I sourced some over a year ago from Husker Mercedes but wow spendy. At the time i took parts off a no. 14 head and installed on a no. 17 head. I did save the old ones but they are pretty banged up as they don't like to turn out easily. I needed to cut one out as it just got stuck and no amount of twisting would help. PM me if you want me to look for old reciepts, the ollder collars or the inclined prechambers and the never style collars.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10