Lug Back
for Compac and Mercer dial indicators Your large 58 mm diameter Swiss made Mercer or Compac dial indicator comes with a plastic flat back. Four large slotted screws hold it in place. If you buy the optional cast metal lug back, you simply remove the plastic back and screw on the metal lug back using the same four screws. Compac and Mercer are identical because they're the same indicator, sold under two different brand names. These metal backs will fit all Compac and Mercer indicators made in Switzerland. They will not fit the old Mercer indicators made in England. Every manufacturer has slightly different specs for their indicator backs. As a result, you will have to use the manufacturer's own. They are not interchangeable between manufacturers. 
Mercer indicator with optional lug back installed. The flat back has been removed. If you're not familiar with the difference between on-center and off-center lug backs, take a look at these diagrams. They speak words.  
on-center (left) and off-center (right) The hole in the lug is 6.4 mm which equals .25". The dimensions given are metric (in millimeters) and correspond to the large (Ø 58 mm) indicators shown above. The smaller models such as Compac 353E have a correspondingly smaller back but still have the quarter inch hole. There's also another difference. The small indicators do not have removable plastic backs. The indicators have a solid case. The optional lug back screws directly onto the case. When you buy the small model lug backs, you will also receive four screws for this purpose. | 
