Wrist pin bent

Mark O'Neal · Post by **Mark O'Neal** » Tue Jun 26, 2018 9:52 pm

A wrist pin is only round at zero RPM.

Jeff Lee · Post by **Jeff Lee** » Wed Jun 27, 2018 10:24 am

Which begs the question...what is a reasonable compromise between a wall thickness for weight consideration vs stability? Let’s say tool steel that is 2.5” long and 7,000 RPM.

ProPower engines · Post by **ProPower engines** » Thu Jun 28, 2018 1:30 pm

I would be concerned about the shape of the piston and its integrity after a blow up.
I would replace them both just to be safe.If it hit the head with enough force to bend the pin what does that say about the integrity of the piston and ring land areas

1972ho · Post by **1972ho** » Thu Jun 28, 2018 5:48 pm

The piston and rod was hosed and replaced.

Mark O'Neal · Post by **Mark O'Neal** » Thu Jun 28, 2018 7:02 pm

Jeff Lee wrote: ↑Wed Jun 27, 2018 10:24 am Which begs the question...what is a reasonable compromise between a wall thickness for weight consideration vs stability? Let’s say tool steel that is 2.5” long and 7,000 RPM.

In a super stock car? .075 to .090 wall.

In a dirt modified? 150 to .180 wall.

With 65 lbs of boost? .990 diameter .225 wall

JohnsnsFord2@aol.com · Thu Jul 12, 2018 3:32 am

A very well known engine builder and engine masters winner , just changed the wrist pin thickness and the engine made more power....... $\:D/$ $\:D/$ $\:D/$ $\:D/$

$\:D/$ $\:D/$

desoto30 · Post by **desoto30** » Thu Jul 12, 2018 4:50 am

Thicker or thinner?

DCal · Post by **DCal** » Thu Jul 12, 2018 9:59 am

JohnsnsFord2@aol.com wrote: ↑Thu Jul 12, 2018 3:32 am A very well known engine builder and engine masters winner , just changed the wrist pin thickness and the engine made more power....... $\:D/$ $\:D/$ $\:D/$ $\:D/$ $\:D/$ $\:D/$

Nearly every time unless you had a sufficiently thick pin before.

JohnsnsFord2@aol.com · Thu Jul 12, 2018 10:14 am

Thicker

Jeff Lee · Post by **Jeff Lee** » Wed Jul 18, 2018 2:14 am

JohnsnsFord2@aol.com wrote: ↑Thu Jul 12, 2018 3:32 am A very well known engine builder and engine masters winner , just changed the wrist pin thickness and the engine made more power....... $\:D/$ $\:D/$ $\:D/$ $\:D/$ $\:D/$ $\:D/$

I was to get .110"” wall for my Super Stock build. When the pistons and pins arrived, there was .220” or .250” wall wrist pins. I called Gibtech and said “hey, this is a drag race engine, not a truck puller!” The laughed at their screw up and told me it would be 3 weeks on .110” or could get .160” now. Thinking things were moving along faster (will I ever learn?), I took the .160” x 2.500” pins. Of course more than 3 weeks have passed but I think I’m ok with it. I think there was a 6 gram weight penalty but my new pistons are considerably lighter and overall is lighter.

Last machine operation is in progress and hopefully done this week. Cam tunnel.

Car went to chassis shop yesterday so we could get the new bathtub oil pan to fit in the car.

Mark O'Neal · Post by **Mark O'Neal** » Wed Jul 18, 2018 2:21 am

DCal wrote: ↑Thu Jul 12, 2018 9:59 am
JohnsnsFord2@aol.com wrote: ↑Thu Jul 12, 2018 3:32 am A very well known engine builder and engine masters winner , just changed the wrist pin thickness and the engine made more power....... $\:D/$ $\:D/$ $\:D/$ $\:D/$ $\:D/$ $\:D/$
Nearly every time unless you had a sufficiently thick pin before.

Yes...but getting a customer to buy the correct pin, when the pistons come with "free" standard pins, has become a fool's errand.

piston guy · Post by **piston guy** » Wed Jul 18, 2018 11:53 am

"I" think you did yourself a favor by using the .160 wall pin. I see pistons that have cracked on the inside where the "pin towers" flow into the under crown all the time. Aluminum cracks because it's flexing . Most often the pin flex is the cause and the piston flexes while trying o follow it. The stiffer pin reduces flex and increases piston life. Piston weight, RPM and horsepower ( to me) are the major factors that determine what wall thickness pin to use. The other obvious fact is if the pin is flexing , power is being lost due to higher energy absorption.

JohnsnsFord2@aol.com · Thu Jul 19, 2018 7:33 pm

^^^^ you are so right sir

Jeff Lee · Post by **Jeff Lee** » Thu Jul 19, 2018 8:23 pm

When I was faced with the decision, my gut told me this was an area were lighter may not be such a good thing. So going to the thicker wall never bothered me. And I just looked at my notes. It’s .165” x 2.50”, not .160”.
My pistons are about 30 grams lighter than previous and the pins are about 5 grams heavier. Net -20 grams.

David Redszus · Post by **David Redszus** » Fri Jul 20, 2018 12:05 am

Mark O'Neal wrote: ↑Thu Jun 28, 2018 7:02 pm
Jeff Lee wrote: ↑Wed Jun 27, 2018 10:24 am Which begs the question...what is a reasonable compromise between a wall thickness for weight consideration vs stability? Let’s say tool steel that is 2.5” long and 7,000 RPM.
In a super stock car? .075 to .090 wall.

In a dirt modified? 150 to .180 wall.

With 65 lbs of boost? .990 diameter .225 wall

Not knowing the weight of the piston and rod, I used a load of 3000 lbs on the wrist pin of various wall thicknesses.
The deflection or bending for each wall thickness is:
.090" =0.00065"
.180" =0.00043"
.225" =0.00037"

what is the pin to piston bore clearance?

Don Terrill’s Speed-Talk

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