Hi all,
I'm in the midst of refreshing the N14 in my 2007 Cooper S and made the unfortunate mistake of tapping a piston out with the block sideways and had a piston and rod fall out of my hand and down about 4 feet onto the concrete floor of my garage. Visible damage to the top edge of the piston as shown in the image below, but no visible damage to the connecting rod. Did I just cost myself $400 for a new set of pistons, or is this salvageable with a soft stone file?


Damage to piston

Motor has just about 160,000 miles on original internals, and part of me wanted to replace the pistons anyway, but I'm running into the limitations of my college student time and money budget. I pretty much gave up for the day after this happened, but plan on taking measurements of the block and this piston to see if: a. I need to go up a service bore size anyway, and b. if there is more damage to this piston than is visible, ex. warping. This is my first engine build on any vehicle, so I have no real sense for how careful I have to be with this stuff. Obviously ideally it would've never been dropped, but here we are.

My Formula SAE teammates say it's fine to just file down the high spots and chuck it back in, but I wanted to get a Mini-specific opinion on the matter before I go any further.

While you're here, opinions on the OEM-style plastic segmented vs. Mahle/traditional single piece oil control rings? I've seen some strong opinions on YouTube about the stupidity of the stock ones, but don't really know much on the subject. Haven't ordered new rings yet.

Thanks in advance!

 

Given the time and money investment for an engine rebuild, re-installing a damaged piston would seem to be unnecessarily risky and therefore should be avoided.

 

If it's a stock piston check out your local used parts lot.