Author Topic: Locating Piston Ring Gaps 350  (Read 538 times)

Offline Ronno6

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Locating Piston Ring Gaps 350
« on: November 22, 2018, 08:18:27 PM »
I'm not sure if this is the correct forum for this topic, but, here goes.
Trying to establish the orientation of ring gaps on the piston is yielding
a variety of results.
Sometimes they are located top/bottom, with RH being one way, LH being opposite.
Basically setting up all pistons the same, regardless of where they go in the block.
Other information places gaps at 3 and 9 O'clock more or less.

The Chevy truck service manual and Chiltons or Clymers all recommend the first method.
Just kind of seems counter intuitive that ring gaps that are up on one bank are down on the other..........
You can lead a man to water, but yoiu can't keep him from pissing in it.

Offline Ronno6

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Re: Locating Piston Ring Gaps 350
« Reply #1 on: November 28, 2018, 08:53:14 PM »
I guess I posed a question the answer for which is too obvious.

Sorry..........
You can lead a man to water, but yoiu can't keep him from pissing in it.

Offline bd

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Re: Locating Piston Ring Gaps 350
« Reply #2 on: November 29, 2018, 12:26:28 AM »
You're allowed some latitude and not every engine builder follows the same routine.  Generally, all ring gaps are located in the diagonal regions situated between the wrist pin and the piston skirts.  The orientation to avoid is gaps, especially adjacent gaps, approximately aligned. 

As a rule of thumb, the upper and lower oil ring gaps are spaced 180° apart, each ~40° from the wrist pin axis.  The first and second compression ring gaps also are spaced 180° apart, each ~40° from the wrist pin axis.  An imaginary line drawn to intersect the oil ring gaps will form a flattened “X” with an imaginary line drawn to intersect the compression ring gaps.  So, looking down on the crown of a piston with the wrist pin horizontal and the circumference of the piston crown divided into 360° clockwise from top center, orient the top compression ring gap at ~50°, the second compression ring gap at ~230°, the top oil ring gap at ~130°, and the bottom oil ring gap at ~310°.

The reason some engine builders orient ring gaps opposed from bank-to-bank is that an engine always rotates in the same direction.  Since the rotating assembly changes the linear motion of the piston into rotational motion of the crankshaft, the piston rod constantly changes its angle with the piston as the crankshaft spins.  On the upstroke, the rod thrusts the piston toward one skirt into its adjacent cylinder wall.  On the downstroke, the rod forces the piston toward the opposite skirt into its adjacent cylinder wall.  The greatest side thrust is applied to a piston skirt and cylinder wall during the power stroke.  The major thrust skirt of a piston is always the side of the piston that is opposite the direction of crankshaft rotation.  Hence, the major piston skirt thrust on one cylinder bank occurs on the opposite cylinder wall compared to the major piston skirt thrust on the opposed cylinder bank.  For example, if an engine rotates CW, the piston major thrust skirt is on the left side of the piston looking down the crankshaft axis as the crankshaft rotates.  A SBC engine rotates CW from the perspective of an observer looking at the engine from in front of the crankshaft.  Major piston thrust occurs on the upper wall of a cylinder located on the driver side of the engine and on the lower wall of a cylinder located on the passenger side of the engine.  The feeling is that the resultant piston rocking can impact ring sealing, thus, gaps are oriented accordingly.  However, fretting over such nuances with regard to daily drivers is splitting very fine hair.  It is unlikely that you would notice any difference or improvement either way unless the engine is clearanced and runs notably loose.

Didn’t you state previously that you have a machinist/engine builder you trust?  Ask his opinion.  Just, don’t be surprised if he differs.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Ronno6

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Re: Locating Piston Ring Gaps 350
« Reply #3 on: November 29, 2018, 08:06:06 AM »
Thanks for the clear and thorough explanation. I appreciate it.

My engine builder said I could follow whatever convention I choose,
saying that the rings will rotate around the piston as the engine runs.
I obtained that input from him after my initial post here.
Your response helped me tremendously!
It is just a very lonely feeling when one gets no response to one's post...........

Thank you again.......
You can lead a man to water, but yoiu can't keep him from pissing in it.

Offline bd

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Re: Locating Piston Ring Gaps 350
« Reply #4 on: November 29, 2018, 09:26:16 AM »
Typically, ring migration is a gradual process. 

Sometimes "no response" is just a matter of timing.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Ronno6

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Re: Locating Piston Ring Gaps 350
« Reply #5 on: November 29, 2018, 09:54:17 AM »
Duly noted, on both counts...........
You can lead a man to water, but yoiu can't keep him from pissing in it.