David...Yes, roll taps require a bigger drill size. And using them to reform the existing threads to the new size is a little bit dodgey. They don't cut, so they don't make chips that clog the tap. At most, you'll see some dust in the oil. It is very important to use a super slippery tapping compound, such as Moly-D, which is expensive and nasty smelling chit. I've used a lot of ENCO #1 tapping fluid with roll taps, but something like STP oil treatment should work fine. Roll taps work wonderfully on 6061T6 and aluminum castings, mild steel, 4340/4130 prior to HT, & 304 CRES. There are definitely some materials that they won't work on. They are simply wonderful for blind holes and soft, gummy materials (like crankcase castings).
The thing to remember is that since you're starting out with a hole that already tapped a different size, you have to use your feel/gut to decide if the roll tap will work or not. Since the roll tap (aka form tap) is almost solid, and there are essentially no chips being produced to clog the flutes (which aren't there!), they're really strong. If you're drilling your own and IF you drill the right size hole (ask for a chart at the tool store), the material is suitable, and the lube is good, it's darned near impossible to break one. That said, you certainly don't want to break one off in an elderly engine case that you can't replace! So caution is advised...
Steve
Topic: old OS engine screw sizes (Read 1807 times)
