Large Drain Plug Size

What size wrench should I be using on the large sump drain plug?
The plug (brass) on my 1968 Fastback is 1.480, which I am guessing is actually a 7/8 BSW size. I picked up a 7/8 BSW wrench the other day, but it was a very sloppy fit. I measured the jaw width, and it came out at 1.515. Which is weird, as this table
http://www.vintagemg.com/ArticlePDFs/WRENCHES.pdf
suggests that a 7/8 BSW should be 1.480 across the flats.
I took the wrench back, tell the guy it is a piece of crap, and he pulls out a perfect unused Bluepoint 7/8 BSW, also 1.515 across the flats. WTF?
I measured the large sump drain on my 73 850, and it measure 1.500 across the flats.
Apparently the early and late large drain plugs are different sizes.
Do I need a 7/8 BSW wrench for the early one?
I am guessing the late (850) plug requires a 1 1/2 inch wrench?
Anybody know why a 7/8 wrench (two actually) would not meaure out at 1.480?
Yeah, I know I can use a sloppy 1 1/2 inch wrench on both. I just wondered where Whitworth fits in here.

Stephen Hill

Stephen Hill said:

Anybody know why a 7/8 wrench (two actually) would not meaure out at 1.480?

Click to expand...

I just measured a slightly worn 7/8W, it was around 1.490" average, but nowhere near as much as 1.515" at any point.

Late hexagons do seem to be 1.1/2"

My 74 850 is 1.5". The 750's I believe were 1 -7/16". There was a guy on eBay selling a very nice combo spanner for the 9/16" drain plug and a big hex for the screen filter plug. It fit my brother's 750 but was too small for my 850.