do you think I could fit a 6-40 set screw into a 6-42 hole?

[greyling22]

I cut a dovetail a little loose (my bad!) converting a marlin from barrel band to mag tube hanger (for a cleaner look) and thought about dimpling the dovetail and running a set screw up through the stud to the dimple to hold it securely like the do with some rear sights on handguns. The dovetail hanger is threaded 6-42 UNS according to ruger, and I cannot find any set screws except 6-40 UNF. nothing is local, so I'd have to buy something online.

 

[REB]

Buy the proper screw!

 

[jdsingleshot]

Have you checked with Ruger to see if they have the right screw?

 

[greyling22]

Buy the proper screw!

Find me a 6-42 set screw anywhere in the world. Because I cannot fine one.

 

[SC45-70]

You could peen the dovetail on the hanger to expand it if it's close.

 

[N555]

Do a hole conversion. Run it through with a 4-40 tap.

 

[greyling22]

Well, a local ace hardware had an amazing selection of screws, and had a 6-40, and it did not fit. I thought a 1/8th inch long set screw might slide through, but it does not. Bummer.

so my new plan! tiny dimple in the dovetail, slide in to mage tube stud, then 2 tiny #9 shot down the hole, then the screw. Tightened it down, and it worked great.

 

[a5werkes]

I am here to learn. Referencing Machinery's Handbook no 6-42 unified thread exists. Same as others above, scoured the internet with no pings for 6-42 anything, fasteners, taps, dies. I came across a 2009 gun forum post discussing 42-tpi threads and it was concluded it most likely was a M3.5 x 0.6 pitch fastener that is used. There is only 0.016" difference in pitch diameter between 6-40 and m3.5x0.6 fastener; for a short fastener it probably doesn't make a difference. Of course early 1900's firearm mfg.'s are known to have used their own propriety size & thread specifications. Unless I see some citation to the contrary, i am believing the Ruger dovetail hanger was not a 6-42 fastener. One of the referenced forum member states his Starrett thread gage had a 42 tpi blade ? None of my thread gages do. Interesting rabbit hole nonetheless.

 

[greyling22]

6-42 from Ruger, in writing. Interesting enough, I have a 1970's marlin that the hanger and screws play nicely with the new Ruger ones. However, I tried a couple 6-40 screws from ace, and they don't thread more than a turn or 2.

 

[a5werkes]

6-42 from Ruger, in writing

Touché!

 

[rdnktrkr]

Harley also has some custom threads on older motorcycles, I had to purchase custom thread chasers for my 31U and 62FL, at some of the antique motorcycle meets there are vendors there that stock the custom hardware.

 

[JohnKSa]

Ruger won't give you a screw?

 

[DrDeFab]

Assuming Ruger didn't make a typo when they sent that, I would drill and tap to a larger standard size, probably 8-32 or M5x0.8.

 

[a5werkes]

So, I now I am the proud owner of the Mitutoyo thread gauges which shamelessly features the 42 TPI on its Amazon sales photo.
Also, the 42 TPI is to my eye visually a mirror image of the metric 0.6 pitch over 0.5 inch length. The gage tool seems handsomely made.
Happy Holidays.

 

[Mizar]

Just for your consideration, OP - when you think metric, you have to think small, like way smaller than imperial... The difference between 0.605 mm and 0.6 mm is 5 thousands of a millimeter, or 5 microns (or micrometers as it is now called in SI), so it's not a big deal at all, identical in any practical way. Same with measurements above the second digit (hundreds) behind the decimal point - with the tolerances shown in the last table, the thousands are simply meaningless. Those type of threads have such low accuracy/big tolerance, that such a precision measurement is absolutely pointless - just get your M3.5x0.6 metric set screw and be good.

 

[a5werkes]

An odd coincidence that the Mitutoyo has a 42 tpi gage and not a 0.6 mm gage?

 

[Tilos]

6-42 from Ruger, in writing. Interesting enough, I have a 1970's marlin that the hanger and screws play nicely with the new Ruger ones. However, I tried a couple 6-40 screws from ace, and they don't thread more than a turn or 2.

So, you asked Ruger about the thread size in the hanger.
Did you ask them if they SOLD that screw?
Maybe where you bought the hanger has the screw!
jmo,
.

 

[entropy]

It'll go in, once, if you force it. Get the right size set screw.

 

[Blue68f100]

Maker a new boss with the screw size you can get that fits the dovetail tight like it should.

 

[greyling22]

So, you asked Ruger about the thread size in the hanger.
Did you ask them if they SOLD that screw?
Maybe where you bought the hanger has the screw!
jmo,
.

They do, and I bought it as part of my conversion. But I wanted to run a set screw up to hold the dovetail I boogered up before I ran the hanger screw in.

I cannot tap a larger hole.
I did find a solution using lead shot.
I do think it is beyond stupid for marlin to use a unique thread pitch way back in the day, and for Ruger to keep using it.