building a 68 390

[jj]

Hi, to all. I have a 68/69 390 I am trying to build to go into 70 amx . I have a few questions I have been pondering, and would some additional imput from some one more experienced than i am . 1. Accordinr to the books I have the 68/69 390 piston sitts .047" down the holewould it worth while to change that to 0 deck height or leave it alone? If it is worth while can a 401 rod be used to do this? I know the piston will have to be machine for proper deck height, or would buying pistons with the correct compresion height be better? 2. Can the 68/69 connecting rods be safely be opend for wrist pin bushings?thank you very much, Joey

[jeepsr4ever]

You can use a 401 rod and piston and get 10.7-1 with 52 cc heads IIRC

[AMX69PHATTY]

I'm not experienced enough to know about the wrist pins.

The deck clearnce is the "modern" understanding as to how to help prevent detonation and good flame propagation and there are many page threads here that go all into that.

Guess it's just a matter of what gas ya wanna run, what compression ya want, how far ya wanna go, and how much ya wanna spend. You could also do some deck milling to reduce the clearance and be sure block is square and true, and/or also have the crank offset ground when you have it freshened up. Tough question to answer, fer me at least being a novice.

Is it "worth it" ? Supposed to allow higher compression with lower octane without getting detonation that can be destructive. And the higher the compression the more HP.
If yer gonna be runnin' pump gas not a bad idea IMHO, and get hardened valve seats put in the heads to work with todays unleaded gas. At least I thinks that's right. Don't think I've seen any discussion here about hardened valve seats and unleaded gas in AMC Heads. Don't have'em in mine, but it sees maybe 1000 miles / year when it's running and haven't had any valve reccesion problems.

jeepsR4ever
any "data" on hardened valve seats and unleaded gas ?

Good Luck with the rebuild.
Hope ya don't get stuck in "limbo" like I've managed to do for three years. Somebody slap me or kick my but or somethin' and make me get after it allready.
No wonder I'm bummed out, not able to do any big smokey burnouts !
:?:

[82Waggy]

Went through this myself on my 69 390 - same as yours.

401 rod is too long for an early block due to lower deck height, unless you go with a custom piston with a high wrist pin (short compression height) - which is not optimal.

The easiest route is to use your 390 rods and go with a custom piston with the compression height you need to achieve zero deck - wait till all other machining is completed (rod resizing, block decking and boring, crank turning), determine final required piston height for zero deck, and then spec the piston dimensions with the proper compression height, diameter, dish size, etc.

Regarding bushed rods, it's probably better to drop down to chevy wrist pin diameter instead of opening the rods up too much. I did this in my 401 so as to use the KB 354 piston - only had to open the rod up a few thousandths for the bushing.

I chose to go with stock diameter pressed wrist pins and custom forged JE pistons for my 390.

[jeepsr4ever]

390 rod 68-69 5.790
401 rod 5.858
__________________
.068 longer

390 piston height 1.56
401 piston height 1.506
__________________
.054 shorter

Using a 401 rod and piston in a 68-69 390 block you will be closer to zero deck by .014". Depending on the cc's in the dish or top of the piston you can easily hit 11-1 or 12-1 to warm it up quite a bit.

[82Waggy]

Sorry, I worded my previous statement poorly. if you use a 401 rod with a 390 piston the piston will stick out above the deck by .023 or so depending upon other machining.

My point is that a stock 401 piston and rod on a 390 crank still comes up short of zero deck in a 68/69 390 block by some 25 thousandths - possibly even more if you allow for some losses to resizing the rods and turning the crank and don't deck the block.

Since you have the 390 rods you might as well use them with a custom forged piston to achieve zero deck as you will need to get custom pistons even if you use a 401 rod - unless you just want to buy a set of 401 rods anyway. Even at .014 closer to zero as MC suggests you will still have a large .059 quench distance with the commonly available .045 head gasket - actually more if you do the math right:

390 stroke (3.574)/2 = 1.787 + 401 rod (5.858) + 401 piston (1.505) = 9.15 Piston height.

390 deck is 9.175 unshaved, so you will be .025 down in the hole even with a 401 rod a piston. This would yield a quench distance of .070 - you want .040 to .045.

Stock 390 piston to head clearance was .093 - which is lousy quench. AMC 360's are almost as bad as the 390.

Bottom line is that in order to acheive a zero deck and .045 quench you will have to buy custom pistons - no matter what rod you use.

[jj]

I guess I should have explaine my situation a little better. What I currently have is a 68 390 block bored .030 over . This is a fresh over bore . The crank was polished and checked good std. std. Icurrently have 2 sets of forged pistons 1 set is the trw L2313F I purchased years ago new and never ran the other set is a set of venolia 9 to9.5 to 1 pistons. The real problem I have is the rods that came with the motor were honed for floating pins upon inspection the wrist pin holes were .002 out of round and .002-.004 bellmouthed. I have got them trued up at .004 over. I have a second set of 68 390 rods but the need alot of work to be usable . I also have set of 401 rods needing much work as well. I know I have to have one set reworked to build this motor . Since I am this far into it I thought I would get some more ideas from some one more experienced than me on an amc motor. If my information is correct a 68 390 stock combination 3.574/2 = 1.787+5.790+1.560=9.137 9.175-9.137=.038 down the hole 390 piston with the 401 rod 3.574/2=1.787+5.858+1.560=9.205-9.175=.030 out the hole 3.574/2=1.787+5.858+1.505=9.150 9.175-9.150=.025 down the hole provided no machine work is downMachining .020-.030 from the top of a piston is not a problem for me .I know it will raise the compression. am useing a set of 72 360 heads with I think 56cc chambers. Sorry to be so long winded, thank you for any and all information Joey

[82Waggy]

Well, down in the hole 020-030 is better than stock.

[jeepsr4ever]

Well, down in the hole 020-030 is better than stock.

Yes and were the pistons you have the Venolia's and Speed pro's for a AMC 390 (68-69) compression height or for a 1970 390?

[jj]

68/69 compression height

[Holeshot]

You should be able to get pins in oversize (maybe MOPAR HEMI cut to length), or just get the rods re-bushed & re-sized.

You need to have the block square-decked to .000" to .002" or so.

Using the offset dowells & '70 & up heads are worth a ton more HP, so good going there. You can "open chamber" those heads to flow better & reduce 12~1 compression (advertised CR w/51cc chambers) pistons down to 9.5 to 10~1 or so.

Then, you need to know how many CCs your head's combustion chamber holds, in order to calc the CR.

THEN you can consider which pistons to buy.