Hello Alan! A correction to your post ... The Koken was the one that worked. The Mastercraft one did not work, but probably not its fault. The stud I tried it on had broken just below the cylinder head, where the rabbit hole narrows just a bit, in the cylinder. It was binding on the cylinder when I thought all my efforts were on the stud itself, so it must have been slipping.
Yes, getting the tool dead straight is dead right! If it wobbles then the gripping end will pull in and out of the broken stud at each point, as it turns.
However, the long hole in the Koken Nuttwister (picture below, from the vid) made that easy. Just turn down the end of your extension/shaft until it is a tight fit in that hole. Then it is dead straight and ready for a bit of MIG welding.
Hello Porchdog! Yes 2187-8 is OK, but better to get the long version that is in the video (pictures hopefully shown here), it has a nice long hole to tap it onto your shaft. I machined off the part number, but it looks like Koken 3128.80-8 ...
https://kokenusa.com/products/deep-nut-twister-3-8sq-dr-8mm-l-80 (~$35 most places. I found mine in a box of bits in a toolshop in Hong Kong!).
Final diameter is as big as possible while getting free turning down that rabbit hole. About 10.2mm for me. I actually tapered it off just a bit so that it is fattest (10.2mm) at the end, where the work is done, but a tad thinner (maybe 9.9mm) as of say 5mm from the end, to help not get snagged on any irregularities. I might not have needed to do that.
Once you know what you are doing, it should be under an hour to make your tool.
I only replaced the four broken studs. My reasoning was that in extracting all those sleeping dogs (unbroken studs), one or more could break, and possibly so near the crankcase that extraction might be difficult. Then you would have the big job to do. Easier to just look each time you adjust the valves, and replace as needed, engine in the car.
Cheers, Brian
