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1970 390 piston choices

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smills61074 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote smills61074 Quote  Post ReplyReply Direct Link To This Post Topic: 1970 390 piston choices
    Posted: Mar/09/2012 at 9:40am
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 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 390spirit Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 11:36am
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote whizkidder Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 11:45am
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.
Ron Frost
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smills61074 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote smills61074 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 12:48pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote 67RogueX-Code Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 12:56pm
You may want to check with SC397, he can probably give you help you out with this. Thumbs Up 
Bob Wilcox

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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 1:28pm
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.
Dan Curtis, AZ AMC Collector Quality Restorations & Parts - amcmusclecars.com
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69 ambassador 390 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 69 ambassador 390 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 8:19pm
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.
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 440sixpack Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 9:10pm
 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote SC397 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 9:18pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote 17tamx Quote  Post ReplyReply Direct Link To This Post Posted: Mar/09/2012 at 10:16pm
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.
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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