So, I spent last month rebuilding my engine after "something" took a chunk out of two pistons (another story, it's in my build thread). Anyway...I cleaned up one of those pistons and it's sitting on my desk at work. Yesterday, during a particularly riveting conference call (yeah, right) I was looking at that piston and noticed this;

There are two of these "gouges", one at 2 o'clock, and another at 10 o'clock on both sides of the wrist pin hole. It looks like someone took a die grinder to these spots and neatly ground these out. So far, I haven't looked at any of the other piston to see if there are similar markings, but I will.

I'm just curious about what would cause this type of wearing. The wrist pins were free-floating on the piston and a press-fit into the rod. I don't know if that's a factor.

Any thoughts?

 

more details. piston and rod manufacturer.

Yes, that looks like die grinder evidence. who built the engine?

 

Wrist pins should not be press fit into the rods on a miata motor. That's an issue.

 

This

 

Supertech pistons, Eagle rods. I built the engine and no grinders were used in the process

 

I didn't think press fitting wrist pins is good on any engine because it will do what you took a picture of.

But I've only built one engine, twice.

 

Not possible if wrist pin is same diameter as hole in piston. That indention was obviously caused by something with a smaller diameter.

 

Piston flexes cause RACE CAR and locks onto pin. Pin is pressed into rod. Wear onto piston sorta like picture is the result.

 

So, it sounds like the "press fit" was the culprit.

When I say "press fit", I don't mean using a 50lb press to put the wrist pin into the rod. It was more like a "stout shove" to get it into the hole (no snickering). This was my first experience with those rods, so I didn't think it was odd that the installation wasn't a slip-in fit (again, NO SNICKERING).

When I removed the pins from the old pistons, it took a bit of persuasion to get them out, but nothing out of the way so I didn't think twice. I did inspect the bores in the small end of the rod and ran a light hone on them to clean up any spalling (which wasn't evident by eye, or fingernail). The rods with the new Wiseco's slipped right in for a nice tight fit without "persuasion" (No...oh, never mind).

Lesson learned.

 

They should pretty much fall out. And fall in, like, blooop.

 

If you've got top-quality parts it's always going to be a tight fit. Proper lubrication definitely helps with assembly. In fact I prefer to assemble and disassemble the components multiple times before I'm fully satisfied with the fit. Especially if it's a bit snug.

 

Not trying to bring this thread back to life, I stumbled across it while looking for some examples of a particular piston modification I need performed. I'm pretty sure the two areas referenced in the wrist pin hole at 10 and 2 are part of the piston design, especially if they are manufactured by MAHLE. I don't know why the areas are rough in the picture, except for the possibility of some debris getting in the space. I noticed the two smoothly, excavated areas in relatively low mileage stock pistons I have from GM 3.6L motors, LY7 and LLT and am convinced they are present for pin lubrication and possibly stress reduction of some sort. That was probably figured out at some point here, so this is for others who may not be aware of it.