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Old 04-22-2011, 04:42 PM
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Offset wrist pins. What to expect ?

I'm starting a build and I see there are two options with the same KB pistons. The set of eight are center type pins and the others come in left and rights individually. This is for a 383 sbc with 5.7 rods. Around 9.2:1 compression, Block is zero deck with a 1.433 piston ht. Which seems to put me out of the hole by .008" I'm wondering what affect the offset pins would have with this build? What would the effect on running be as well? Durability and so on ? Any difference with fuel octane requirements mpg. ?

I just tanked up today 150 bucks in a pick up and the pump didn't even click off before I chickened out. $ 5.02/ gal.

My first pick up was a 69 gmc. right from empty only took $14.00 to fill.
My pay sure didn't go up no 10 times.

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Old 04-22-2011, 05:42 PM
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offset pins wont make a difference. The piston compression height that it was manufactured with will be what changes the "in hole depth" other than decking the block.
The offset is for noise suppression and it tends to be a little easier on cylinder wall wear.
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Old 04-22-2011, 06:05 PM
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Quote:
Originally Posted by mumbles
I'm starting a build and I see there are two options with the same KB pistons. The set of eight are center type pins and the others come in left and rights individually. This is for a 383 sbc with 5.7 rods. Around 9.2:1 compression, Block is zero deck with a 1.433 piston ht. Which seems to put me out of the hole by .008" I'm wondering what affect the offset pins would have with this build? What would the effect on running be as well? Durability and so on ? Any difference with fuel octane requirements mpg. ?

I just tanked up today 150 bucks in a pick up and the pump didn't even click off before I chickened out. $ 5.02/ gal.

My first pick up was a 69 gmc. right from empty only took $14.00 to fill.
My pay sure didn't go up no 10 times.
X2 to what latech said. Offset pins are there to take (eliminate) the snap over of the piston thrust faces at direction reversal. 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 ('cause where else would you spend that kind of money on dimensional and directional correctness) it probably has a tiny effect on power output; 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 single point of precision.

Bogie
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Old 04-22-2011, 07:26 PM
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Originally Posted by oldbogie
X2 to what latech said. Offset pins are there to take (eliminate) the snap over of the piston thrust faces at direction reversal. 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 ('cause where else would you spend that kind of money on dimensional and directional correctness) it probably has a tiny effect on power output; 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 single point of precision.

Bogie

So, No real down side to offset pins other than cost.
But piston and cylinder wear is reduced, and noise or piston slap is also reduced when installed in recommended direction.

What about when you put them in backwards. Isn't that an old trick that causes the piston to reach higher in the bore and sends the rod down at a sharper angle to create more torque at the crank?
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Old 04-23-2011, 06:10 AM
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Nope. Compression height (piston pin height) and deck height are the only two factors to consider there. offset does not affect that.
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Old 04-25-2011, 10:24 AM
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Quote:
Originally Posted by mumbles
So, No real down side to offset pins other than cost.
But piston and cylinder wear is reduced, and noise or piston slap is also reduced when installed in recommended direction.

What about when you put them in backwards. Isn't that an old trick that causes the piston to reach higher in the bore and sends the rod down at a sharper angle to create more torque at the crank?
Yes reversing the offset was an old trick, it gains about as much a going form 5.7 to 6 inch rods which may show in hundredths of a second on time slips but isn't apparent to the driver as to how the car feels in the seat of your pants. The down side of this trick is a really bad piston slap which will eventually break skirts but for a competition engine that sees frequent tear downs and parts replacements this isn't a problem. For an engine that isn't opened up frequently it would be a problem because that cracked or busted piston skirt is out there waiting for you to arrive.

Bogie
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Old 04-25-2011, 10:43 AM
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Thanks Bogie,

The most important thing to me is to build a fun daily driver with lots of low end.
That's what I thought offset pins installed properly should enhance, and help with longevity.

I just wasn't sure if the extra dough was worth spending, since center pins come in 8pack.

Now if putting them in backwards = longer rods , which is better for high rpm.

Then am I to assume that that putting them in normally I could expect them to be better at low rpm. Not that there would be much of a difference, but some?

And running quieter too?

Also we still had snow up here a couple days ago. And our engines see allot of cold start ups.

Are you thinking like me ? Ya spend the few extra bucks.
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Old 04-25-2011, 11:39 AM
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Quote:
Originally Posted by mumbles
Thanks Bogie,

The most important thing to me is to build a fun daily driver with lots of low end.
That's what I thought offset pins installed properly should enhance, and help with longevity.

I just wasn't sure if the extra dough was worth spending, since center pins come in 8pack.

Now if putting them in backwards = longer rods , which is better for high rpm.

Then am I to assume that that putting them in normally I could expect them to be better at low rpm. Not that there would be much of a difference, but some?

And running quieter too?

Also we still had snow up here a couple days ago. And our engines see allot of cold start ups.

Are you thinking like me ? Ya spend the few extra bucks.
Yeah this is getting to be a long winter, we got one sunny day Saturday, Easter went back to rain on the coast and snow in the mountains.

I think you're best off using an offset pin in the normal orientation. That will give the most part life and quietest operation from the otherwise constant click of zero and reverse offset pins, especially when the engine is cold. Structural longevity of zero or reverse offset pins requires a forged piston. This really requires the engine be warmed up before operating loads are put on it otherwise the constant snap over of the skirts will crack them if not actually break them off. These pin configurations also make it hard for the rings to adequately seal compression and control oil on the cylinder walls. The gain in torque and horsepower by doing this is minimal, it's the kind of thing that professional racers who are top competitive in their class worry about as they are in a world where extremely small power differences can make or loose the weeks paycheck. This is not the kind of change that's going to transform a so-so weekend warrior into America's or Canada's next Top Model, er ahh, Race Driver.

The difference of long rods and piston pin placement are only each in the order of a fraction of to 1% or so on power output. For the money spent there just isn't much torque or power there. If you just have to do something go with a longer rod. It does a better job of stabilizing the piston in the bore which reduces blow-by, oil consumption, and cylinder wall/piston skirt wear. They give a little extra power but what they do best is to really extend the life of the piston, rings, and cylinder wall, so that the engine keeps running at its best power potential for much longer than will happen with zero or reverse offset pin placement.

Bogie
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Old 04-25-2011, 02:28 PM
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Thanks again Bogie,

The pistons I have chosen are hyp-u-tec. And come with a choice of two different ring thicknesses. 1/16, 1/16, 3/16. And 5/64, 5/64, 3/16 . What I was wondering is if the thicker rings add more stability as well? Or do the narrower set leave more distance to the top of the piston. I may have to remove some to stay within safe quench, since these pistons 1.433" are .008" above 9" and 5.7 rods are supposedly 5.703" in reality. My deck is also .0015" below zero on one side and .001" above on the other. With a .040 gasket looks like I'll have to take off some .008" and adjust chamber size or take some out of the dish on the high side.
Fun Wow!!
Thoughts ?
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Old 04-25-2011, 04:16 PM
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Quote:
Originally Posted by mumbles
Thanks again Bogie,

The pistons I have chosen are hyp-u-tec. And come with a choice of two different ring thicknesses. 1/16, 1/16, 3/16. And 5/64, 5/64, 3/16 . What I was wondering is if the thicker rings add more stability as well? Or do the narrower set leave more distance to the top of the piston. I may have to remove some to stay within safe quench, since these pistons 1.433" are .008" above 9" and 5.7 rods are supposedly 5.703" in reality. My deck is also .0015" below zero on one side and .001" above on the other. With a .040 gasket looks like I'll have to take off some .008" and adjust chamber size or take some out of the dish on the high side.
Fun Wow!!
Thoughts ?
Hyperutectic pistons can be run with tighter skirt clearances than standard alloys because they don't expand and contract as much with temperature change. Since the tighter clearance reduces piston rocking around the pin it will let you run an off set pin piston closer to .035 squish/quench clearance without risking banging the piston into the head. For a street engine this is tighter than I like but not a disaster. You can use multiple layers of standard head gaskets to adjust their thicknesses to dial in the squish/quench clearance. For example always run multiple head gaskets to adjust the thickness. A set of 3 .015 inch gaskets would give .045 inch minus your lowest cut deck of .00015 under 9 inches would leave a .0435 Squish/Quench clearance. Or you can stack a couple .015s and .019 to get .049. This eliminates having to surface the pistons to get clearance and results in pretty close to the magic .040 minimum clearance on a street engine. Essentially you're building your own MLS gasket, this is good because it provides greater conformity with the gasketed surfaces than does a one piece shim style gasket.

Actually the dimensions of rod length, piston crown height, even rod throw length differs from the standards enough with production items that trying to work this out to the knats butt mathematically before the build is not necessary, the best you can do with the published data is develop an estimate of the probable dimensions. In the end critical dimensions need to be measured to determine the cumulative effect of statistical production tolerances on every machined part and surface. So I just wouldn't worry this stuff too hard, the cost to machine parts to "Blue Print" tolerances is unaffordable to the average person and doesn't produce that much of a reward in power or economy compared to a nominal production accuracy.

Generally thinner rings are used in high speed competition engines where the thicker ring is more at home on the street. The big difference is wear, the wider ring spreads the load over more area so is subject to less wear. It also is designed to apply more force to the cylinder wall without the aid of operating pressures to force it against the wall because in normal day to day use those kind of pressures just aren't being developed with-in the combustion process. The thinner ring applies more force on less surface area so it wears out faster, but at the same time it depends upon combustion pressures behind the ring to get that force as when left to themselves the spring action of the just the metal develops less force than the wider ring. So these narrow rings tend not to seal up to the wall as well at lower engine speeds and loads as are usually seen on the street. They also are rather dependent on forced crankcase evacuation to lower the pressure on the underside of the piston to form a good seal. To that end they need more than PCV, they need either a positive exhaust vent or a crankcase suction pump to work effectively at low RPMs and nearly closed throttle positions such as are found at highway cruise speeds.

Bogie
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Old 04-29-2011, 04:35 PM
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A very good comprehensive look into wrist pin offset.
I have been wondering a couple other things as well.
Like the fact that combustion forces on the drivers side of the engine seem to be in line with increased pin offset. But the pass. side has the intake and exhaust on opposite sides respectively. Which would indicate less pin offset to align combustion forces with chamber location.
Have you heard of piston sets designed in left and right configurations with differing amounts of offset, for this reason? Or is the offset simply designed in for the mechanical aspect of piston movement? And not combustion forces.

Should the engine have been desingned as a radial multi- cylinder, much like what is seen on aircraft. These forces would be symmetrical, and not needing of any consideration.
As for my build this is going well over the top. But maybe not for all.
Thanks.
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