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Bollhoff-Armstrong

Bollhoff-Armstrong is one of the world's leading manufacturers of threaded insert equipment and materials, more commonly known as Helicoil repair kits. They offer such kits in a huge array of thread sizes, including many of the standard model engine thread types such as UNF, UNC, Metric, BSF, BSW and BA. The range of sizes goes all the way down to those typically used in model engine assembly, such as the 6 BA thread commonly used in classic British engines. The company seems to have a fairly comprehensive worldwide distribution network. In addition, helicoil kits in various thread sizes are offered with surpising regularity on eBay and Amazon. A search there will often bring up the very set that you need. 

Although not cheap, these kits represent a really good approach to the repair of stripped threads which allows you to retain the engine's standard fasteners. I've used a 6 BA kit many times for this purpose, with excellent results. I've also found a 1/4-32 glowplug thread kit to be extremely useful given the frequency with which stripped plug installation threads are encountered. A helicoiled thread is actually far more durable than one cut directly in crankcase alloy, making a recurrence of the stripping problem most unlikely. A recommended option if you can afford it and have some degree of mechanical skill.  A drill press is pretty much essential, but that's about it.