Scoop, I ended up with a new casting 940 which was drilled and tapped for a bypass tube. I have decided to use it on my freshly built 1360 Cooper S. It did not have a tube so I ordered one from MSC. I doped it up and put it right in. I should have gotten a bag of them.

Doug, please email me you mailing address, Scargo popped by the shop with something from you. I had forgotten all about it. I know I owe someone and I'm thinking it must be you. Thanks for your friendship it's guys like you I missed while gone.

Steve 

There are only about 4 threads in that hole altogether.
I routinely drill old rusted ones out with a 9/16" drill, then tap them 5/8-18 UNF.
For 20 years I have made replacement fittings from hex brass- and they never rust out.

16tpi is part of the UN constant pitch series. Of course, standards don't mean you can buy them. 5/8"-16 is mostly used for drill chucks. You can try metric M16x1.5 it might be close enough for a relatively short thread length and you're going to Loctite it in place, right?

Kelley

I don't wish to stir up the thread argument again either.  We had a long run on this topic on TMF a couple of years ago.  However, the definitive answer was printed in the factory manual:  "Whitworth form".  Not a true Whitworth but a 55 degree thread with radiused crowns and roots.  However, I went so far as to make CAD models of each thread form (UNS and Whitworth) and superimpose them on each other.  The two have about 0.001" of interference at various points.  They are SO close in this size that you could certainly use them interchangeably by the time you factor in thread fits.  I don't care to enter a discussion on relative thread profile strengths.  Enough on that.

If I find a source for the plugs I will let you know.  McMaster lists a die for 5/8-16 but it is $70. 
A Google search shows that Global tools has the die for $15
//www.globalindustrial.com/p/tools/industrial-tools/Dies1-Round/5-8-16-hss-tmx-round-adjustable-die-special-thread-1-1-2-inch-od
I've never used Global so that's not an endorsement.

Regardless, you could buy a die and re-thread some brass pipe plugs to fit.

thanks Doug. Yeah I have done NPT on some common heads.  These AEG163 heads I am messing with now might have a bit of vintage value, so I prefer to not modify them.  The Churchill 18G1128 tap was the BMC tool for chasing crank threads during rebuilds, its a bit pricey these days so I just bought a Qualtech 5/8"-16 tap to do this job, less than $15. 

I don't want to get into that Whitworth argument, there are people saying it is, some say it isn't.  Most places list this thread as UNS, unified special.  I would care about the precise thread form if I was cleaning the crank, but for chasing the head hole its not that important.

[edit] OK, back to plugs and fittings.  Shown here are two 5/8"-16 cylinder head bypass hose fittings.  



One is equivalent to the 12A2075, but it is better than the standard BMC one as this one has a hex so it can be removed with a socket. The other is a plug from a 12G1316 smog head which I use for pressure testing heads, but it is the only one I have.  As I said above, I may have to make some.  I would like to find more of both of these items, but have not found a source.  If you have a source for 5/8"-16 plugs and fittings, let me know.  Thanks.

I have never seen a source for any 5/8-16 parts except the crank bolt (which has a Whitworth "form" thread).  You could buy a piece of brass hex stock and ask a machine shop to make you some threaded plugs.  Those could be turned on a lathe without their having to invest in a die. 

EDIT:  Or of course... you could re-tap the heads for 3/8NPT and use a pipe plug.  That would be your least expensive option.  You would never be able to use 12A2075 again in the head but you could replace it with a standard hose nipple if you wanted to revert to using the bypass again.

The older motors had bypass hoses.  The 12A2075 bung in the head is 5/8"-16 thread, the same weirdo thread as the crank bolts.  We used to just braze up the bung and plug the water pump.  I have a head here which has a proper threaded plug in it, I would like to find more of these.  Anybody know where to find 5/8"-16 threaded plugs, or should I cut a piece off an old crank bolt and make my own plug?