I was facing this yesterday, but since I was the last guy to replace the injectors, I knew all the heat shields were there. There is a lot of carbon in the crack and behind the shield sticking it to the prechamber. Take just a little penetrating oil and soak the joint. Scrape around the seal with an awl or small jeweler's screwdriver. I did this and mine still wouldn't budge. So I took a deep socket that fit the prechamber hole almost perfectly and gave it a sharp tap with a hammer. That finally broke the carbon seal. One advantage of using the small 7.5mm seal is there is more of a lip to pry against.

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