two versions for sale on this site, both are 12mm, one is 94mm long, the other is 54mm. Which one should I invest in?

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1991 300D

The short one is "For models from 1983 to end of 603 production. Fits Engine: 601, 602, 603, 617 Turbo" and the long one is for "For models from 1996 to end of production. Fits Engine: 602, 605, 606, 648". So unless I am misunderstanding, it would seem that the short one would be the appropriate choice for your current cars.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

thats good to know. thank you

in the past, I've used a 7mm drill bit to good effect & understand it is possible to fashion a reamer from an old plug, of which I'll have several shortly.

hmmm.

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1991 300D