Thanks, I have a 5/8 x 8 right hand acme tap, but its not acme. I also thought about it being left hand but it is a right hand thread. The 5/8 x 8 acme wont work. I used an acme thread gage and its not a true acme. Then I checked with an older machinist who was in the business since being in the navy in WWII and he said it was probably a modified thread, just by looking at it. Thanks for the help , I will keep looking