Need to Re-Tap the Mast

[StringBimini]

I have a question about tapping existing holes on the mast to a larger thread size. I have three blocks on eyestraps that are attached to the mast by #10-24 half-inch machine screws.

Ultimately I didn't like the setup, so I have decided to replace the blocks with a different type, that require #12-24 half-inch machine screws instead. Luckily the center to center distances to the existing screw holes are exactly the same as needed for the new blocks. I want to reuse the holes in the mast if possible, and simply tap to the next larger size.

I have never tapped anything in my life, so I am a bit nervous about doing this. First, can a good tap be made into an existing threaded hole, that is so close in size? If not, I guess I will have to tap new holes, and use epoxy putty to fill the old ones.

Assuming it can be done, any advise to a novice on the steps to properly make the tap (or re-tap)? I will do it by hand, and be careful to stay perpendicular with the mast. I assume I should use tapping oil, correct? It is an aluminum mast, and fairly thick -- I estimate somewhere between 1/8 and 3/16 thick.

What spec of tap should I use? I have SAE tapset, but I haven't researched the exact tap specification needed to mate with a #12-24 screw. Also, I assume a right-hand tap is needed for righty tighty, correct? Any particular metallurgy for the tapset, or tap configuration for tapping into aluminum?

[capngeo]

Yes you can, just be sure that the new thread is not crossed wit the old. Typically aluminum will not require oil when tapped by hand. Wax or soap may help. Turn the tap 1/3 to 1/2 turn, reverse to break the “chip” then advance again, repeat until desired thread depth is reached. Always use a electrolysis preventer like tefgel or deoxit when using ss fasteners in aluminum

[Dsanduril]

Shouldn't be a problem, especially since your thread pitch is the same. The 10-24 has a minor diameter of ~.1449 and a major of ~.1900. The drill size for 12-24 tapping is 11/64, which is ~.1718. So, if you drill out the existing hole to 11/64 you'll remove about half of the existing threads (you could probably over-tap without even drilling, but would run more risk of galling, so why risk it unless you just don't have the drill).

Once you've drilled it out the 12-24 will run in the hole more easily than normal tapping because there will be some existing thread for guides (and some of the material you would normally remove will already be gone). Just about any SAE tap will do, since you're not tapping thick material. For aluminum a spiral flute tap is generally recommended, so, if you have to go buy one anyway... but I've done similar tapping in aluminum with straight flute taps. The key is to go slow (at least by hand - machine tapping works better at medium speed in aluminum), keep it lubricated, and keep it straight. Hopefully the existing threads will help with the latter. If you really want to go all out there are special lubricants (Tap Magic Aluminum, etc.) that help with aluminum flow when tapping.

Or maybe you could just use 10-24 screws in oversize holes on the block straps?

[Weyalan]

I'd consider using helicoil inserts to reduce the chances of stripping the relatively soft aluminium threads in the mast. Use an anti-corrosion agent between the stainless and the aluminium.

[StringBimini]

Thanks everyone. Glad it can be done. I tried using the 10-24 in the new block holes, but the problem is, they are pan-head screws, and the taper of the smaller screw wasn't wide/deep enough to hold the block secure to the mast (there was some play when the screw was all the way in). So, I must go to the 12-24 pan head screws.

I have no idea what helicoil inserts are. Guess I better look that one up.

I do know that there is corrosion potential between the aluminum and the SS screws. I have both lock-tite, and tef-gel. I have heard that lock-tite will also isolate the two metals, so I will probably use that. The new block I am using is a Schaeffer flip-flop type, and the SS mounting strap lays flat against the mast. I just painted the mast, but I will still put a strip of tape between the mast and the SS strap so that there is no possibility of future corrosion (or so I hope).