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I BELIEVE it is a copper compression washer. I also BELIEVE to have read that by taking the washer, heating it up till it glows and cooling it down in water, it returns to its original shape and seals like new.
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NO... The compression washer provides the seal between your head and the injector (during compression). Spray some water on the seam (where the injector and head meet), and look for bubbles. If you see bubbles, you have a small compression leak. Tighten the 14mm bolt 1/4 turn.
If you are leaking diesel fuel tighten the nut where the fuel lines meet the injector. I believe a 17mm crows foot is the easiest to manipulate. There's no O-ring or anything like that.
If your injector return lines are leaking, replace them with some 1/8 internal diameter, 3/8 outer diameter VITON tubing from McMaster.com. 1 foot ($18, not cheap) will do the whole car... It's biodiesel friendly and should last forever.
If you change injectors, I've found you should tighten these at leats twice, as the heat cycling tends to loosen both.