Small Diameter Wrist Pin Life

[Mark O'Neal]

With a piston pin, the ultimate strength condition is usually NOT the key.
"Toughness" IS ... how strong is it when it distorts and how much distortion can it endure before failing ?
Also, when it distorts, how much does that affect the piston ?

It usually tears out the bottom of the pin tower.

[piston guy]

Or pulls the pin out of the piston completely if the pin bosses are rigid like the "box" style forgings are.

[piston guy]

Let me also add this. Many racers are fixed on running very light , thin wall wrist pins. With the furnished charts on page one it's easy to see that these thin, long pins flex. Mark noted pin flex causes pin boss cracking and it certainly does. The one byproduct of all this flex is LOSS of energy transmitted to the connecting rod through the pin and piston going through all this flex. Why do you think Pro Stock engines went back to steel rods??

[clshore]

Sure,
I wasn't suggesting it was a new idea.
But we have modeling and simulation tools now that can make design more effective.
And we have production methods that can make building them more practical.
And we still have a competitive environment where the gains they enable can make them economically viable.

But if you could have a lighter pin that was proven to be just as strong and tough in real world use, wouldn't you use it?

[Brian P]

Just as an aside ...

Late model production VW TDI engines use a tapered small end of the connecting rod which fits into a tapered recess in the piston.

By "taper" I am not talking about diameter (the pin is one fixed diameter its whole length, as usual) but the con-rod itself in side view. The extreme upper end of the con-rod is thinner than the thickness of the con-rod below the bottom of the wrist-pin hole.

Doing it this way allows some of the gas pressure loads (which are rather high in a modern turbo-diesel engine) to be transferred through the wrist pin directly through compressive loading rather than entirely in shear.

[In-Tech]

LS-7 Ti rods are the same.

[axegrinder]

This is said to be from an 800hp V8 F1 engine.

http://wheelnutsjournal.typepad.com/.a/ ... 4e8970c-pi

I still think pin strength is the easy part. Lubrication has got to be a nightmare when the pin is that small.

[Adger Smith]

They usualy pressure lube through the rod on pins like that. The real test for pins is in an engine like an A/fuel or a top fuel or funny car. They run into conditions that most engines never see. The worst being able to compress a liquid. Randy touched on a real problem. The material the pins are made of and how they react to the stress they are put under.
There are so many cheap pins sold with good pistons and the customers think they have something that is bullet proof. I see them all the time in shelf stock pistons. A good piston design and a 2 dollar pin. The pin bores all torn up from either going square or just outright deflection. When I see that I will buy an oversize pin from PPPC, either coated or uncoated, and select the correct material and thickness. It all depends on the application. The Chinese and India pins are all over the place. There are even some being coated with DLC and touted as the "Greatest"... Yea, like a shinny dime up a goats a**. well, it's late. A real good pin can even help in ring seal. Think about it!!! I have more... later...Too late.. time to give my pillow a hit....

[piston guy]

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".

[axegrinder]

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.

[Kevin Johnson]

This is said to be from an 800hp V8 F1 engine.

http://wheelnutsjournal.typepad.com/.a/ ... 4e8970c-pi

I still think pin strength is the easy part. Lubrication has got to be a nightmare when the pin is that small.

I think this might be a side view of the same piston pin:

pistone.jpg

[axegrinder]

Kevin, I bet your right. Those pics are pretty popular around the net. It's so small and some text somewhere said it was from a 2001 Ferrari, so I bet it's from a V10.

[clshore]

Anyone know how the pin is retained on that one?

[Kevin Johnson]

Anyone know how the pin is retained on that one?

http://www.ferraridatabase.com/The_Down ... arossa.pdf

On the Testarossa engine (page 27 of manual) a circular clip is used. See pages 382-3 of the parts manual. If you look closely at the picture you can see what appears to be this same type of part.

[clshore]

OK, I see it now, missed the relief slot at the top of the pin boss, I thought it was the shadow.