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Piston wrist pin length

General engine tech -- Drag Racing to Circle Track

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Mark O'Neal
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Re: Piston wrist pin length

Post by Mark O'Neal » Sat Jun 30, 2018 9:57 pm

We used to use a minimum of .562 a side of engagement. But it would depend on the application, the material the pin is made of....and all of that.

Material and machining accuracy is far better now than they were when I was thin and had hair.
Last edited by Mark O'Neal on Sat Jun 30, 2018 9:58 pm, edited 1 time in total.

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Re: Piston wrist pin length

Post by Mark O'Neal » Sat Jun 30, 2018 9:58 pm

Mark O'Neal wrote:
Sat Jun 30, 2018 9:57 pm
We used to use a minimum of .562 a side of engagement. But it would depend on the application, the material the pin is made of....and all of that.

Material and machining accuracy is far better now than they were when I was thin and had hair.

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Re: Piston wrist pin length

Post by Mark O'Neal » Sat Jun 30, 2018 9:59 pm

The second post was made because I'm a moron, which probably is an indictment of the first......

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Re: Piston wrist pin length

Post by user-23911 » Sun Jul 01, 2018 5:53 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...... :P :P
Who went to the moon?
Really?

No, they couldn't do it and have been bullshitting about it ever since.


Anyway, Subaru and 23 mm pin,

They just use the same pins used in the 2.0 with 92 mm bore.

I'd be more concerned about the wall thickness.
OEM pins tend to have much thicker walls than their upgrade / downgrade replacements you get with aftermarket pistons.

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Re: Piston wrist pin length

Post by ptuomov » Sun Jul 01, 2018 6:04 am

joe 90 wrote:
Sun Jul 01, 2018 5:53 am
Anyway, Subaru and 23 mm pin,

They just use the same pins used in the 2.0 with 92 mm bore.

I'd be more concerned about the wall thickness.
OEM pins tend to have much thicker walls than their upgrade / downgrade replacements you get with aftermarket pistons.
These pins that come with Mahle and Manley pistons are neither light nor thin wall, in my opinion:
10414116-6C96-44EC-A9F2-AD89B126536D.jpeg
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Re: Piston wrist pin length

Post by user-23911 » Sun Jul 01, 2018 6:57 am

What about Ross, JE and stock pins?

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Re: Piston wrist pin length

Post by ptuomov » Sun Jul 01, 2018 12:46 pm

joe 90 wrote:
Sun Jul 01, 2018 6:57 am
What about Ross, JE and stock pins?
Don’t know. I can google the weights, but don’t know about the lengths.

I’m not an expert on pistons, but these Mahle pistons look like they are on the edge in every possible way. I guess if you design something to be on the edge, design it to be as close to the edge everywhere.

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Re: Piston wrist pin length

Post by peejay » 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....

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Re: Piston wrist pin length

Post by hoffman900 » Mon Jul 02, 2018 7:24 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....
Not only that, but the Apollo computer was programmed in metric, with provisions for the inputs to be US.

It seems to me a lot of the talented engineers and scientists who were (and still are) at the cutting edge of research all come from somewhere else where metric is used. Look at all the former German scientist who were the backbone of the early NASA programs...

If you’re a practical person, the metric system is leaps and bounds easier to do mentally (and anywhere for that matter).

... nothing by old men puffing out their chests over nonsensical pride...
-Bob

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Re: Piston wrist pin length

Post by ptuomov » Mon Jul 02, 2018 7:53 am

SI units are easier.

On pistons. The way I see the piston design for a specific application working is that one first picks the bore, stroke, and max rpm. Then, the max rpm and stroke gives one the ring width based on predicted flutter critical rpm. The stroke and rpm give one the maximum weight for the piston assembly, which with bore size then determines the compression height, piston design, and how expensive the piston materials and manufacturing will end up being to get under the maximum weight. The power level, ring width, and top land width will give one the heat flow equations and how much additional cooling one needs to keep the top ring and top ring groove below critical temperature. The additional cooling need estimate gives one the answer to whether the piston cooling jets are required and what is the required oil flow rate (Mahle book gives the heat rejection rates as a function of oil flow rate). There are 100 other things to figure out, too, which I don’t really either understand or know about.

The above design problem is so complex that I don’t think I could do it (no shocker there). I don’t even think that some lesser aftermarket piston manufacturers can really do it correctly, if one goes right to the edge in terms of requirements. And as a hobbyist, ordering say 16 pistons, it’s probably impossible to get attention from anyone in any company who can actually do the design work correctly.

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Re: Piston wrist pin length

Post by piston guy » 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.

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Re: Piston wrist pin length

Post by ptuomov » 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?

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Re: Piston wrist pin length

Post by piston guy » Thu Jul 05, 2018 11:47 am

Notice I put the very first I in quotation marks. What "you" do can certainly be different , no problem. Life would be boring if everybody thought alike.

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Re: Piston wrist pin length

Post by ptuomov » Thu Jul 05, 2018 1:58 pm

piston guy wrote:
Thu Jul 05, 2018 11:47 am
Notice I put the very first I in quotation marks. What "you" do can certainly be different , no problem. Life would be boring if everybody thought alike.
I'm just reading books, measuring parts, and asking questions because I've no learned enough about pistons to know that I know very little about pistons.

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Re: Piston wrist pin length

Post by modok » 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.

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