Before I try this thread chasing gimmick, ??

[PBSmith]

Have a couple of 8-40 scope base threads in a receiver that are slightly messy. Would like to clean them up.

Tried to no avail to purchase tap in local hardware.

Several sources suggest an el-cheapo way of chasing: cut two small grooves in the end of a screw with the same thread size as those you want to chase. Leave the burr on the cuts. Thread the cut screw into hole with damaged threads.

The people talking about this trick are not gunsmiths or machinists.

What say you experts about this trick? Sounds risky to me.

I can afford to buy and wait for shipment on the 8-40 tap if that is advised.

Thanks.
PBSmith

[metricmonkeywrench]

I have used that method before in less critical applications, but it was always with the intent to either drill oversized or install an insert should the "fix" create further thread damage.

Being overly cautious on something that needs "good" threads to keep a scope from moving I would opt for the correct tool.

[Jedman]

I have bought many taps on eBay, including 8-40. Look at all of the listings to find best prices, you should be able to get one including shipping for $ 5.00 or less.

Jedman

[country gent]

Chances are if the threads are in the receiver its harder than the screws. I would order a couple 8x40 bottom taps from brownels and use them for this. Had a guy in one shop that tried this several times and ended up breaking the screw or bolt off o r making the threads worse. What may work if a screw can be started and ran in is a brass screw with a little flitz abrasive on it and light oil run in and work back and forth. back out 1/2 turn and repeat the back and forth.

[KCSO]

Order the tap from Brownells way cheaper than taking a chance on messing up the threads. I have made my own taps from drill stock but cutting the threads on a screw is BYM.

[Texas by God]

Brownells used to offer slightly oversized screws for these situations

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[slughammer]

Taps are for cutting new threads in a tap drilled hole. I always try to chase the threads with a thread chaser before I run a tap into a hole that is already tapped. Threads may be deformed, but is cutting out half of them the best answer? Try a chaser first.

When I make a chaser, I cut one slot and I DO remove the burrs. Run it in and out of a nut or a good female thread first. Lube it up and be gentle, work it in and out, stop before it brakes or you mess something up. Many times it works, but don't be so committed that you force it and brake or seize it up in the process.

If I feel it's not working, then I use a tap and loose some material.

[bob208]

no way will you find a tap like that at a hardware store. go to msc.

[MostlyLeverGuns]

8-40 available at Midwayusa, Brownell's, 8-40 came from old category National Extra Fine, finer threads are more resistant to vibration, also allow multiple threads in thinner materials for greater thread strength in shallow holes or thin materials, coarse threads are normally preferred for strength. I have used notched screws to chase threads, I notch the thread with a Dremel cut-off wheel, I DO DEBURR the notch with a fine file before using the screw to chase the thread. You can also slightly taper the screw thread to assist in thread alignment. Like slughammer said, go easy.

[PBSmith]

8-40 available at Midwayusa, Brownell's, 8-40 came from old category National Extra Fine, finer threads are more resistant to vibration, also allow multiple threads in thinner materials for greater thread strength in shallow holes or thin materials, coarse threads are normally preferred for strength. I have used notched screws to chase threads, I notch the thread with a Dremel cut-off wheel, I DO DEBURR the notch with a fine file before using the screw to chase the thread. You can also slightly taper the screw thread to assist in thread alignment. Like slughammer said, go easy.

How, exactly, does the cut or notched screw work to chase the female threads? I thought the burrs on the cut were what cleared the damaged threads. Obviously I'm mistaken about that.

[slughammer]

The idea of a thread chaser is to move metal and clean out junk, not remove metal.

Within limits it can reform bent threads back where they need to be.

It cleans out debris without cutting out the existing threads.

The straight vertical wall created by the slot does the forming and scraping work.

The slot collects debris.

[steve urquell]

I've done the slot cut with tapered end bolt used as a thread chaser before on big bolts that were expensive to buy a proper tap or chaser for. I think I have a 3/4" in the toolbox.

I cut the slots and tapered the end then used a thread file to smooth the slots and threads. I wouldn't leave burrs on one if I built one as it would likely cause more thread damage. My 3/4" one has worked OK for me when needed. I use oil with it and go slowly. For a small tap I'd just buy one.

[PBSmith]

The idea of a thread chaser is to move metal and clean out junk, not remove metal.

Within limits it can reform bent threads back where they need to be.

It cleans out debris without cutting out the existing threads.

The straight vertical wall created by the slot does the forming and scraping work.

The slot collects debris.

OK, so the sharp edges of the slot do all the work. In my case, I believe the receiver holes for the girl threads might have been drilled using the improper, over size drill. So I definitely don't want to remove metal if I can avoid doing so.

I might give this a whirl. It's going to be delicate, with the small size here (8-40). Maybe I can cut the slot using needle files. I have a Dremel, but with the small screw size I might not have any screw left after I cut the slot.

Thanks for your good explanation.
PBSmith

[Texas by God]

By the time you create the slotted screw and modify it to work correctly, you could have bought the proper tap. It's what you really need anyway. I say this with love because I have been there and done that haha.

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[country gent]

If you can find one we used a special tap for cast aluminum parts At work it didn't cut but "ironed" the threads in forming them with the force. I believe it was called an expungment tap. This style would reform the metal back where it needs to be. But be careful they really turn hard in materials other than soft cast aluminum.

[ulav8r]

Chip-Free Machine Tool Taps for Steel and Stainless Steel

Also known as cold-forming and fluteless taps

The above from McMaster Carr, but they don't have them in 8-40.

[JSnover]

We only knew them as thread forming taps. They needed a larger drill (about midway between the minor and major diameter of the thread) because the crown on the tap threads would displace metal down into the root.
One advantage was some work-hardening of the finished thread.

[Char-Gar]

This jury rig shade tree mechanic fix is to be avoided. Buy the proper tap and do it the right way.

[JSnover]

This jury rig shade tree mechanic fix is to be avoided. Buy the proper tap and do it the right way.

Amen! It only takes a second to screw this job up, followed by a few days/weeks of kicking yourself in the butt. Wait for the tool, get it done right in a couple of minutes.

[William Yanda]

I needed to chase the threads in a 1/2-13 application, without a tap, I used a hacksaw to cut 3 slots perpendicular to the threads. It worked well. 8-40, I don't know. If the screws are harder than the receiver it could work.