Pin tightness in piston.

[MBellRacing]

How tight should the rod pin / wrist pin be inside the piston? When I disassembled my motor, it took a 10mm socket with a 1/4" extension plus a mallet to knock them out, now it seems impossible to get them into my new pistons. When in, the pins do not rotate or move on their own AT ALL. I see on the bottom of them there are little spots where it seems oil should be brought up to them from underneath, signaling to me that they should rotate in the piston. Is this correct?

[e3steve]

In my experience, they should be a good fit -- not interference-fit. They should press home by hand. The bearing surface is the pin-to-rod interface, not the pin-to-piston; but the machining should have produced equal sizing between the pistons' and the rods' orifices, thus the pin becomes fully-floating (like a turbocharger's shaft bushing, only not so stressed!).

[jensen]

Hi,

First of all, never ever knock on a piston pin out, if stuck press it out.
The con-rod is a very strong piece of metal, but only to forces in the length of it.
Also the big end bearings can have a large impact, but only perpendicular to it's axis, not angled.

The piston pin is part of a bearing, together with the small end. Never ever knock on bearings, they are not made for that. The piston pin / small end is the most sensitive bearing in the whole engine, and the most precise, don't screw around with it. A bad small end means a bad conrod and that means a bad crank. Replacing a conrod is very expensive, replacing a crank also.

Always use new piston pins when renewing your pistons, and always use new clips.

Having said these words of consideration, I ll go further.

Before you install the pistons, take the new piston pin, clean it carefully, and oil it (lightly) before you fit it in the small end. Also clean the small end and oil it. Feel with the tip of your fingers for irregularities on the small ends as well on the piston pins. When his is not a slick fit, or you won't be able to get it in (gentle), look at both parts. The pistons pins are angled 45 degree's at both ends to make it easy the get them in.

Like I said the piston pin is part of a bearing, together with the small end, so the piston pin should be able to move through it smoothly and rotate smoothly in the small end. New piston pins are smooth, so check the small ends for damage. Try to stick the new piston pin in both sides of the hole. If one side is letting the pin in and the other one not, the small end is not straight any more.

Jensen