Well I took a look at one with a ruler... seems to be closer to 51mm long (including the head). We'll probably just take a ride out with one in hand and see what we can find.

Edit, so I gather you're suppossed to not include the head in the length measurement. So they are more like 45mm long.

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My brother-in-law has a '81 300D w/ Vegtable oil conversion. I'm just a Ford guy foolish enough to help him work on it.