Hi,

I have a few questions of the brain trust

Background: We own a 2007 R56 Mini Cooper with an N12 engine. It has 77K miles & was bought from a Mini dealer in a non running condition via public auction under the mechanics lien act. It is a father / daughter project with my 17 year old, she is Mini crazy. The story was it had broken the timing chain which was repaired (not by the Mini dealer) & was now back with them in a non running condition. With new plugs they got it running, but badly & setting misfire codes. They performed a leak down & showed only one good cylinder, one a little low & two really bad. They recommended removal of the cylinder head suspecting bent valves, at that point the owner walked away from it & the $900 diagnostics & storage bill.

I removed the cylinder head & replaced 6 exhaust valves, 2 intake valves & rebuilt with new stem seals (all OEM parts) The bores & piston crowns looked as I would expect for a well maintained 77K engine.

Upon reassembly, it starts & runs but obviously unhappy & essentially on 2 1/2 firing cylinders. I did some more research & found this thread, plus other tales of cracked pistons (a common problem ?)

https://www.northamericanmotoring.co...-leg-help.html

A cold compression check shows 50, 185, 140, 30

I now suspect cracked pistons on cyls 1 & 4, so to my questions;

- Does this diagnosis make sense to those familiar with working on N12 Mini engines ?
- When these pistons crack, it is not visible when installed in bore, correct ?
- After pulling the cyl head (again) & the oil pan & pump, can I get the pistons & rods out the bottom with the short block still in the car ?
- Do the pistons crack due to fatigue or due to the piston / valve contact ? (no witness marks on crowns)
- With pistons & rings approx $250 each, do I replace only 1 & 4 or all four ?
- Why does my $110 Bentley shop manual not cover engine teardown & rebuild ? (pistons are not even in the index !!!)
- I assume the rod bolts are one time use only as pretty much every fastener on this thing are that way ?
- What are the torque specs on the rod bolts ?

Thanks in advance & sorry for so many questions at once,

Bill D.

 

If your going through all the trouble to take apart the engine you might as well replace all the pistons, rods etc. Sometimes when the timing chain causes valves to collide with the pistons the real damage could be hidden for a time and come back to haunt you later. The OEM valves are very brittle and tough, they can splinter like glass.

I have even seen pistons just start breaking apart for no apparent reason, there's always a reason and that's a casting flaw which is a known issue with the R56 Mini Coopers. The pistons are weak and made from brittle alloy. There's a bridge plate in the way so removing the rods and pistons isn't going to be as easy as you're used to, I know where you're coming from. The good old days, German engineering is going to make things difficult for you. It's the nature of the beast!

You also need to make sure your cylinders are true and free from vertical scorching, you don't want to find out that your cylinder walls need attention after you put everything back together.

 

Try doing a leak-down test and listening for where the air is coming out of the combustion chamber. If it comes out of the oil filler, the rings aren't sealing or the piston has a gap (or hole!!) in it somewhere. If the noise comes from the tailpipe, the exhaust valves aren't sealing. If the noise comes from the air intake snorkel, the intake valves aren't sealing.

That's just a way to double-check where your compression is going, to make sure you're not looking in the wrong direction.

 

I did a piston / rod change using only the Bentley manual, no other help. All the req'd info is in there someplace, you just gotta root around for it. Anything unique to one of the engine types will be written separately. Otherwise, all the instructions apply to the N12. edit 9/28/15: OOPS, WRONG --- my piston / rods came with their own instructions, bolts and torque settings. My apologies --- As for pricing, my new pistons are not oversized, and lots less than half the total $1700 bill (rods were outta sight). If only replacing pistons, be sure to do a trial fit --- assemble 1 cylinder then slowly turn the crank by hand, CW only, watching for piston / rod interference.

Piston and rod assembly won't pull down without removing the crankshaft. With the head off, they easily push out the top. The Bentley manual will tell you when to replace the TTY (stretch) bolts. When multiple torque settings / turns are spec'd for a given bolt, it's a TTY bolt and you should use new ones.

As for piston replacement, pull all 4, inspect, then decide on re-use. If the cylinder walls are still round and not scored, re-use the good parts. For a daily driver, replacing only the defective parts should be OK. I highly recommend using new rings on all 4, after a good de-glazing process. If you find scored or out-of-round walls, it's machine shop time and new parts are req'd --- all 4 cylinders.

 

Update: Did a leak down with the following results, 70%, 30%, 35% 88%, with all leakage coming out of the cam cover.

Pulled the cyl head (again), as before when I replaced the bent valves, the piston crowns & bores look as you'd expect for a 77K motor.

Pulled the oil pan & was able to get to #4 rod bolts, so removed #4 piston (yes, I did mean access rod caps from bottom / pistons out top, my bad)

As expected, ring land broken away on the thrust face for about a 1" chord, both between 1st & 2nd compression rings + between 2nd ring & oil control ring.

Well at least I know where all my compression was going ………..

I assume that once I remove the oil pump & the windage tray, I'll be able to get to the other 3 rod cpas / bolts.

So, based upon the limited web research I've done, the only std bore replacement pistons are OE, so if this is / was a known manufacturing flaw, how do I know if the new OE pistons now have this fixed ?

Best price I've found for pistons & rings are $258 each, does anyone know of any better sources ? (Ugh, over $1K for all four )

Thnx,

Bill D.

 

That's were all the pistons disintegrate around the ring landings do to very little material underneath, instead of extra material to support the landings you'll find hollow cavities, very poor design! Here's a Cooper S piston, a little more severe but same result, no support underneath. There's a newer casting for the Cooper S model but not sure about a non-S.

 

google "r56 standard bore pistons" yields a number of options.

 

Ref. Google search, yes lots of options for Cooper S & even some options for gen I base Cooper, but as of yet I have found nothing but OEM for gen II base Cooper.

What am I missing here ?

Thnx,

Bill D.

 

sorry i didn't realize the non-s had different pistons. Kind of bizarre there are no aftermarket pistons, but maybe worth making some calls to some of the resellers.

 

I've read a couple posts that indicate Mahle makes (made?) the OEM pistons. You might want to check their web site. Yes, they will probably be the same design, but they might be less costly without a dealer markup.

 

Did some rooting around in my files and found this.
https://www.northamericanmotoring.co...1&d=1444498825

Might help when you start putting it all back together.