Hi,

I recently bought my first Mini Cooper, and it has started to develop some strange issues. The idle fluctuates wildly, and the engine stalls whenever I try to throttle.

I ran a diagnostic and got the error code 2771, indicating a misfire in cylinder one. I inspected the cylinder with a borescope and noticed that the exhaust valves are damaged. Additionally, as shown in the attached image, it seems like part of the piston is either missing or has melted.

Has anyone encountered something like this before?

Do you think the entire bore is damaged? Would it be better to replace the entire engine, or would repairing the piston and valves suffice? Or maybe just the valves?

Thanks!

 

Which year? Which engine?

 

2007 N12 engine

 

Mileage?

N12 & all N series engines are known for dropped valve seats and burned exhaust valves. I've restored 5 MINIs, 4 Gen 2s and 1 Gen 1. 3 of the 4 Gen 2 cars had to have the cylinder heads removed and repaired; one was a complete engine rebuild. If your cylinder head needs repair, I'd recommend Allied Cylinder Head in Atlanta, GA. When sending in your head, make Sure they replace all the .022" and .023" valve seats with the deeper .035" valve seats. Seats tend to fall out when the engine is overheated or right after the valve seats are ground.

As far as cylinder bore damage, you won't know until you disassemble the engine.

 

It has about 80,000 miles on it, and it's a Gen 2. The exhaust valves appear to show signs of running a lean mixture.

I really hope it's all of the issues; dropped valve seats, damaged valves, pistons and the bore.

I'll be opening the head this weekend to inspect it. Lets se if it is tears of joy or sadness.

 

Okay, I've removed the head now.

It was a disaster. Both the piston rings and the piston need to be replaced.

I was hoping the head would be enough, but the cylinder walls are also damaged.

Is it even possible to salvage this with honing?
Is replacing the sleeve an option?

The shine thing is oil, poured some 5w30 oil to see how it behaved.

 

Ouch! That's a melted piston. My last rebuild of an N12 Mickey at Pro Comp Machine bored out the block because my piston gap was out of spec. I got a set of MAHLE +0.20" pistons from Ebay; see site below. After 10000 miles and three oil samples sent to Blackstone labs. The engine is performing perfectly. Don't forget your break-in oil change at 500 miles once you finish the rebuild. Also, change all of your fuel injectors when you rebuild; I suspect your lean condition was caused by a clogged/restricted fuel injector.. According to Mickey, N series valves can't be ground because there will be an insufficient margin; so replace all the valves. Lastly, while you have the head off, make sure the machine shop changes all the valve seats with the deeper version; if you don't, I'll bet you lunch they will drop out on you before you hit 2000 miles.



Here's the difference between OEM and SBI. OEM in the middle. Tell the machine shop you want a .010" press fit.



These come from China, but all MAHLE pistons are made in China now. I believe this seller has +.020" and +.050" pistons.

https://www.ebay.com/itm/235348990147

 

Hi, thanks for the replies. The material on the cylinder wall was actually aluminum transfer, so I removed it and measured the tolerances with a bore gauge. Miraculously, the cylinder bore seems to be fine, with no deep scratches or other issues. However, the piston is another story—just wow...

The other pistons and cylinders look okay.

So, I’m considering just replacing the damaged piston with a stock one. I weighed the damaged piston at 329.36 grams. Is it safe to go with the OEM spec of 331 grams from RealOEM, or should I remove one of the undamaged pistons to check?

https://www.realoem.com/bmw/enUS/par...&q=11257566478
Does anyone happen to have an OEM piston lying around or know its exact weight?

Thanks

 

I'm not sure why you're worried about piston weight, especially one that's lost some mass due to melting. At a minimum that damaged cylinder will have to be honed to restore the cross hatch, so while you have it apart, why not do this properly and get the block to a qualified machine shop? A good shop will know what to check and will have the technical references. They will let you know if any of the pistons can be reused. As a minimum, you'll need 1 new piston and 4 sets of rings. My N12 had 150K miles on it, and the piston to cylinder wall gap exceeded the max allowable tolerance of .006", so Mickey said I had to go +.020" over. My finished product was perfect with a .001" piston to cylinder wall gap; I did have to buy a ring grinder (see link below) to set the piston ring gap correctly, but that was easy. It was harder to find the spec for the piston ring gap than it was to actually grind the rings. If your cylinders are within spec (using factory spec pistons), you won't need the ring grinder.

How many measurements did you take with your bore gauge? Cylinders can wear differently at different heights along the piston stroke. Cylinders will need to be checked for axial concentricity, out-of-round etc.