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Here's the latest. I pulled 2 prechambers and measure the bore. The new head has a shallower bore than the old head. The p/c hits the angled part before the top sealing lip seats.
The correct fix is to bore out the recess in the cylinder head. I'm not removing the head (again) on my car, so that's not an option. Using the spacer (seal ring) would pull the p/c up too far, it is supposed to be within a 0.5mm range of protrusion from the head. The spacers are available in 0.3, 0.6, and 1.0mm thicknesses. They are designed for use when the p/c sealing lip is resurfaced, to correct the alignment (remove 0.3mm of material, insert 0.3mm spacer.)
What I'm going to try is what Dale did - grind down the p/c body where the radius begins. It should only require about 0.5mm of material removed. The total difference is definitely less than 1mm. It's impossible to measure with the head in the car, unfortunately.
Metric Motors confirmed that the new OM603 heads vary widely in accuracy. Some have this problem, some don't. You need to measure BEFORE installing the head! (Ooops.) TheBenzBin claims you need to install new prechambers but that will NOT cure it, the new p/c are the same physical size and if you're lucky to have a "bad" new head (like me), new p/c's will have the same problem!
Regards,
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