In the mean time I'm going to re-use the right angle coolant plug for the rear of the cylinder head. The o-ring was very hard and split in two pieces on removal. Being proactive I went to the local Grainger which surprisingly had nothing that was close enough. Tyler on the D24 ML mentioned an A/C repair kit from Harbor Freight having the right o-ring. Looking at their website I see a "397 Piece Metric O-Ring Assortment" or a "225 Piece Metric Nitrile O-Ring Assortment" each under $10. I think I'll go this route and report back.
I mean really, you can never have enough o-rings!
