Posted this many years ago for my 1340GT, and built another for my ERL-1340 the other day. It uses a test indicator attached a Zero-Set which is normally used in mills for hole centering. Versions of the Zero-Set are on eBay and through Shar's for $20-30. The test indicator holder slides on two bars mounted to the lathe belt cover, so remains in the same position vertically relative to the stock in the spider. The tip of the test indicator is always in aliment and the Zero-Set/indicator slides forward when not in use. The rails are 1/4" rods, on mine the spacing is 3/4", the cover aluminum mounts are angled so they are level horizontally. They attach with two small screws behind the cover. I use a bit of red loctite when I insert the rods into the mounts. I have an extended reach indicator which are a bit harder to find/cost, one could use a inexpensive test indicator and put a longer point on it (check the thread size), the key is to have a test indicator with enough travel (i.e. more than one revolution). Some examples below. Be sure to extended the spider bolts fully to make sure they do not interfere with the test indicator.
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AGD 2 standards. Easy to read aluminum dial face with anti-glare finish. Reverse Counter. Lug Back: 1/4" Holes. Tolerance Markers. at Fixed Prices.
Fits Mitutoyo dial test indicator :513-415-10，513-215N，513-215F，513-215FE，513-415，513-115，513-515，513-477-10H etc。.