Cox aluminum piston?

Brad how does the compression feel with the aluminum piston? Is the theory correct that to consider the higher thermal expansion of aluminum than steel the piston fit when cold was set loose?

Brad do you think the engine would benefit with using extra castor ?

Thanks Brad , information taken in , i have 3 pistons and 1 rod coming , i am pretty sure somewhere in all my stuff i have some rods maybe even alum. one ?

1/2A Nut wrote:Grish 5x4. Cox Aluminum Piston Bench Test Break In .049 1oz rich run prior to this one aluminum surface was dusting off small particles as it seated into it's thermal state. If leaned up too much it expands to the next level of heated state subject to binding enough to shut down. Will require patience with some rich runs till fully broken in.

Compression seems good still bit less than when new. I imagine another 2oz of rich running to baby the piston into it's best tuned thermal state. You want to compress the outer skin of the piston's molecules into a hardened condition, if rushed you scrub off too much
material too fast. The reward is a piston that will last. Plus you don't want to stress the ball socket during break in, it is aluminum after all.

Thanks for sharing this Brad and for documenting your experiences. Regarding thermal expansion got me a little curious about the theoretical empirical dynamics behind the metallurgy. (I say theoretical because values given are in general and not toward the specific grades of materials involved, and arbitrary assumptions made for the sake of discussion so values can be quantified.)

https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Thermal Expansion Coefficients in 10^-6 in./(in. Deg-R):

• Steel, 6.11 - 6.94
  
• Cast Iron, Gray 6
  
• Aluminum, 11.7 - 13.3
  
• Brass, 10 - 10.6

Degrees are in Rankine, to convert Fahrenheit, add 459.67.

Cox .049 bore is 0.406 in. Hypothetically, say steady state methanol combustion temperature with 15% - 25% nitro is 500 deg-F (where most synthetic oils vaporize and combust), then we multiply these coefficient values by 1e-6 * [0.406 x (500 + 459.67)] = 0.00039

Arbitrarily rounding values for back of the envelope calculations:

Aluminium (12) = 0.005 in.
Steel (6.2) = 0.002 in.
Cast Iron (6) = 0.002.

We see that at 500 deg F, we have about 3 thousandths of an inch piston expansion toward cylinder walls. Of course, we have not also taken into account that because the piston head is thinner, it doesn't have the heat sink effect of the thicker cylinder, which will not expand as much due to its slightly lower temperature. Piston top will be hotter.

It is not hard to see that one may have to start the engine with lower compression, so that by the time it reaches steady state temperature, the aluminium piston has expanded to the point where it can now maintain a proper compression seal.

Now we can surmise that Leroy Cox went to a steel piston for obvious reasons. The ABC engines work well with aluminium pistons because the brass is a lot closer to aluminium than steel for the sleeve.