Correct drill size / tap for 8mm x 1.25 thread repair?

[sonicnofadz]

A few of my exhaust stud threads on my head have been totally stripped. Does anyone know the correct size I should use for the drill and tap to fix them? Thanks!

 

[eclipsh]

If they are completely toasted you may just want to upsize to the 10mm 1.25 bolts. They'll fit through your manifold. It's what 2nd gen cars use stock. I can't recall what size drill bit to use but the package for the tap should say.

 

[Defiant]

The tap will have the drill size marked on the side of it. 6.8mm.
http://www.desouttertools.com/HTML/Tap-Drill-Sizes-Chart.htm

 

[sonicnofadz]

Thanks for the replies! Wow that chart really comes in handy, thanks Defiant!

 

[Red91EclipseGSX]

Determining tap drill size for metric taps is easy without a chart! Tap size is the same as the bolt size, as most forum members know. To determine tap drill size: subtract the thread pitch from the bolt diameter and add .05mm for clearance giving you the correct size hole to drill. It is best to drill the tap hole undersize and then ream to final size (or just run the proper size drill into the predrilled hole). Also when drilling a hole, back out the drill to break chips loose from time to time. When reaming or drilling to final size, push reamer or drill in only one time and back out quickly to get the truest hole. After drilling tap hole, chamfer hole slightly larger then bolt O.D. before tapping hole, this makes it much easier to start the tap and less likely the tap will break. When hand drilling into the engine or other parts in confined spaces, do not trust your sighting the drill straight, use a drill guide of some sort. To fabricate a no cost drill guide, get a piece of scrap material of some kind, such as a block of wood, aluminum, or steel, in a drill press drill your block with the size drill you need to drill in the engine, clamp it in place, then drill the hole in the engine. Use plenty of cutting fluid when drilling and tapping. The only cutting or tapping fluids I use come from industrial supply houses, they work much better than that purchased from home centers, plumbing supply houses, and most auto parts stores!

 

[Defiant]

Heh. If one in ten modern DSM owners know what a reamer is, let alone own one.

Or, have room to use it on a DSM motor. !