88-91 300-SELs use a Bosch H9DC or H9DC0 plug. Std. setting is .032; however, there are some on this list who believe a better idle is attainable using a slightly wider gap. I haven't experimented with widening the gap yet.

The H9DC0 plugs have a slightly wider side electrode and in my opinion, last a little longer than H9DCs.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"