Hi everyone!

I'm new to the Mini craze, so bear with my ignorance and naivete.

I snagged an '03 R53 for my wife, cheap, and for good reason. #2 misfire, and the previous owner experienced blowing the plug out of said cylinder.

Now, before I go any further, to determine there were no other issues associated with #2 I verified spark and fuel, injector was bad and replaced. Pulled the valve cover and checked the springs, everything is tight topside (haven't stuck a camera in the cylinder to look at the valves for carbon buildup or indications of a cylinder hit. Spark is good to the cylinder, but this is where it gets hinky.

Seems the prior, after blowing the spark plug, retapped for a FORD plug, the large version for their big V8 engines, so there's a large spacer in the plug tube, and the tube itself was swapped with a larger version, presumably from something sitting in his yard!

As of today I was unable to do a compression test because I couldn't get the compression tester to thread.

Could this foreign tube and oversized plug be the cause of loss of compression to this extent?

If so, would it make sense to replace the tube and potentially retap the plug hole for a more compatible plug? Unfortunately I don't know why he did what he did, though I suspect he used what was available rather than buy parts, so it's an unknown to get in there and pull the tube, etc., I may find even more damage than expected.

If the cylinder head needs to be replaced then we'll go that route, but if there's a possibility the plug/tube issue may be the culprit, I'd rather try that first.

If this is a good direction, are there directions for tube removal? I found some references to removing it, and also the Mini repair manual, but it's not very specific about the how-to.

Thanks in advance everybody!

 

IMHO, take it to a machine shop that does head repair, and look into a used head. But I think either way the head needs to come off.
Other thoughts, If the current spark plug can be removed and replaced I don't see why a compression tester, of like thread pitch, can't be screwed in, unless the re-tap job was botched. So verify the compression tester has the same pitch as the odd plug. If so, you are back to square one and the head needs to come off. If not you just need a proper adapter.
Good Luck

 

Hold up, so there's one cylinder tapped for a different spark plug? So 3 plugs are the same and 1 plug is different? That's going to cause major issues, especially on a boosted motor. Whoever decided that was a good idea needs to never touch a tool again.

 

How would increased head pressure be involved here? Just asking the question. Would think the larger plug hole is symmetric. Yes the head pressure mapping would be slightly different but does a Mini have sensors that would pick this difference up? Would think any difference would be similar to ring leakage and so small that it would not matter. The issue I can see is that the larger diameter hole is now closer to some head/piston clearance point.

 

Here is a picture of the head as well as the block. The pistons are basically flat and I cannot see how a larger spark plug would impact the dynamics of the motor. The difference in plugs from smallest to largest is like 4 mm or .08 inches.

 

Only way the plug would effect it is if it's a different heat range or gap and if it isn't sealing otherwise very little would have changed , I think you need to bring it in and have it magnafluxed and surfaces but only if they can save that plug hole otherwise you should look into another head , when your that far in you don't need a head wig a band aid

 

That's what I was talking about. It would need to be the exact same heat range and gap and everything.

Just changing the pulley to something smaller on an r53 can start causing the car to break up under throttle, casting you to need to go to the jcw plugs, which are just barely a different heat range.