Re: Re:Piston identification inquiry

["Dan Curtis" <d.curtis@xxxxxxx>]

Randy,
Actually, as the post stated, this is a 69 block, not a 70 block.  Also,
this motor has 50.6 cc heads not the stock late 401 58cc heads.

You have confirmed my initial suspicions that these were later model 8.5:1
401 pistons and not 70 390 "dish" pistons as claimed.  On the other hand
when using my compression spread sheet, if I enter the 69 specs of cylinder
bore: 4.195 (.030 over), stock stroke: 3.574, block height: 9.087 (shaved
.088), stock rod length: 5.79, with late model 401 piston height: 1.508, the
deck does come out to .002 which judging from the near equal alignment of
the piston with top of the deck might be the case.  With a .045 head gasket
and a 41 dish relief piston using 50.6cc heads, the swept volume is 809.482
and the compression volume is 102.245 resulting in a final compression ratio
of 8.91708.

Thankfully and as you stated, these are NOT Ross pistons provided by Mad Dog
in that they were bought nearly 5 years ago when the engine was built.  I
have the blueprinting spec sheet with all of the tolerances from this
reputable engine builder but his odd piston and deck combination proves he
really knew squat about AMC engines.  From the calculations I have from the
spreadsheet however, it should work out OK with an 8.9 compression ratio out
here on Phoenix's 91 octane "oxygenated fuel" as long as the 69 rods work OK
with these different piston and pin configuration.

Best Regards,
Dan Curtis
Cell 602 317 2018
d.curtis@xxxxxxx


----- Original Message -----
From: "Guynn" <amx69@xxxxxxxxxx>
To: <d.curtis@xxxxxxx>; <mail@xxxxxxxxxxxx>
Sent: Wednesday, December 29, 2004 4:29 AM
Subject: Re:Piston identification inquiry


> 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
>