Long or short piston pin

[David Redszus]

That would be true if the pin is supported by the very ends. Inside a wrist pin bore I would think the pin would be somewhat supported on each end, so most of the loading of the pin would be between the gap from the rod to the pin bore.

That is true. It is also true that a bending wrist pin applies loads to the piston bore in several concentrated locations.
The actual pin deflection depends not only on pin length, OD and ID, but also on the geometry of the piston pin bores.
The topic has been subject of intense FEA analysis studies by major piston manufacturers for many years.

[piston guy]

Piston guy------if you know, would you please explain how the piston guided set-up works, i have never had it explained to me. Thanks Mark.

"Piston guided" is where the rod side play is limited by the top of the connecting rod between the pin bosses of the piston and not by the side clearance on the two connecting rods to the "cheeks" of the crank. OEMs have done it over the years and it has been popular in race engines for around 20 years. It is NOT for everyone. While on the subject , pin length should ALWAYS allow a minimum engagement into the piston pin bosses equal to 1/2 of the pin diameter. In the case of a .927 pin , .464 of each end of the pin ( minimum) should be in the piston.

[Jeff Lee]

Piston guy------if you know, would you please explain how the piston guided set-up works, i have never had it explained to me. Thanks Mark.

"Piston guided" is where the rod side play is limited by the top of the connecting rod between the pin bosses of the piston and not by the side clearance on the two connecting rods to the "cheeks" of the crank. OEMs have done it over the years and it has been popular in race engines for around 20 years. It is NOT for everyone. While on the subject , pin length should ALWAYS allow a minimum engagement into the piston pin bosses equal to 1/2 of the pin diameter. In the case of a .927 pin , .464 of each end of the pin ( minimum) should be in the piston.

What would be a good example of “It is not for everyone” ? There sure isn’t a lot of info out there on piston guided rods!

[Mark O'Neal]

Pin length is determined more by forging pinboss span and pin tower design than anything else. Pinboss span is determined, primarily, by rod width. Minimum engagement into the pin tower is .562 for most. I have no idea where that number came from.

You guys have probably put more thought into it here than manufacturers have in the last ten years. Shortening the pin probably has more to do with weight reduction and external influences (like using 2.250 pins to clear LS reluctor wheels) than any technical influence......unless you consider that to be a technical influence.

From a manufacturers standpoint pins, irrespective of length, don't cause problems. Ludicrous cylinder pressures and missed tuneups cause most issues, and those are usually solved by increasing diameter and/or wall thickness.

While I'm on the subject, someone should make a SB Chevy rod with a .990 pin for the 60 lbs of boost crowd....but it ain't gonna be me......

[swampbuggy]

Thanks piston Guy i appreciate the explanation, Mark.

[piston guy]

Mark,
They're out there I've done them already.

[piston guy]

Jeff,
Opening up big end rod clearance ( since rod end play is now limited by the piston) allows more oil to be "thrown out" and cause oil control issues. In some cases the oil hole in the crank is exposed . All of this for a small weight and friction loss. I just don'r see the plus side.

[Warp Speed]

Jeff,
Opening up big end rod clearance ( since rod end play is now limited by the piston) allows more oil to be "thrown out" and cause oil control issues. In some cases the oil hole in the crank is exposed . All of this for a small weight and friction loss. I just don'r see the plus side.

Unless you are uncovering a feed hole, the larger rod side clearance hlin a pistom guided design has nothing to do with oil flow/control.

[MadBill]

2X. You can restrict the flow from a bearing with too little side clearance but can't increase it with too much; only more radial clearance can do that.