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401 pistons in a 70 390 block will be .060" down the hole and you will have pinging problems if not addressed.
I decked my 70 390 block .060" to get to zero deck height. I am using the Speed-pro forged 401 pistons with the .170" dish. I took .024" off of my 291C heads to get me to 10.71:1 and I run standard 91 octane pump gas here in Phoenix without any detonation. It took 312 grams of mallory metal to switch from external to internal balance. If I had used the lighter more expensive forged pistons the pistons would not have been $333.00 from summit but I would have saved money on the balance work. If I was doing it again in (2000) I would have bought the lighter forged pistons (Ross or J&E at $600+) rather than the Speed-Pro (TRW $333) from Summit. Now that we have the perfect AMC piston design from BullTear the $635 seems like a great deal and the best way to go IMHO.
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Kirk P. Fletcher
70 AMX BBG w/Shadow 390 4sp 71 SC360 Wild Plum Ram Air 4sp 67 Rogue Convert 343 4sp 66 Rogue Hardtop 290 Auto 66 440 Convert 232 Auto |
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