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Piston wrist pin length

General engine tech -- Drag Racing to Circle Track

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ptuomov
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Re: Piston wrist pin length

Post by ptuomov » Fri Jul 06, 2018 10:05 am

modok wrote:
Thu Jul 05, 2018 8:07 pm
ptuomov wrote:
Mon Jul 02, 2018 1:02 pm
piston guy wrote:
Mon Jul 02, 2018 12:34 pm
"I" use 1/2 the pin diameter as the absolute minimum pin engagement , so on a 23mm pin it would be .453. MORE is always better especially in boosted applications. .500-.650 is an average number.
That probably works in practice. However, shouldn’t we rather state the minimum overlap requirement in terms of fractions of pin length? If we just drive it off the diameter, then would increasing pin diameter by 1mm and making it thus stiffer increase the need for overlap?
It makes perfect sense IMO to go with a ratio. the length should be a function of diameter, Diameter should be a function of load...... IF you are making a lightweight design and a rigid piston.
I can see how it might work out that way for a clean sheet design, so I'm starting to slowly get what Piston Guy is saying. You need to hold some load, and that load amount determines both how much overlap you need and how large diameter pin you need.

But how about, say, boring out the cylinder and retaining the same rod? Now the bore is larger while the pin diameter is the same. You'd probably want not only a little thicker wall pin but also a longer pin. Should the overlap between the wrist pin and the pin boss in the piston be increased? I don't know, but seems to me that the answer is yes.

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Re: Piston wrist pin length

Post by modok » Fri Jul 06, 2018 8:09 pm

Actually, a day later, i still don't understand why you think that, Many pistons are made with the pin bosses all the way at the bore, But there is no rule that it has to be built that way.
For a racing piston, however wide the rod small end is, that how wide the pin bosses should be apart.

but now I see a flaw in the logic.
Stiffness goes up at the square of diameter.
If the rules for bearing width are related to diameter, then as the pin gets larger, the bearing area is less for a given load. I'm not sure if that's 100% ok

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Re: Piston wrist pin length

Post by Mark O'Neal » Sun Jul 08, 2018 3:43 am

peejay wrote:
Mon Jul 02, 2018 6:58 am
Mark O'Neal wrote:
Sat Jun 30, 2018 9:50 pm
digger wrote:
Fri Jun 29, 2018 11:14 pm
Have a set of mahle here it's about 10-11mm from radiused edge to the start of the lock groove. 20mm pin 48mm pin length . Bosses are 26mm apart. No turbo though
There are countries that measure using the metric system...and there is a country that went to the moon, built the most powerful military in history, as well as the largest industrial base ever...... :P :P
Which country is that?

The US has pretty much always been metric. Ever notice that an inch is EXACTLY 25.4mm, a pound is EXACTLY 454 grams, and so on? US measurement is defined by the metric system....
A pound is EXACTLY 453.59237 grams...and the 25.4 mm to an inch relates to absolutely nothing.

The US Measurement system is actually the British Imperial system.

I take you you were taught common core math.....?.. :P :P

I am, also, sorry you were born with no sense of humor....or no sense. I'll figure it out later.
Last edited by Mark O'Neal on Sun Jul 08, 2018 4:06 am, edited 1 time in total.

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Re: Piston wrist pin length

Post by Mark O'Neal » Sun Jul 08, 2018 4:00 am

joe 90 wrote:
Sun Jul 01, 2018 6:57 am
What about Ross, JE and stock pins?
Depends on the application but generally, the small blocks are 2.500....or 63.5mm if you're a snotball kid that thinks America sucks (not directed at you). Big blocks are 2.930, usually. LS engines are 2.250 to 2.300 depending on the forging design. They've been like this forever and I have never known how those numbers were arrived at.....but I've only been at it for 40 years. DCal is older than me....maybe he knows.

Standard wall thickness is .150, but that seems to be changing, and 5115 is probably the most common material, though I preferred 8620, and 9310 is in there somewhere. Breakage is incredibly low on any of them.

52100 Bearing steel is the most common upgrade, while S7, H13, and a number of other tool steel alloy pins are made.

I'll run a .562 engagement .150 pin at 9.0:1 and 15 lbs of boost any time.

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Re: Piston wrist pin length

Post by Mark O'Neal » Sun Jul 08, 2018 4:02 am

I messed up
Last edited by Mark O'Neal on Sun Jul 08, 2018 4:05 am, edited 2 times in total.

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Re: Piston wrist pin length

Post by Mark O'Neal » Sun Jul 08, 2018 4:02 am

twice
Last edited by Mark O'Neal on Sun Jul 08, 2018 4:05 am, edited 2 times in total.

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Re: Piston wrist pin length

Post by Mark O'Neal » Sun Jul 08, 2018 4:03 am

Thrice

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Re: Piston wrist pin length

Post by Mark O'Neal » Sun Jul 08, 2018 4:09 am

modok wrote:
Fri Jul 06, 2018 8:09 pm
Actually, a day later, i still don't understand why you think that, Many pistons are made with the pin bosses all the way at the bore, But there is no rule that it has to be built that way.
For a racing piston, however wide the rod small end is, that how wide the pin bosses should be apart.

but now I see a flaw in the logic.
Stiffness goes up at the square of diameter.
If the rules for bearing width are related to diameter, then as the pin gets larger, the bearing area is less for a given load. I'm not sure if that's 100% ok
Stiffness also go up as you reduce the length of the pin.

Overall it's hard to say. At 7K rpm the piston is shaped like an egg, the rod looks like it's made of al dente spagetti, and the pin isn't round or straight.

user-23911

Re: Piston wrist pin length

Post by user-23911 » Sun Jul 08, 2018 7:06 am

A Subaru piston, it's got a fully floating pin.
That's how you pull them apart?

I've got a couple of pistons lying around at home, no pins though, but 23 mm.
The pistons are from FJ20s, (92 ? mm bore)the length of the pin is determined by the space between the circlips. About 60 mm long.
Because I don't have a pin, I can't measure the wall, however I've got plenty of 22 mm Mitsubishi pins which are 63 ish mm long with 91 ish mm piston bore.
Aftermarket pins tend to have a 3.8 ish mm wall where stock has a 4.5 mm wall.

The aftermarket pins are known to bend, the stock ones don't.
3.8 mm is about 0.149 and 4.5 mm is about 0.177


But anyway, you'd want to focus most of the load closer to the middle of the pin where the conrod is?
That's the opposite to making the pin longer.

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Re: Piston wrist pin length

Post by DCal » Tue Jul 10, 2018 10:18 pm

(/quote)
I'll run a .562 engagement .150 pin at 9.0:1 and 15 lbs of boost any time.
[/quote]

Hey Mark!!
I dug up a 6-7 year old billet Pro Stock CP piston for a Mopar and it has an .866 X 2.125 pin bore with an .890 pin boss span. That leaves about .577 of pin bore per side after the pin hole radius, at 1400HP and 11,000 RPM. It's funny how close it is to your standard.

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Re: Piston wrist pin length

Post by gruntguru » Wed Jul 11, 2018 12:11 am

Mark O'Neal wrote:
Sat Jun 30, 2018 9:50 pm
There are countries that measure using the metric system...and there is a country that went to the moon, built the most powerful military in history, as well as the largest industrial base ever . . . . . . in spite of being hampered by an archaic system of weights and measures. =D> =D>

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Re: Piston wrist pin length

Post by ptuomov » Wed Jul 11, 2018 10:08 am

Mahle aftermarket piston pin has 4.5mm wall thickness and Manley aftermarket piston has 5.5mm wall thickness. They look built like a brick house.

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Re: Piston wrist pin length

Post by Mark O'Neal » Thu Jul 12, 2018 6:35 pm

I'll run a .562 engagement .150 pin at 9.0:1 and 15 lbs of boost any time.
Hey Mark!!
I dug up a 6-7 year old billet Pro Stock CP piston for a Mopar and it has an .866 X 2.125 pin bore with an .890 pin boss span. That leaves about .577 of pin bore per side after the pin hole radius, at 1400HP and 11,000 RPM. It's funny how close it is to your standard.
Guys will call me and tell me they won't run a .150 wall pin at 13.0:0, then put 15+ lbs of boost on the .150 wall and think nothing of it.

Back in the day. I'd hear tell that you don't run a BB Chevy rod journal at .030 under.......but turn the journal to 2.100 with an offset stroke is just fine.

People are funny.
Last edited by Mark O'Neal on Thu Jul 12, 2018 6:41 pm, edited 2 times in total.

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Re: Piston wrist pin length

Post by Mark O'Neal » Thu Jul 12, 2018 6:39 pm

ptuomov wrote:
Wed Jul 11, 2018 10:08 am
Mahle aftermarket piston pin has 4.5mm wall thickness and Manley aftermarket piston has 5.5mm wall thickness. They look built like a brick house.
The .177 wall is fine for most applications, the .216 wall is a lot of weight for no gain on most applications. Having said that....what are they made of? 5115, 52100, S7, H11, H13, 4340 card chromed?

Stock pins used to be quite thick too, it was justified becasue the materiel wasn't near what it is today.

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Re: Piston wrist pin length

Post by ptuomov » Fri Jul 13, 2018 7:43 am

0.117mm is about 3mm and .216 is about 5.5mm. I am equally uncomfortable with imperial and metric units, having lived almost exactly half my life in Europe and half in the US. The 5.5mm wall thickness, 23mm diameter Manley pin is made of 9310, I think.

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