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Piston orientation???

Mopar Muscle
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Piston orientation???

jclaus68 jclaus68
New User | Posts: 33 | Joined: 09/09
Posted: 01/23/14
10:56 PM

Gents, hoping someone can shed some light on this or me, as I've searched high and low and can't find any quality information.  I'm in the process of building my first engine and I'm puttng together a 440, .040 over with cast pistons, stock rods and a stock cast crank.  I have paid very close attention to all the details, used the facorty service manual for all the proper clearances and specifications and have the short block completely assembled.  However, here's where I shot myself in the foot; I installed all the new sealed power pistons with the notches toward the REAR of the engine, not the front like everything I can find says they're supose to be.  The pistons did not come with any instructions whatsoever, and the only thing the box has to say is that they require a minimum of .0015 in piston to wall clearance.  I have something like .010 in all the way around all of them and I have all the rods on their proper journals and oriented correctly.  What I'm wanting to know is there any mechanical reason why a flat top piston with no valve reliefs, no offset skirts, and no machined differences from one side of the piston to the other would NEED to be oriented a certain direction in the bore???  I've seen forum posts where people have said that turning them around backwards in their bores will increase HP?!?  At the cost of more piston slap???  How could that possibly be if the piston is round and I have equal clearance all the way around the piston in the bore?  I've racked my brain and I can't think of any reason why it would be necessary for flat tops to go in a certain way, but that's why I'm coming to you guys for guidance.  If I have to tear everything back apart and spin them around so the litte notch atop each piston is pointing towards the front of the engine, then I will do so to ensure the $1000's of greenbacks I have into this aren't wasted the first time I go to fire this thing up.  But if I don't absolutely have to, I'd rather not.  Properly centering the wrist pins into the pistons and rods was definitely a chore and would prefer not to repeat that process unless absolutely necessary.  Thanks in advance for any info you can share.  

wayne712222 wayne712222
User | Posts: 121 | Joined: 10/13
Posted: 01/24/14
12:04 AM

if you have only assembled the short block..

its only an hour to turn them all around..

there is an old trick of installing the 440 pistons backwards..

because the pin is offset.. not in the exact middle of the piston.. there is a mechanical advantage..  kinda like having a longer rod.. it changes the rod angle vs the rod throw.. i seem to recall it made more torque but was hard on the pistons..

several people over the past 35 years have mentioned this. backward pistons in a 440..  it has to be written down in some mopar or magazine article from the late 60s or mid 70s..

oh... and in a top fuel motor.. its only a few minutes to pop the pistons and rods out and check them then slam them back in...  complete tear down and reassembly in under 45 minutes..  

marcohotrod340 marcohotrod340
Enthusiast | Posts: 470 | Joined: 09/08
Posted: 01/25/14
08:42 AM

most stock engines, ford mopar chevy, had cast pistons slightly offset to be slightly quieter when cold. forged pistons have the pin on center. an old trick for less friction was put the offset slugs in notch to the rear. It takes more than an hour to reverse PRESSED pins, unless u wanna just put rod + piston #1 in hole #2, and 2 in 1 etc., which is fine be me. and if it is a resto, maybe swap them, if u have more cam and exhaust than stock u will never notice any difference  

tuffnuff tuffnuff
Moderator | Posts: 324 | Joined: 03/11
Posted: 01/26/14
11:44 AM

First off, Offset pins are there to take (eliminate) the snap over of the piston thrust faces at direction reversal. it makes the engine quieter and reduces the chances of breaking a piston skirt over the TDC and BDC points. Racers like to use zero offset pins in the theory that there is more power through better alignment of the rod angle with the angle the bore makes with the crankshaft. If the engine is carefully blueprinted for racing (because where else would you spend that kind of money on dimensional and directional correctness) it probably has a tiny effect on total power output.,. but for an engine NOT subjected to such a rigorous and expensive blue printing, it's doubtful there's any effect, as the average build machining tolerances would quickly eat up any gain from such a single point of precision.
Now, since you installed the pistons with the notches to the rear.,. it raises the question of whether the large chamfer on the big end of the rod agrees with the fillet radius of the crank.,. this is very important!!!
Check it out and IF the big chamfer of the rods face each other on the crank journal.,. swap the even bank piston/rod assemblies,,,,,,, that is IF you want the notches to the rear.,. IF not, then get the rods turned on the pistons.
Typical pin offset is about .060"
By the way, pistons are "NOT" round.,. they are barrel ground where bulk mass of the piston has more skirt clearance for expansion (coefficient of lineal expansion) under running conditions, when hot.

When The Flag Drops.,.


The Bull ***t Stops.,.

P. Engineer, Engine Builder

3404spdvaliant 3404spdvaliant
New User | Posts: 20 | Joined: 01/14
Posted: 01/27/14
12:11 PM

Leave them alone, they might make a hair more noise...but you actually just freed up some torque by means of less cyl wall loading, old racer trick.

as for the wall clearance ... its a stacking, so you have .005  

3404spdvaliant 3404spdvaliant
New User | Posts: 20 | Joined: 01/14
Posted: 01/28/14
01:03 PM

Listen to tuffnuff on the chamfer, but technically you can switch them around and keep chamfers where they need to be.

the number stamps on the rods means nothing other than an easy reminder for chamfer orientation, but you can put the rods in whatever holes you want as long as the thrust sides face each other.  

3404spdvaliant 3404spdvaliant
New User | Posts: 20 | Joined: 01/14
Posted: 01/28/14
01:03 PM