while rebuilding my 25 t engine I have pistons with two rings above the pins and one below it. when I look in the model t ford service manual the exact pistons are shown in the book. However, in the parts catalogs from common suppliers, my original style is not offered? why?
The original pistons were cast iron with two rings above the wrist pin and one beneath it. The pistons you see in the catalogs are aluminum with 3 rings above the wrist pin. The aluminum pistons are lighter in weight. They are made more like the pistons in modern cars. They work very well and if you are going to rebuild the engine they will do the job.
From what I have heard from a good T engine rebuilder, is with the new aluminum pistons and rings you need to have the cylinder walls perfectly round. If not you will get piston slap. There are suppliers that carry rings for the original cast pistons. Ray
The idea is that the lighter aluminum pistons are better for your engine. They have to go from zero to whatever and back to zero twice every revolution. That's a lot of mass to accelerate and decelerate. It pulls and pushes pretty hard on the crank and bearings. Some say the cast iron pistons give a better idle. I've not noticed the difference, and can make either of ours idle well below 300 rpm.
The cylinder walls wear to an ovoid shape. It doesn't matter what kind of pistons you use - if the cylinders are worn too far, you will have piston slap.
For the insignificant price of a set of pistons it makes very little sense to re-install a set of cast iron pistons. The benefits of aluminum pistons cannot be overstated. The engine will last longer, wear less, accelerate more rapidly, have more power, while using less fuel.