Small Diameter Wrist Pin Life

[Kevin Johnson]

I have been curious as to whether an interference fit of two concentric hollow pins would allow a pre-stressed net combination that could use a thinner combined wall thickness that would exceed the material properties of a single hollow pin of that wall thickness. Might be too exotic/expensive for engines but the lower mass might be useful for spacecraft. Just a thought.

Found an example used in constructing gun barrels. http://www.google.com/patents/US6615702 2003 -- Give it a few years to become someone's brainwave in F1 or Nascar. Then promptly banned.

Instead of space I need to think weapons.

[Abbottracingheads]

I will happily go on record as being a heavy pin guy. Less flex = more power in my experience. Common sense has to excercized though. I'm not suggesting a .350 wall top fuel pin be used in a 10,000 rpm N/A motor any more than a .100 wall pin be used. Weight reduction is critical but not at the cost of performance or durability. It's still "to finish first , first you have to finish".

I agree. All of my 800 to 975hp NA motors have .150 to .180 wall, .927 dia pins (all aluminum rods). .125 wall pins are not even a consideration.

I have not yet used the X-forging piston and the small short pins that can be used with them. I dont want to go down that road without being more educated on how to make them live.

You will find some more power when you go to the x-forgings. They maintain their shape better, thus better ring seal. My engines sure run better with the new style pistons.

[piston guy]

"X" as you call them, forgings bring the pin towers closer together and strengthen the center of the piston. That is the area most suceptible to deck flex. Shorter pins also stiffen the assembly as a whole. The tricky part is getting the ovality of the piston right so "strut scuff" on the skirts is eliminated.

[DCal]

Randy, This is where you and I have an advantage, we work within the industry and can test ideas on our personal cars without making something a company spec or standard. And we have complete control or at least know what happened if there was a problem. I'm currently testing a 4.600 bore box forging on the street with .004 ptw. We have FEA and other programs to help with the design of the skirt and so far those pistons have survived a break-in , 11 dyno pulls, a stuck float and there's no noise out of the ordinary. I'm also testing Gas ports and oil delivery and return ideas along with using our .927 ceramic treated shelf wristpin. Hopefully these efforts will provide useful info for the company by monitoring oil consumption etc.

[piston guy]

Dave,
I have no doubt you'll make it work!!! I'm going to run them in my Ford as well. Never got around to making them at the last place, LOL I'll do a set here.
Good to hear from you my friend!

[axegrinder]

Dave, your street project has me thinking about high end sports cars. Do you know how small the European cars are going with wrist pin size? If I (or anybody) wanted to build a high end sb2 street machine with the latest lite parts, what would be the draw back? Money is the obvious negative but it would probably still come out cheaper than a new Ferrari V12!

[piston guy]

.728 and .747 are both common.

[DCal]

Denis. I also see Ferraris at .669 and .708. but I see Honda at .827 and their rod width isn't as tight as the European high end cars. Another reason may be that the Honda have more stroke. I'm not sure other than getting the rod small end pared down what the benefit would be to go much smaller.

[machstang70]

I've been getting many different opinions on this.

Drag race application: 70 Mustang 3150#, 357 Cleveland 13:1 compression, 3.50 stroke, 4.03 bore, 500g piston w/1.645 pin height, shift at 7800, (8000 max). Thoughts on a .912" x .132" wall x 3.030"length H13 Pin ?

Thanks

[piston guy]

machstang70
I don't like running less than .150 wall pins. A 500 gram piston has a lot of inertia at 8,000 rpm and thin pins flex more than thicker pins. The piston tries to "follow" the bending pin and can cause cracks in the piston. Plus there is energy loss anytime things bend , so a stiffer pin will transfer more energy to the crank. Unless you are using a TRW/Speed Pro piston, I would use a shorter pin ( 2.750) for weight reduction purposes.

[machstang70]

I was thinking the same and just wanted to get some more opinions. I am using a Speed Pro piston so a shorter pin is not an option. I was thinking of going to a .927" pin, but that little change may not be worth the trouble.

Mike

[ptuomov]

A question to this old thread. How important is the side clearance or the gap between rod big end and piston pin boss to the piston pin requirements?

[PackardV8]

To understand what pins need to look like to withstand pressures, spend some time looking at OEM diesel rods and pins. They're always large diameter, thick walls. When that's not enough, there's aftermarket tool steel, $65 each:



To understand the RPM inertial forces, spend some time looking at F1 and superbike pistons and pins. Titanium Ti-6Al-4V with a titanium nitride coating has been tried and until regulated out, there was a very exotic pin developed many years ago for Formula One and Pro Stock applications which used an aluminium beryllium inner and a steel outer sleeve.

The tradeoff for most OHV8 racing applications is somewhere between the light and the strong.

[ptuomov]

To understand what pins need to look like to withstand pressures, spend some time looking at OEM diesel rods and pins. They're always large diameter, thick walls. When that's not enough, there's aftermarket tool steel, $65 each:



To understand the RPM inertial forces, spend some time looking at F1 and superbike pistons and pins. Titanium Ti-6Al-4V with a titanium nitride coating has been tried and until regulated out, there was a very exotic pin developed many years ago for Formula One and Pro Stock applications which used an aluminium beryllium inner and a steel outer sleeve.

The tradeoff for most OHV8 racing applications is somewhere between the light and the strong.

Interesting. I'm half-way between, given 100mm bore and 24mm wrist pin diameter (right now). That makes sense given that mine is a gasoline turbo engine. I've read the Mahle piston book and it goes into detail on these differences between high-rpm atmo gasoline engines and turbodiesels.

However, one thing that the book doesn't comment on or give any design guidelines is the gap between the rod big end and the piston pin bosses. What's the design guideline there?

[modok]

A wide survey of modern engines they have clearance from .060"-.200"
And I have never seen any relation to that and anything else.
Besides..... if you want to run tighter then the rods NEED to be straight.

A "wide survey" of old chevy 350 piston pins will show you that they are often utilizing that extra wiggle room.