I previously posted about my JCW project car, 2010 S dealer conversion here


https://www.northamericanmotoring.co...n-cyl-2-a.html


and after a thorough cleaning, put it back together and *thought* I was done.


To recap, I had low compression cylinder 2, it would barely run, and here was the initial compression test:
125-50-145-150


And after cleaning and reassembly, it still had a rough idle and misfire on cylinder 2, but compression:
150-30-150-155
hmm.

The car was running sub 6 seconds 0-60. I assumed that I f***** up <major expletive deleted> and had a valve problem, so I tore it apart again, did an even more thorough inspection of the bottom end (without disassembly...) - no scratches on cylinder 2, even had a strong cross hatch mark -same as the rest of the block.


SO I tore the head apart, took a skim cut across the bottom of the head = couple of thousandths, hand lapped in all the valves, and glass beaded the bottoms of the valves (NEVER THE STEMS or SEATS!!!), checked each rocker arm, each lifter, valve guides, new seals, and here is what I ended up with:

Mini Cooper 2010 S/JCW N14 stock head after cleaning



Since this is the second time I have pulled the head, I figured I would exercise great caution and pulled a vacuum on all the valves - 100% pass. woohoo.


Putting the head back on and timing it, I figured I would try compression testing by hand turning the engine just to ensure I had solved the issue before putting everything back. I might add, I replaced all the fuel injector seals, and replaced the fuel injector in cylinder 2 just in case.

Well Sh*t,,,, no compression cylinder 2 again. WTFO?
I decided to use some 60psi air to pressurize each cylinder in turn - (and I might add, had I performed a leak down test on the first rebuild try, I would not be here!) - cylinder 2 leaked into the bottom of the block. All of the other cylinders actually turned the engine - and thats with just some oil I splashed on the cylinder walls.


The PO said after this rough running incident, and a round through the dealer's repair dept, they pulled the pan and it was clean. Car prob has 200 miles after the incident, including the 125 miles or so I put on it after rebuild #1. After pulling the oil pan, which I SHOULD HAVE DONE THE FIRST TIME, looky what I found on the magnetic oil plug:






And the surprise, in the bottom of the pan:






Well shoot. At least I know what is going on - especially since the compression ring is intact when inspected from above, and this car runs so well. Its cracked right under the compression ring, but it was a clean break. Probably hung on for awhile, and during a bad misfire came loose. Couldnt see it from the bottom or top.



Ill post an update once I track down a suitable used piston and reassemble. Not sure if Im going to crack the block and do a complete rebuild, or just replace this piston - depends on how worn the bearings are on piston #2. Judging by the clean cylinder walls- and less than 0.001" of wear on the diameter, and the general cleanliness of the engine, probably just this piston.


Any thoughts on this?


Score - Mini 2, me 0.

 

And here is what was stuck in the oil pump strainer...





And the piston...


And with all that damage, the cylinder bore is perfect - not a scratch. Cant figure that one out.

 

well well well...Glad it wasnt mayhem in cyl 2. this should be easy now

 

Pictures of the bore, please.

How did you determine that there is less than .001 wear on the bore?

What are the taper and roundness specs for a new cylinder? Does this bore meet them?

How do you intend to break the glaze and remove the honing residue from the bore wall?

 

Here is a bore picture - cant see much, but its really in great shape.

N14/JCW Cylinder # 2 after removing damaged piston


In order to measure a piston bore - you need a special type of caliper designed for this purpose. I did not take pictures of the measurement on this engine, but you can see a picture of this device in a previous post:
(picture # 5 I believe in newest-oldest order)
https://www.northamericanmotoring.co...destructo.html


I also verified the bore diameter on all four piston bores - measuring ~ 1/2" from the top - both directions, i.e. front to back, left to right. I compared the measurements of all four cylinders - found 1, 3, 4 to match, and cylinder 2 was 0.0005" less. They were all within the spec in the shop manual, which is:


11 11 Engine Block, Cylinder Crankcase N14

Bore hole ∅a) mm 77,000...77,018 Permitted out-of-round of cylinder bore a) mm 0,005 Permitted conicity of cylinder bore a) mm 0,01 Permissible total wear tolerance between piston and cylinder (engine operated) mm 0,130


Here is the piston spec - btw, I purchased a used piston to match the other pistons better. I probably should have replaced all four pistons with 0.020" oversized pistons and bored the block, but decided this shortcut was acceptable. The used piston measured 76.92 mm in its smallest dim. Could not (chose not to) measure the other 3 pistons still in the block.


11 25 Pistons with Rings and Pins N14

Piston and pin are paired to each other - replace together only.

Measuring point ”A” (position) mm 10 Piston diameter at measuring point ”A”

Original (new dimension) mm 76,964 ... 76,992 Piston running clearance mm 0,008 ... 0,050 Permissible total wear tolerance between piston and cylinder
(engine operated)
mm 0,130




Regarding boring the cylinder - I did a very quick hone using a lisle cylinder hone (refer to that top link, same tool) but took no material off - just put a nice cross hatch pattern, maybe 20 seconds . No change to the measured dimension.

 

In any event, the engine runs even better than before (with the damaged piston) and is measuring:
165-170-170-165. Almost new?