MTFCA Forum

Just a reminder on why it's important to have oil holes in the pistons for the wrist pins.
Caught this engine, barely ran in, just in time. (Fitting a scat to it)

I have seen that on engines with the flywheel stripped, not enough oil being vaporized.

You need to check fit of piston pins before you put rods on. They need to be honed.

Dan is absolutely correct! NONE of any of the Model T pistons are ready to use right out of the box. They all have to be honed. Typically a regular automotive machine shop would set clearance on a piston like this at .0005”, but they actually need a little more than that. I generally set them up at .0008” clearance because the bores are generally a bit out of alignment and that is the point that the bores will totally clean up at. If a dry pin will not drop thru a dry piston under it’s own weight, the bores are too small or out of alignment. Oil holes are a secondary consideration. Without oil holes, capillary action will still properly lubricate the pin if the clearance is correct. The method of checking pin fitting in the Ford Manual is out of date. That was for an iron piston with brass bushings. A fit with aluminum pistons should be based on a precise measurement and feel dead free.

If those pins are too tight it will actually cause a knock! The piston will rock back and forth with each revolution of the crankshaft. However, if they are too loose, the pins will knock, so that is important, both the oil supply, and the fit.
Norm

I believe there is more than one factor here, the original cast iron pistons were fitted with brass/bronze bushings and the bushings extended inside the piston to allow greater surface area for the bushing. The normal splash oiling system was sufficient to lube the pins. With aluminum pistons today the pins are pressed into the pistons and the rod moves on the pin. The T pistons appear to be backwards that is, the rod should move on the pin and the pin should remain fixed. As the aluminum piston heats and expands the holes the pins gets smaller that is why the pins will bind and turn blue, the only fix is greater clearance which could create excessive noise and further premature wear. When removing some round gears from shafts, heating the assembly only makes it tighter, however with a 50 ton press almost anything can be taken apart but the parts are most likely ruined. With the T cast iron pistons the bushing may be a means to transfer the heat to avoid the bushing becoming smaller and binding. The aluminum piston has no bushing and as it heats up it can only get smaller. Most machine works have a tolerance range in some cases up to .002, to maintain .0005 with no +- would be very difficult. The pistons would be made to a +- tolerance and it would be up to the person doing the assembly to do the fine fit. When playing with .0005 the air temperature is an important factor as is the heat from machining and your hands. I feel that the pins should be fixed and the rod should rock on the pins when aluminum is used, that way both metal surfaces are compatible.

David, a solid cylinder will increase size across it's diameter when heated and a hole (circle) will increase its circumference when heated. Since the value of a circumference of any given diameter is greater than the same basis diameter, when heated, the hole will always become larger than the rod. It cannot become smaller.

As you stated, there are other things at play that will work against the assembly which has a less-than-perfect fit, primarily lack of lubrication in addition to steel/aluminum being a much poorer design than steel/bronze with respect to coefficient of friction in a marginally lubricated state.

This piston locked up the engine and stopped the car. The pin bonded with the piston and I had to cut the piston to get the pin out to remove the rod. The other three never had a problem but try and buy one piston from a dealer.
Rich The engine ran fine for a few hundred miles before this happened. It was a very cold day but I doubt that affected anything.
I wouldn't have believed it if it hadn't happened.

It was a very cold day

Rich

that had EVERYTHING to do with it

what was a too-close fit at 70F and too few miles to wear in, a very cold day would shrink the piston right onto the pin and eliminate virtually any boundary lubricant

holes grow more than pins when they heat up...what happens when they get cold?

Yup.

Rich, those where the Jahns pistons, they where made out of any aluminum they could find, I saw one that did not have crap for compression in a cylinder, tore it down and I could see through it on top from porosities in the aluminum.