I think there are two, if not three types of bolts used for the radiator-but
definitely two.

One, and I think was on the early models has a hole through the shaft> I
can't remember exactly, but I believe it has a hole in the bottom and then
"vented" out the side through the threads.

Another was just a regular bolt, although I think it may have been brass (?)
not sure. This one was matched with a radiator that had a groove cut in the
female threads that worked as a bleeder "canal" when the bolt was loosened.

There may have been a somewhat standard bolt with a groove cut in it for
fluid to bleed.

From my experience there were a couple of radiator configurations. My Bus is
an early water-cooled (83.5) and I swear it had no provisions in the female
threads for bleeding. Plus it had a regular bolt so bleeding was precarious
because the bolt had to be about all the way out to let fluid by.

I did find one of the "vented" bolts in a wrecking yard.

Having installed a new radiator a few years back, it had the groove in the
female radiator threads so I think it's just a bolt. It may be brass. I can't
recall.

But as I recall it's true what maxjoyce said in that it's 8M bolt as far as
the threads. And yeah, the copper washer is key.

There may be an odd configuration in your radiator that accommodates the 7mm.
But I sticking with the story that it may vary depending on the date of
manufacture.

I'm gonna try to run ETKA a little later to check the part number. My
computer's acting funny so it may not run, but let us know what year your bus is.

Best,
Jeff
83.5 Westy
LA,CA

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