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A friend of mine has acquired two new rifles apparently they are threaded at the muzzle in M18x1. He has asked if I can make him a couple of muzzle protectors. Am I correct in thinking the tap drill size will be17mm.
Is M18x1 a common muzzle thread size.

16.5mm more likely

https://www.trfastenings.com/products/k ... etric-fine

Minor diamter is Ø16.917 so Ø17 will do for a full thread. If you wanted something that threaded on a little more loosely then you could go for Ø17.2ish and that will give you an 80% thread or so. But you will be fine with Ø17.

Thanks.

I machined a couple of the thread protectors in Aluminium. I knurled them etc. Any way I sent him a pm telling what I had done he did not sound happy that I had made them in Aluminium said he would prefer steel preferably stainless. The aluminium I used is the same aluminium David uses at the Shooting Shed and I know its good quality with good machinability other wise david wouldn't be using it. The aluminium came out of his scrap bin. Is there a good reason why you would not make a muzzle protector in aluminium or his he just being piccy. Or should I make them in stainless and charge him stupid amounts of money when it comes time to pay for them.

Does the alloy one do the job ,is it ugly , if you make stainless ones your tooling will be more worn , yes charge him plenty !

Perhaps he's concerned about the different rates of expansion when hot? I think you'd have to shoot an awful lot quickly but theoretically the aluminium will expand quicker than the steel and could loosen on the threads. I'm using my bike mechanic's head here - if you need to get a bearing race out of aluminium that's tight waft a torch around the ally for a bit and the race will just drop out.

I think he is just been piccy I think when he sees them all will be well. He is 80+ I have known him forever and shot side by side with him all over we keep bumping into each other at different Clubs we attended over the years and renew the shooting thing together. He still travels to Bisley twice a year with his Son who is also a keen shooter.. Last time I spoke to him he was made up because he actually got to speak to Christel in person.
(On the phone)

I think he was ordering shiny things.

Dissimilar metals can cause a galvanic corrosion circuit. If it gets even the tiniest bit of moisture in the threads, then it will cause one material to corrode and the material to be lifted and deposited on the other material. In the case of steel/aluminium, aluminium is lower on the galvanic series so the aluminium will corrode and deposit aluminium oxide onto the steel.
In some situations this can be beneficial. I used to use this in my old job (sub sea oil and gas engineering) by putting sacrificial aluminium anodes on a steel structure so the aluminium oxide deposits would protect the steel structure from corroding.
In this case it will jam up the threads and get it stuck. In reality, if its kept clean and dry with a small amount of oil in the threads, its unlikely to cause a problem. But the potential is there.

Thanks.