Curti

A question about pistons

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A very good friend of mine who retired from Crankshaft Supply is a machinist and engine builder has told me that pistons are .010 -.015  are out of round fore and aft.  So I put this in my book of facts.

Last fall when we were out to the SEMA show, there was a large display of very technical engine building equipment.  I struck up a conversation with the technician there and mentioned pistons being 'out of round'

His response to me was: ' our pistons are round sir'  I am confused.

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Hi Curti,

The process is known as "cam ground" and there is plenty on the net about it.  It is regarded as being desirable but probably refers more to aluminum pistons.

 

Regards

Al 

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The way it was explained to me decades ago, due to the greater mass of metal around the piston pin, the rate of expansion of the piston when it's heated by combustion is greater in one direction (parallel to the piston pin) than in the other direction 180 degrees opposite -- or perpendicular to the piston pin. Thus, pistons should be "cam ground" to compensate, so that when the heat expansion occurs, they end up being round. I'll be interested to see other responders verify whether this is still done with modern pistons.

 

 

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A new set of Auburn pistons was just received from Ross, and they are 'cam ground'  .  

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Here's a drawing for a cam-ground aluminum piston for a 1937 Studebaker President straight 8 of 250 cu in displacement.  The nominal block bore is 3.0625.  In one dimension, the piston diameter is 3.061 maximum diameter, and at 90 degrees, it is 3.05225" maximum.  That is about an 0.009" difference.  I think the cam-ground pistons date back to the age of steam engines, well before 1900.

 

studebaker piston cam grind 1937 President.png

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From the information above and knowing the competence of my buddy, I have to conclude the guy at SEMA  was full of BS.

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When I needed pistons for my Pontiac I could find lots of "semi-finished" and "unfinished" cast pistons.  I could also find lots of machines to "cam grind" pistons but could not find anyone who knew how to operate the machines.  So rather than being able to purchase $12.00 - $20.00 cast pistons I had to get $200.00+ forged pistons, of course I would not expect these to ever have to be replaced again.  Just to get back to post #1, I learned about "out of round" pistons back in 1955 in school "Auto Shops".  I was really surprised to find that anyone in the automotive hobby was unaware of that basic mechanical tenent.

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Do I then assume the bores are perfectly round?

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2 hours ago, Curti said:

From the information above and knowing the competence of my buddy, I have to conclude the guy at SEMA  was full of BS.

 

Imagine that... :rolleyes:

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2 hours ago, Curti said:

Do I then assume the bores are perfectly round?

 

18 hours ago, jrbartlett said:

The way it was explained to me decades ago, due to the greater mass of metal around the piston pin, the rate of expansion of the piston when it's heated by combustion is greater in one direction (parallel to the piston pin) than in the other direction 180 degrees opposite -- or perpendicular to the piston pin. Thus, pistons should be "cam ground" to compensate, so that when the heat expansion occurs, they end up being round. I'll be interested to see other responders verify whether this is still done with modern pistons.

 

 

 

Yes to both.

 

The guy at the SEMA booth was probably just a fill in guy, not a technician.  I see the uninformed at shows all the time.

Edited by JACK M (see edit history)

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