Having just answered a thread about Allen Key sizes I thought that the following information may be of help to members. I hope you find it useful and that the information will make life easier for you. I have put together a small tool kit to help me in the maintenance and repair of my instruments. Item 1 Pro Bolt Thread Identifier. Used to identify the thread size and bolt/screw length of Pan Head and Grub screws used in telescopes etc. http://www.amazon.co.uk/Pro-Gauge-Bolt-Measuring-Tool-Essential/dp/B009SQACP0/ref=sr_1_1?ie=UTF8&qid=1431363853&sr=8-1&keywords=pro+bolt+thread+gauge Item 2 Hex Key Chart. Used to match the correct size Hex Key to the bolt/grub screw. http://www.allenhex.com/faq Item 3 Hex Allen Keys. There are many available on the market. This set is more expensive than others but the quality of the tool and the fact they have an offset driver head at one end make them a joy to use. They are strong do not flex and good quality. http://www.axminster.co.uk/wiha-magic-ring-hex-key-set?gclid=CjwKEAjwpsGqBRCioKet--bp_QcSJADCtbsbm19_hR_LPC0mPGppYTwfFFgWlyFsL3jF88OIEOEuyBoCWWfw_wcB Replacement Grub screws and Pan Head bolts. The items below are available on line and are part of my spares box. http://www.amazon.co.uk/gp/product/B00BSYKQB0?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00 http://www.amazon.co.uk/gp/product/B000LG0TXG?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00