In this situation a tap will work best, but only run the tap into the damaged threads and no further, then chase the threads with a good bolt or thread chase.The very beginning of the threads, is preventing the banjo bolt from starting to screw in.The threads are undamaged beyond the beginning, Somehow, the start of the thread got slightly damaged when the head was off, don't know how. The banjo bolt is not damaged, it easily screws in on the left side. I ordered the tap. 73+ year old doing the re-assembly, could be the problem.
I have BSF & BSW thread chasers, I've have had these for many years as needed for my old trade. But like most things you don't see them for sale anymoreFor Metric fasteners, this type of chaser works well, without cutting bigger. Is there something suitable for the Whitworth threads?
Failing that, a bolt with a notch filed in it has worked for me when afield and cleanup is needed.
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But if the banjo bolt won’t start neither will a slotted bolt surely?
I took a look at that earlier, but the link you posted just goes to a bunch of “slotting threads” videos. Was there a particularly helpful one?It would need a starting taper as per the video in Post # 5.
Nice one Concours !!So, Concours came over and fixed the thread, but he noticed something I did not. The threaded hole for the banjo had been hit with something hard that put a dimple on each side of the opening, pushing the starting threads in. That's why I could not start the bolt. He strightned out the threads, then filled in the dimples with JB Weld, smoothed it out flush with the rest of the surface and tomorrow I will put the oil lines back on. I don't know who, how or what hit the surface. When I get it all set and running, I will update on the repair.