There are several other posts I didn't see linked. It seems later OM617.952 engines have a straight cylindrical pre-chamber, so you can view straight down. Early engines (don't know when) have a "throat" in the pre-chamber. I think that makes them more susceptible to carbon build-up, which is why there is a "glow-plug reamer" tool. I upset some people by saying I didn't see a need for that tool since I ran a tap thru and could clearly see it, but my engine(s) may differ (don't recall on which ones I saw what). I suspect the change to a straight pre-chamber was for lower emissions and better performance at the cost of more engine noise.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans