My first guess is a spark plug crush washer.
Can you get a picture of the side of it, does it look like it has two layers ?
Also aren't the crush washers for banjo type bolts copper ?
I'm also bad with mm to inches. Is 28mm just over a 1/2" ?
If it's bigger then that, I'd guess oil drain plug washer like MAP said.
Thinking about it, isn't 2.5cm = 1 inch ? The oil drain plug is 20mm so something with a ID of 28mm is going to fit on it.
174,000+ miles, 5/20/12 new ej257 short block, 8/20/11 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. Also CHECK YOUR OIL, these cars use it.
Last edited by Max Capacity; 01-11-2013 at 03:30 AM..