Bad news.... good news....better news

Bad news:

Got a call from my buddy Guy (correct pronunciation is Gee, which is french Canadian) .  Despite his best efforts to remove the broken studs he is struggling a bit.  He tried welding a nut to them but they just snapped off further down and now they are sitting close to flush to the block.  This leaves no option but to drill them out using a milling machine and end mill bit.

Fussy, temperamental work with a fairly high risk of wrecking the threads if not careful.

Good news:

After finding limited success using the weld method, he tried centre drilling the stud in preparation to back them out with an EasyOut bit. This proved to be very difficult because the studs are a hard material to drill, but he did manage to centre drill one of the three.  The other two he's going to use the milling machine as it should be easier.

The next concern will be how to clean out the remaining debris from the threads that gets left behind.  The conversion manual is very specific that the lower end of the studs is a special thread called 3/8"-NC5.  So at this point I believe I'll need a 3/8"-NC5 tap to clean out the threads.  Even an experienced machinist like Guy had never heard of this particular thread (he checked with his suppliers too) and suggested it will be expensive to obtain due to it's rarity.

While my buddy worked on end milling the holes, I decided do do some research.online.  Although I wasn't able to find a tap or die that matached this unique thread, I did come across an online archive of GM production drawings that show the machining dimensions of a Corvair engine!  I really love the internet!

This is where it gets a bit more confusing.  According to the dimensional drawing showing the machining instructions of the casting, the stud holes are supposed to be tapped to a dimension 3/8"-16 UNC:

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But how can that be?  The hole and stud should be the same thread as the stud..... hmmm.

I sent an e-mail to the internet Corvair conversion forum seeking some guidance.

​Better news:

Not long after sending the e-mail, I got a telephone call direct from William Wynne (the Conversion Manual author and recognized Corvair guru).

We had an almost hour long conversation about the conversion process, my overall plans and this particular issue regarding the studs among other things.  He's very supportive of new builders like me that want to learn and his overall philosophy about home-building and being in the arena speaks to me.

He is an amazing person to speak with and very quickly confirmed that the GM drawings are correct, the stud holes are in fact 3/8-16 UNC.  The reason the studs are slightly different is GM engineers wanted an interference (extremely tight) fit to ensure the studs would remain in place.  Using a common 3/8-16 UNC tap would be appropriate to chase the debris from the holes.

Guy happens to have that tap (it's common) and I called him afterward to confirm what we know now to be correct.  He's going to finish cleaning things up.  In the meantime, I'll bring him the other half of the block and have him clean it up too.  On the advice of William I'll also be contacting Dan Wesseman of FlywithSPA.com, William's recommended supplier for info on obtaining 12 new (to me) OEM matching long studs as the ones I have are too corroded to re-use.

To make things even better, Brenda tells me a fellow Corvair conversion builder called for me while I was at work and invited me to visit his shop near Barrie.  He is building a Zenair 650 with a Corvair engine.  It is almost complete at this point and offered to help answer any questions I might have along the way.  I'll make contact with him tonight when I get home and maybe arrange a time to visit this weekend when I'm in the area for a family function.

Progress!