I feel sure someone will have the answer to this simple question, so here goes! I had on my eBay watch list a boxed 2MT boring head kit, nine boring bars, Allen keys etc., which I was not really intending to buy but then the vendor offered it at such a ludicrously low price that I could not resist. Although I will find a use for it (eventually) with its 2MT fixing, I would ideally like to swap the arbour for a 30INT for my Harrison mill's vertical head. Such items are available but my problem is I cannot establish for certain what the arbour thread on the boring head is because I only have a cheap set of thread gauges and the old eyes aren't so good as they used to be. So the thread, as far as I can determine, is either metric 38mm x 1.5mm pitch or 1.5" x 16TPI. Chronos sell such an item but they quote it as being 18TPI which may be correct for the boring head they may be selling but not for mine. So can anyone tell me which thread I might require, please? (I think this must be a fairly standard item across all brands and price ranges for what appears to be exactly the same thing).