offset wrist pins

Dave1695 · 04-21-2011, 12:48 PM

Is it better to offset the wrist pin on a 350 Chevy engine, IN toward the center of the engine or OUT toward the exhaust manifold ? Is there an advantage to one over the other? Have asked a few racers and gotten varying answers.

Alan Roehrich · 04-21-2011, 04:14 PM

When running flat top pistons with symmetrical valve reliefs, we turned the arrow that indicated the front of the engine to the rear, offsetting the wrist pin to the exhaust side.

GTX JOHN · 04-21-2011, 06:36 PM

On our Mopar Stuff only the cast production style piston had an offset pin which we reversed on perf. stuff.... I have not noticed a pin offset on the forged stuff. Am I missing something because the pin appears to be centered?

Alan Roehrich · 04-21-2011, 08:22 PM

Some the forged replacement pistons do have a pin offset.

John Dinkel · 04-21-2011, 10:17 PM

Seems like I remember that the offset pins were to make the engines quiet on cold starts.
Reversing them would make a little more power. An old Indian trick I think.

Dan Lattimore · 04-22-2011, 08:28 AM

Quote:

Originally Posted by John Dinkel

Seems like I remember that the offset pins were to make the engines quiet on cold starts.
Reversing them would make a little more power. An old Indian trick I think.

John-------You are correct ! Go to the head of the class! I think that old Indian was called Smokey and had a unusual last name.

Dave1695 · 04-22-2011, 12:23 PM

Thanks for the feed back guys, as I indicated the topic came up while bench racing and nobody knew which way the offset worked best. It's too late to try that trick this year but when I do, I'll post the results! If anybody else has other opinions or information, I would appreciate some feed back.

Adger Smith · 04-22-2011, 11:43 PM

(I copied this good explanation using others comments from another forum)
The offset direction is based on the direction the crank rotates. A.S.

"Offset Pins
Some of our pistons are supplied with offset gudgeon pins. This is to help reduce piston “slap” and have a pin that is offset to the thrust side. Although all piston crowns are marked accordingly, additional machining processes can inadvertently remove these markings. Offset pin pistons fitted incorrectly will result in mechanical noise and reduce efficiency. Race pistons defer almost exclusively to a central pin design, a necessity for engine speeds over 10,000 rpm.
Accralite Performance Forged Pistons "

"Most engines today utilize pistons with an offset pin bore. That is, the pin bore is "moved" a specific distance from the centerline of the piston. In gas engines the offset is always to the major thrust side of the piston. The piston thrust side is the part of the piston perpendicular to the pin bore that carries the majority of side loading during the power stroke.
The primary reason for pin offset is to prevent the piston from slamming into the cylinder bore after the connecting rod journal passes top dead center. This problem is referred to as piston slap. The desired scenario for piston movement is to gently rock from side to side within the bore. This rocking motion eliminates slap. " by Hunter Betts

"
Modern powersports engine pistons are not mounted on their connecting rods exactly on center. They are offset radially, that is, perpendicular to their rotational axis. In other words, in a vertical, single-cylinder engine, the piston is offset toward the exhaust side of the cylinder. That is, the piston pin hole is bored off center toward the intake side, usually less than 1 millimeter. It is frequently stated with a sort of bored complacency that the purpose of this offset is to reduce piston slap. The truth is, while piston slap is reduced through piston pin offset, but that is not the purpose of the technology.
The piston engine has three major parts: crankshaft, connecting rod, and piston. Each has a different job to do. The reciprocating part -- the piston -- makes the crankshaft -- the rotating part, uhh, rotate. The connecting rod is simply the part in the middle. It translates the piston's recip motion into the crankshaft rotary motion. The neat thing is that in the process, it shares in the motion of both. That is, the connecting rod is both a reciprocating part and a rotational part, at the same time. (In fact, when balancing an engine, it is common to divide the connecting rod's weight in two, thus permitting half its weight to be calculated as recip and half as rotation.) The point is, the upper half of the connecting rod reciprocates with the piston, the lower half rotates with the crankshaft, and this is important to understanding the stresses on all three parts, but especially those on the connecting rod.
The piston and its half of the connecting rod stops twice per crankshaft revolution, even though the crankshaft continues to turn. This means the piston and top of the rod also start back up twice. This stopping and starting imposes stresses on all three of the parts, stresses that increase with crankshaft rpm. To reduce these loads, the piston is mounted to the connecting rod slightly offset. This causes the piston to reach top dead center at a different time than the connecting rod,
effectively spreading the shock loading over a greater number of crankshaft degrees. In short, the real reason for piston pin offset is that it softens reciprocal loading, permitting lighter more power-efficient parts to be used, and the engines to be capable of higher rpm.
However, there is another phenomenon at work also, a kind of side benefit. Because the connecting rod spends most of its time in the engine at an angle, the piston engine has what is called minor and major thrust. Major thrust refers to the downward-stroking piston's force against the cylinder wall during combustion, due to the rod being angled in that direction. Minor thrust is the piston's thrust against the opposite cylinder wall during compression, because the rod's angle is opposite also. These thrust forces push the piston firmly against the cylinder wall. The important thing is that at TDC, they flip-flop. Major thrust turns into minor thrust, and visa-versa. In older engines, this flip-flop caused the piston smack the cylinder, resulting in a noise. Fortunately, the piston pin offset in today's engines, besides reducing inertia stresses, does two things that reduce this noise. First, because the piston is mounted off center, the transition from major to minor thrust is less sudden. There is less impact. Second, instead of a sudden lateral shift, the piston actually rolls from major to minor thrust. That is, the piston shifts first at the skirt, then gradually the rest of the piston makes contact, instead of all of the piston at once.
To summarize, piston pin offset is the manufacturer's way of reducing stress on reciprocating parts. It permits these parts to be lighter, which results in more efficient manufacture and less power loss in the engine, as well as higher rpm. A complementary result of piston pin offset is reduced piston slap due to the more gradual shift from major to minor thrust. "

04-21-2011, 12:48 PM	#1
Dave1695 Member Join Date: May 2009 Posts: 160 Likes: 0 Liked 15 Times in 11 Posts	offset wrist pins Is it better to offset the wrist pin on a 350 Chevy engine, IN toward the center of the engine or OUT toward the exhaust manifold ? Is there an advantage to one over the other? Have asked a few racers and gotten varying answers.

04-21-2011, 04:14 PM	#2
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,114 Likes: 1,572 Liked 1,827 Times in 414 Posts	Re: offset wrist pins When running flat top pistons with symmetrical valve reliefs, we turned the arrow that indicated the front of the engine to the rear, offsetting the wrist pin to the exhaust side. __________________ Alan Roehrich 212A G/S

04-21-2011, 06:36 PM	#3
GTX JOHN VIP Member Join Date: Jun 2009 Location: Boulder City, Nevada 89005 Posts: 2,733 Likes: 2,799 Liked 2,486 Times in 697 Posts	Re: offset wrist pins On our Mopar Stuff only the cast production style piston had an offset pin which we reversed on perf. stuff.... I have not noticed a pin offset on the forged stuff. Am I missing something because the pin appears to be centered?

04-21-2011, 08:22 PM	#4
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,114 Likes: 1,572 Liked 1,827 Times in 414 Posts	Re: offset wrist pins Some the forged replacement pistons do have a pin offset. __________________ Alan Roehrich 212A G/S

04-21-2011, 10:17 PM	#5
John Dinkel Senior Member Join Date: Nov 2005 Location: Pinckney, Michigan Posts: 507 Likes: 399 Liked 263 Times in 68 Posts	Re: offset wrist pins Seems like I remember that the offset pins were to make the engines quiet on cold starts. Reversing them would make a little more power. An old Indian trick I think. __________________ John Dinkel 3295 STK

04-22-2011, 12:23 PM	#7
Dave1695 Member Join Date: May 2009 Posts: 160 Likes: 0 Liked 15 Times in 11 Posts	Re: offset wrist pins Thanks for the feed back guys, as I indicated the topic came up while bench racing and nobody knew which way the offset worked best. It's too late to try that trick this year but when I do, I'll post the results! If anybody else has other opinions or information, I would appreciate some feed back.

04-22-2011, 11:43 PM	#8
Adger Smith VIP Member Join Date: Dec 2001 Location: Texarkana Ark/TX Posts: 2,446 Likes: 575 Liked 880 Times in 311 Posts	Re: offset wrist pins (I copied this good explanation using others comments from another forum) The offset direction is based on the direction the crank rotates. A.S. "Offset Pins Some of our pistons are supplied with offset gudgeon pins. This is to help reduce piston “slap” and have a pin that is offset to the thrust side. Although all piston crowns are marked accordingly, additional machining processes can inadvertently remove these markings. Offset pin pistons fitted incorrectly will result in mechanical noise and reduce efficiency. Race pistons defer almost exclusively to a central pin design, a necessity for engine speeds over 10,000 rpm. Accralite Performance Forged Pistons " "Most engines today utilize pistons with an offset pin bore. That is, the pin bore is "moved" a specific distance from the centerline of the piston. In gas engines the offset is always to the major thrust side of the piston. The piston thrust side is the part of the piston perpendicular to the pin bore that carries the majority of side loading during the power stroke. The primary reason for pin offset is to prevent the piston from slamming into the cylinder bore after the connecting rod journal passes top dead center. This problem is referred to as piston slap. The desired scenario for piston movement is to gently rock from side to side within the bore. This rocking motion eliminates slap. " by Hunter Betts " Modern powersports engine pistons are not mounted on their connecting rods exactly on center. They are offset radially, that is, perpendicular to their rotational axis. In other words, in a vertical, single-cylinder engine, the piston is offset toward the exhaust side of the cylinder. That is, the piston pin hole is bored off center toward the intake side, usually less than 1 millimeter. It is frequently stated with a sort of bored complacency that the purpose of this offset is to reduce piston slap. The truth is, while piston slap is reduced through piston pin offset, but that is not the purpose of the technology. The piston engine has three major parts: crankshaft, connecting rod, and piston. Each has a different job to do. The reciprocating part -- the piston -- makes the crankshaft -- the rotating part, uhh, rotate. The connecting rod is simply the part in the middle. It translates the piston's recip motion into the crankshaft rotary motion. The neat thing is that in the process, it shares in the motion of both. That is, the connecting rod is both a reciprocating part and a rotational part, at the same time. (In fact, when balancing an engine, it is common to divide the connecting rod's weight in two, thus permitting half its weight to be calculated as recip and half as rotation.) The point is, the upper half of the connecting rod reciprocates with the piston, the lower half rotates with the crankshaft, and this is important to understanding the stresses on all three parts, but especially those on the connecting rod. The piston and its half of the connecting rod stops twice per crankshaft revolution, even though the crankshaft continues to turn. This means the piston and top of the rod also start back up twice. This stopping and starting imposes stresses on all three of the parts, stresses that increase with crankshaft rpm. To reduce these loads, the piston is mounted to the connecting rod slightly offset. This causes the piston to reach top dead center at a different time than the connecting rod, effectively spreading the shock loading over a greater number of crankshaft degrees. In short, the real reason for piston pin offset is that it softens reciprocal loading, permitting lighter more power-efficient parts to be used, and the engines to be capable of higher rpm. However, there is another phenomenon at work also, a kind of side benefit. Because the connecting rod spends most of its time in the engine at an angle, the piston engine has what is called minor and major thrust. Major thrust refers to the downward-stroking piston's force against the cylinder wall during combustion, due to the rod being angled in that direction. Minor thrust is the piston's thrust against the opposite cylinder wall during compression, because the rod's angle is opposite also. These thrust forces push the piston firmly against the cylinder wall. The important thing is that at TDC, they flip-flop. Major thrust turns into minor thrust, and visa-versa. In older engines, this flip-flop caused the piston smack the cylinder, resulting in a noise. Fortunately, the piston pin offset in today's engines, besides reducing inertia stresses, does two things that reduce this noise. First, because the piston is mounted off center, the transition from major to minor thrust is less sudden. There is less impact. Second, instead of a sudden lateral shift, the piston actually rolls from major to minor thrust. That is, the piston shifts first at the skirt, then gradually the rest of the piston makes contact, instead of all of the piston at once. To summarize, piston pin offset is the manufacturer's way of reducing stress on reciprocating parts. It permits these parts to be lighter, which results in more efficient manufacture and less power loss in the engine, as well as higher rpm. A complementary result of piston pin offset is reduced piston slap due to the more gradual shift from major to minor thrust. " __________________ Adger Smith (Former SS)