So, I am a little confused as to what exactly I am dealing with in terms of my wrist pins. On my factory piston and rod assemblies, and on all the assemblies I am seeing on Youtube/reading about online, the wrist pin is interference fit into the small end of the rod, and rotates freely in the piston, and can be passed easily through the piston by hand, if the rod is not there. On the Sealed Power 2362 pistons I just bought, the pins seem to be interference fit into the pistons. No amount of force applied with my bare hands can cause the pins to rotate or slide in their bores... which seems terribly incorrect to me. If the rod is fit to the pin and the pin is fit to the piston, that creates a rigid assembly which I imagine would not really be ideal beyond about 0 RPM... These are NOS, been sitting on a shelf for years and years I'm sure, is it possible they've seized? What am I missing here?
That was a good buy. Don't try to force the pins in the piston pin bores. As long as there is no rust they will come out. Instead use try penetraiting oil with a paper towel on the pins in the middle of the bosses to remove any buildup from over the years and oil in the slots and around the pin from both sides where the piston pin bosses are Use a heat gun on the piston pin bosses. Try not to heat the pins. Go easy. Just keep going around the bosses for about 15-20 seconds with the gun not staying in one spot. They should move pretty quickly as the aluminum expands, rotate freely then apply the penetraiting or motor oil to the pin in the middle and from each end. I would leave them like this for your machinist to deal work with. I would not take them out. You don't want to go monkeying around with pins and their bores as you can damage the bores easily if there is a slight burr and you try to remove or slide them back in. Ray
Mike, As Ray said, your unused speed pro pistons have not been pin fit, which means the pins will go into the piston, but they don't have the proper running clearance.. over time , the lube used by the manufacture has dried up, seizing the pins up. Those pistons/pins always fit tightly when new. The bores are just big enough to get the pin in, to make the pin/piston package more compact for packaging. Let you machinist get them out, when he pin fits the pistons, and presses them on the rods for you. JW
Alright, that makes sense. So before the assembly is ready to operate, the size of the bores in the piston is checked, and then the pin is milled down to fit freely but precisely? I was going to ramble on about some other things, but I think I will make up a new thread for those... so look forward to that!
No there isn't any pin milling involved. A good machinist using the pins as you said for a factory style press fit will measure the small end of the rods and the pins. They will determine if they should open up the small end a bit. With the right equipment it doesn't take long. Or if using bushed rods (w/bushing) the small end is honed to the proper size for pin clearance so the pin is "floating" like how the factory piston floats on the pin. However pistons that use this method have spiro locks or c-clips that go into a groove in the pin bosses that stop the pin from sliding out of the piston as the pin is free floating on the rod not pressed. Ray
If they are forged, the factory only reams them to have .0003" clearance. This is way to tight. Have your machinist hone them to .0008" to .001" clearance. If not, they will seize and wipe out the skirt. Make sure to clean out the dried up cosmoline from the bores before you mic them. Laquer thinner works well.
The rods are checked for proper interference fit for a pressed applicaton, and then the piston pin bores are honed for the fit that Ron suggested. This allows the piston to "float" on the pin. The process is called "Pin Fit", and has to be done to all aftermarket pistons, either by the piston manufacturer, or the end user. Speed pro never pin fit that shelf stock pistons, it's a machine shop operation that has to be done to them. JW
I was told by a TRW rep many years ago that the reason their pins are so tight from the factory that is they adhere to the factory clearance spec. It may work in grandma's car, but not in any kind of performance application. Many TRW, Speed Pro, Fed-Mog (all the same) forged pistons even call for factory piston to wall clearance too, which is usually too tight for any kind of performance use.
I made a reply about the piston boss being machined for clearance but deleted it. I thought I'm not writing a book (There is many asking threads that need one). Afterward I thought JW will have my *** on this one and reply back with a responce on piston boss clearance. Sure enough. Good info on the replacement piston clearances. The newer ones with the skirt coating are a little larger as well. Since they don't expand as much. Ray
Ray, I just needed to write a little longer "book" with my first response, as I sometimes just assume that everyone knows how things work, which is of course not the case. I forget that what is second nature to a lot of us, has never been experienced by many folks out there, and we all tend to gloss over the actual answer to the question sometimes. We were all answering the question correctly, but none of us actually spelled out the exact process of piston pin fit. :TU: JW
