So, this is a prelude to my upcoming "lessons learned while sorting the cooling system" thread... which will be published when I've finished and got her running again. In the mean time:

Without going into the gory details, the radiator has two fittings on top. Right side is the banjo bolt for the recovery system. Believe it or not, mine was not plugged. I will replace it eventually, but for right now it seems in order. I will be annealing the copper washers when I put it back together.

On the other side: shop says they tapped it to the stock "British Standard Taper Thread. My plug from the old radiator does not fit. I believe it should be "British Standard Parallel Pipe", just like the banjo bolt. Mr. Palm is quite clear on that point. Comments? If it helps, 1991 5.3 V12, VIN 178XXX.

And, if it should be BSPP, any issues with simply running a BSPP tap to enlarge/remove the BST taper? They should both be 55 degree pitch, 28 pts/in, right?

 

Yes, it is a parallel thread. Sealed with a washer as the banjo. On mine it is the same thread size as the banjo. This is used to bleed the radiator as part of the fill and bleed procedure, about which there are several very clear threads on the forum.
FYI, I have had great success using a fibre washer, rather than a copper washer, on this plug.
Greg

 

Thanks. My plan was to use the fiber washer. I may not have that choice. It's pretty clear the manufacturer used a tapered plug, so I will source one of those. He does not recommend re-tapping to BSPP.

 

A BSPT in a radiator??? BSPT requires PTFE tape to seal it as a washer might not seal if the plug does not screw down far enough due to the tapered thread. Plus this could have the unsuspecting screwing that plug down too far and damaging the thread.

Greg's right it should be parallel.

 

I cannot see ay reason not to be able to retap it successfully.
Greg