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Avoid the cheap pass through ones that connect inline with the HT lead, you may get away with them at idle but when raising the revs to check advance you may get a few shocks, they also tend to be quite dim.Any one that connects to the battery and has a pickup which clips to the HT lead should be fine, I think mine is a Draper.You probably don't need a high end one where you can advance or retard the strobe, as you'll normally just be checking that the standard marks align.