Re:Piston identification inquiry

[Guynn <amx69@xxxxxxxxxx>]

Dan,

The L2381 piston is a deep dish 401 piston. The dish is as deep as 3 
nickles stacked on top of each other. This is a Sealed Power/Speed Pro 
piston.

someone at the machine shop does not know what they are talking about 
here on 1970 model 390 deck heights. ALL 1970 model 390 engine had a 
deck of .002, not .010.

What someone did was to put the wrong piston in something and then mill 
the block to compensate for it. They unknowingly built a very, very low 
compression 390 engine. The engine will have less compression than a 
stock 401 with that piston. The stock 401 would be 8.5 at the max with 
this piston and the correct heasds belonging on a late 401.

Here is the problem. The 390 pistons, all of them, had a pin to deck 
measurement of 1.56 all 401 engines have the measurement at 1.51. So, 
let's look at this mathmatically. Stock deck height on a 1970 block is 
9.208, stock rod length is5.858, and 1/2 the stroke is 1.787. Styart 
with block height:
            9.208
less       1.787 1/2 stroke
leaves    7.421
less        5.858 rod length
leaves    1.563
This 1.563 is all there is left over to put the piston into. The stock 
piston is 1.56, so by math, there is .003 deck clearence, not .002 as 
Every factory book calls for. No factory AMC book anywhere that I have, 
this would include all factory TSMs, the Performance American Style 
book, nor any Chiltons or Motor Manual lists a 1970 390 as having .010 
deck clearence.

Now, when you couple the fact that you have 1.563 left, then you install 
a piston with a deck of 1.51, you can easily see that you have an extra 
.050 MORE deck than stock.  This means the piston would be Down the hole 
by .053 total, not .002 as Factory calls for, not the .010 the shop set 
it at.

This block, if it is a true 1970 block, has been milled quiet a bit, at 
least .040 to get you anywhere near a .010 deck.

In 380 terms, if you stick a stone stock 1969 piston in a 1970 block, 
you will Increase the compression by 1.279 points.A 1960 piston in a 69 
block  carries .037 deck clearence. A 1970 carries .002, so you can 
easily see that with the big 4.165 bore size, that .035 deck will affect 
a compression by over 1 1/4 point. If a 401 piston was used, and the 
block not milled, instead of the say, 8.5 compression a 401 would give 
with the piston, you can subtract compression to nearly 2, yes TWO 
complete points of compression less.

So, whoever morphed up the 390 you bougfht decided to mill the block to 
get enough compression to at least start and run the engine. I bet in 
all reality if you ''blueprinted'' the parts you have there you would be 
very lucky to have 8 to 1 compression there if you are lucky

These are exactly some of the points I tried to raise recently about the 
WRONG info which Mad Dog Racing has attached to thier 390 pistons. They 
cliam in a 1970 engine the piston carries 10.2 compression, and in a 
1969 engine it carries 10.0 compression. WRONG, CAN"T BE!!!!

It makes absolutely no difference what the starting compression of a 
piston is, i.e. 10.0, 9.5, 12.5, or any other ratio, when you install 
the same piston in two different 390 engines, If you use the correct 
block, and rod for the year of engine, the 70 style engine will always, 
everytime, have just over 1.25 MORE compression.

Every bit of this added compression comes from the deck difference of 
.035 between the two engines.This is exactly why both 1969, & 1970 390 
have 10.2 advertised compression, but the 1969 uses a flat top with .037 
deck clearence, and the 1970 model uses a Dished piston and carries .002 
deck clearence. Different pistons, different deck clearences. But again, 
if you decide to install the SAME piston in each respective engine, you 
will have just over 1 1/4 point in compression difference.  Not the .2 
difference that is claimed by Mad Dog on their Ross 390 pistons. Mad Dog 
was provided the wrong info someplace along the line. They in turn 
passed the info to Ross which built and figured the compressions. Ross 
made the mistake, but on;ly because they were fed the wrong info to 
begin with. The bad thing is, these pistons have been sold to folk 
thinking they were getting a certain compression ratio, but they are 
not. So, are the 390 pistons they are selling 1 1/4 point higher or 
lower?? and in which engine is the real 10.2 compression provided with 
the 390 piston they sell??

This is just the sort of problems that come up when dealing with folks 
that do not know AMC engines or the correct specs for them. They make 
mistakes and sell the wrong parts to folks. Even worse is that there are 
AMC vendors out there providing the wrong info, and then selling the 
parts and they do not even know they messed up. If the KNEW  AMCs, 
rather than just Think they Know something about them, then as soon as 
they saw a piston advertised like this they would KNOW it was wrong, 
they would put a stop to the problem.

Curtis, I suggest you get your hands on a book called AUTO MATH. This 
book will give you everty formula you will ever need to figure 
compressions on any engine. Next I suggest you get a good AMC related 
book that has Correct engine specs. With incorrect specs it does you no 
good to have the Auto Math book . And I assure you, .010 deck on a 1970 
390 engine is WRONG.

Good Luck,

Randy Guynn