Stick a magnet on it?

isn't the drain plug magnetic?

It is really easy for it to get from the head to the pan. there are holes that go directly from the head to the pan for oil drain. broken pieces of that size would be easy to get down there.

How it broke is an entirely different question.

Things we know it isnt- Bearing, crush washer, thrust washer,

Yeah I now agree valve spring shim is probably the most likely culprit

What valve springs are you running OP? I think most like Supertech use a thicker shim which would be larger than the one pictured. the one i posted was a brand new stock one i had lying around.

What's this? I have 2 on my third cylinder, either side.

https://cimg0.ibsrv.net/gimg/www.mia...5d7e4c934b.jpg

if you are taking apart your engine, you should probably consult your service manual for such info. Are you rebuilding it yourself? seems a bit scary if you dont know these basic parts.

Its a thrust washer.

OPs piece is about half the width and radius.

That's my destroyed engine. Opened her to check for bent rods out of interest.
Cyl 1 is cactus and heads trashed.

Did you see it come out of the drain or was it in the pan after draining? Drain plug crush washer?

Yup, magnetic.

It wasn't before, but is now AND caught this part. See below for more explanation.

I WAS running early 1.8 hydraulic lifter valve springs, then had an overrev earlier this year so we had to rebuild. I did not have a magnetic drain plug at the time. Of course we changed the oil, but we did not pull the drain pan. In the rebuild we used Supertech double springs with titanium retainers and SUBs. I agree (and hope) the theories are correct that this is a stock shim, and it broke earlier this year and sat in the oil pan until we installed the magnetic plug during the last oil change.

OR, we are wrong and it's a part from something else post-head rebuild.

It was stuck to the (now magnetic) drain plug; it's not copper.

This is what i am betting it is.