Piston:

Tony's speed tips.

 

Q: What should the side clearance be for aluminum pistons?

A: All of the aluminum pistons I have used in the last 10 years or so have had the proper clearance built in to the size of the piston itself. By this I mean that a .030" oversize piston will actually measure about .026" oversize at 90 degrees (right angle to) the wrist pin. All modern cam-ground pistons, including new T pistons, are measured for clearance this way. Thus, if the block is bored exactly .030" oversize, the clearance will be what the manufacturer intended, usually around .004" to .005" at a point perpendicular to the wrist pin.

The coefficient of thermal expansion for cast iron is 0.0000061, for aluminum it is 0.0000128 or twice as much. For Aluminum pistons I have used 0.006" clearance measured through the diameter of 0.040" oversized (AL) pistons. This setting may have been a bit tight as I later pulled one piston out for checking and the mill marks on the sidewalls of the piston were about rubbed off at 800 miles. So 0.006" clearance is my personal minimum for aluminum pistons of standard size up to 0.030" oversize. For Aluminum Pistons of 0.040" to 0.060" oversize I would try 0.007" and 0.008" of clearance respectively next time.

When it comes to the rings, try inserting the rings one at a time into one cylinder bore and with a feeler guage check the end gap of each ring and write it down. Do this at a constant distance down from the top of the deck (like with an upside down old piston with a ring in place). I have found that the ring end gap can vary by 0.002" to maybe 0.005" in a given set. I mix and rematch the top, middle and bottom rings respectively trying to obtain the same ring end gap result in each cylinder bore. I don't file ring ends!

A machinest buddy of mine says no less than 0.015" ring end gap for model T's and he is correct! Ring end gap in thousanths is roughly figured by 4 times the piston diameter in inchs. That is 4x3.75" = 15 or 0.015". I hope this helps. Let me know if you need more info.

Q: I believe that .0025 clearance is for cast iron pistons. The clearance for aluminum should be twice that or .005 to .006 . Can you confirm these measurements?

A: These are the skirt fitting clearances from the 1940 edition of Dyke's Automobile Encyclopedia for split-skirt aluminum pistons.

cyl. dia. 2 1/2" to 3"---.002"

" " 3" to 31/2"----.00225"

" " 31/2" to 4"----.00250

" " 4" to 41/2"---.003"

" " 41/2" to 5"----.00375

The clearance for the stock cast-iron Model T piston is given as .0025", (replace piston or regrind, (rebore), and fit new pistons if .006" or more). From this it would appear that modern replacement aluminum pistons for the T would be set at the same,(.0025") as the iron. The block is bored to true diameter, the clearance is turned on the piston, ie, a 4" piston would be 3.997" in skirt diameter. Modern pistons are either of the split-skirt or cam-ground design, these allow for expansion due to heat without loose fit when engine is cold and, hence, a quieter running engine on start-up.

Q: Tony --- Thank you for your answer to my question about aluminum piston clearance in the T. The coefficients of linear expansion that I used are pretty close to yours 0.00000589 Fe; 0.00001233 Al) What did you use for a temperature?

A: By using the minimum ring gap of 0.015" we can solve for the change in temperature. If we use the formula Li=C*T*L

Where Li is the length of increase due to temperature change,

C is the coeffient of linear exp for cast iron rings= .0000065

T is the temperature differential in F

L is the cool original length of the member

and when we solve for T, the formula becomes:

T = Li/(C*L)

and if Li = 0.015 inches for a Diameter of 3.75 inches (which is a

recommended minimum ring clearance)

C = 0.0000065 thermal increase per deg.F

and L = Pi*D = 3.1416*3.75 = 11.78 inches

***** then T = 195F which is the difference in temperature between the piston and the block as per one recommendation.

The fuel that we use today contains much more BTU's and burns hotter than what Henry used so I ended up setting my ring gaps at about an average of 0.023" of cool end clearance with excellent results at 0.040" o.s. pistons.

The temperature of the piston at the top is important and I think that I was using something around 400F to 450F. The block will also heat up to something around 200F to say 250F in the cylinder and the piston may average around 400F to 500F. For the overall net temperature differential I probably used 200F. I have no measurements on this.

Perhaps your machine shop could give you some numbers too.

Q: Piston alloys 18S and 32S have copper and magnesium added. I have not been able to find what alloy is used in the Taiwan pistons. Any idea? I do have the coefficients for these alloys if I can find what they are.

A: If you took the piston diameter measurement at a known temperature and placed it in your wife's oven and heated it to a known temperature and obtained the new diameter you could solve for the new coefficient of thermal expansion. But, the amount of alloy in the new piston should be so low that it shouldn't be much of a difference.

Perhaps the above data can eliminate most of overheating due to possible piston to cylinder wall rubbing problem. The pistons and rings will wear themselves into the best fit over time from this point forward and it is likely that you may have to suffer for 500 to 1000 miles while doing it. Change your oil often so that the magneto does not get shorted out by the metallic filings. Also place a strong magnet on the bottom pan at each dipper indentation and on the oil drain plug as so to trap all metal filings before they get to the mag.

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