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[JamesV6CDX]

Taking the piston out isn't really an option 

I don't know how big of a deal it is to remove on your car but usually just drop the sump and undo the 2 big end Conrod bolts and out it comes?

The issue is with the sump removal rather than taking out the piston - I'd have to lift the motor or drop the subframe.... and I'm not ready for really heavy work

[X30XE]

Missing circlip on the end of the pin? 

Eg the pin holding the piston to the rod? How would that have happened? 

If we're certain this scoring happened when the car was running, before it was dismantled, that makes me feel a bit better. I was worrying it's something I did during the repair process 

Yeah. Pass... Manufacturing error?

I would bet money I don't have on it occuring well before you opened it up. And also that whatever did it has been "rubbing" against it for a long time rather than grinding/forcing against it for a short period of time.  Aside from which you'd have heard it if you'd caused that much marking in a short space of time.

[JamesV6CDX]

also that whatever did it has been "rubbing" against it for a long time rather than grinding/forcing against it for a short period of time.

So a chance something could still be in there?

[X30XE]

I pressume it isn't mirrored on the opposing side of the bore?

[JamesV6CDX]

I pressume it isn't mirrored on the opposing side of the bore?

Let me run out and have a look

[JamesV6CDX]

I pressume it isn't mirrored on the opposing side of the bore?

Nope, just went out for another look, just that one side.

I filled the cylinder with WD40 earlier, and it doesn't seem to have leaked down at all, maybe that's a good sign.

Also interesting how the score is dead vertically straight on each side?

[Andy B]

....
Also interesting how the score is dead vertically straight on each side?

not really, the piston only goes up & down 

As suggested, idealy, you need the piston out to examine the piston ring grooves for crap or broken ring

[X30XE]

Hmm. The fact that there seem to be hundreds of marks all reaching various altitudes in the bore further backs up the theory that it is/was something floating around in the end of the wrist pin hole in the piston skirt thingamy.

Unlikely to be the wrist pin itself I would have thought as that would never have touched in the middle.

Have you got a higher res picture just out of curiosity. 

[JamesV6CDX]

I'll upload one in a tick and post a link rather than embedding, so I don't screw up this page

[JamesV6CDX]

http://www.uk-chat.co.uk/bmw/bore.jpg

This is the image without being shrunk

[X30XE]

Ah it's not central to the pin then. But very nearly the same width as the aperture...



Also looks like a weird casting mark horizontally 1/3 the way down? The honing looks a bit healthier in the top of the cyl behind, but maybe that's camera focus?

[X30XE]

Actually, looking how far it is up the bore it could only have been something in the top ring 

[Doctor Gollum]

Not sure that I would worry about that vertical marks are finer than the lateral roughing marks (also present on adjacent bore), and both are still smooth enough to reflect the crown of the piston...

[JamesV6CDX]

Not sure that I would worry about that vertical marks are finer than the lateral roughing marks (also present on adjacent bore), and both are still smooth enough to reflect the crown of the piston...

To be honest - if we're sure it happened before I took it apart, then it's not so much the scoring we can see, that I'm worried about - as I think that's fine - it's more whether or not there's anything stuck in there likely to do more damage... 

[106pete]

All well just put the piston to tdc and use a torch, there should be a tiny bit of play and give you enough clearance to see if there's anything still there?