Series 2 Crankshaft Sludge Plug threads


I’m trying to figure out what size tap to use to chase the threads on Series 2 engine’s crankshaft sludge plugs (the later, large size plugs). The nearest I can figure is that it is a 5/8-14 BSPP. Can anyone confirm?



Check out the archives with the Search function. The consensus there is that the thread is 5/8-14 BSP. That is, the tapered thread sometimes referred to as BSPT, not the parallel thread BSPP. That makes sense, as a tapered thread will seal when it is tightened, whereas a parallel thread requires a sealing washer and a corresponding sealing surface.


I believe it is 5/8 11 BSP …which is British Standard Pipe, I think. I bought one on line from England some time back in prep for when I get to the engine during my 69 Coupe restore.

Mark M
Murrieta, Ca.

Hi David, Thanks for the reply. I found that forum thread when I searched on ‘5/8-14 BSP’ after you mentioned it but couldn’t find it earlier before I posted - I think I searched on ‘sludge plug’ or something.

Looks like I figured correctly that it was a 5/8-14 BSP thread but I wasn’t sure of the taper since the plug doesn’t seem tapered which is why I though it was parallel thread… granted there are only 3 complete threads so it’s hard to tell. Plus when I put the plug in between the jaws of a caliper all the threads seem to touch on both sides making it seem parallel. I did think it was odd if it wasn’t tapered - thanks for confirming.


Hi Mike
Yes that’s the thread size. If you use a first tap and screw it in until you feel a strong resistance that will be fine. The threads will be cleaned but you won’t lose the taper.
I refit them with green loctite 270 rather than staking with a centre punch as we did back in the day!

Excellent - Thanks to everyone for the very helpful replies! Saved me from having to waste money buying multiple taps.


69 Coupe

I’m glad I read this post, My comment above was clearly incorrect! Sometime in the past, I asked the same question and I must have mis-read the answers. I ordered a 5/8 x 11 and have had it sitting on my shelf, waiting for my restoration to reach the engine phase! I just ordered the proper size! Sorry for an incorrect post!!

Mark M
Murrieta, Ca.

Not just the thread pitch I’ll bet. 5/8-11 is a standard UNC size–it’s diameter is 5/8. The diameter of 5/8 nominal pipe, BSP or NPT, is about 0.9 inches.

Thread form is subtlety different. It’s a BSP thread.

Hi Mike,
The correct tap is 5/8-14 BSP
I found mine on ebay uk years ago.