1/4-18 thread machine screw?

[cockeyedMFer]

We're cleaning up some old metal window frames in one of the buildings here at work and the boss wants to replace the hinge hardware because the heads are chewed up. They're just coarse thread round head slotted screws, 3/4" long.

Problem is, all the screws are 1/4-18 thread. Checked them with a thread gauge and its definitely 18tpi. Its close to 7mmx1.25, but not quite. Windows are galvanized steel with wire mesh inside the glass and can't be older than the 1950's?
Is this a specialized fastener or something? I can't modify the window frames to accept different screws - the windows open inward similar to a pair of closet doors, and have nut-plates inside the frames where the hinges attach.

 

[ultrarunner]

Can you run a tap and convert to a standard size?

The only place I can remember 1/4-18 is for pipe and grease fittings

 

[Soundguy]

npsf? ie.. straight thread?


what are the heads?

 

[LD1]

Certainly wouldnt be 1/4-18 NPT. Even straight pipe. A 1/4" pipe has an OD of closer to 1/2". Certainly wouldnt run a 1/4-18 pipe tab in a hole for a 1/4" bolt.

Are you positive they arent 5/16-18 ??
How are you measuring the diameter?

Got any pictures?

 

[oldnslo]

7mm would be larger than a 1/4" & smaller than 5/16. I suspect LD1 is on the right track with 5/16-18.

 

[Baby Grand]

7mm would be larger than a 1/4" & smaller than 5/16. I suspect LD1 is on the right track with 5/16-18.

Ditto that. Double check that diameter.

 

[duroc]

Can you run a tap and convert to a standard size?

This is probably what I would do, but I'd have to take a look at it first. Another option could be to drill out the nut plate(s) and install rivnuts. Pictures really would help.

 

[cockeyedMFer]

Here's a comparison - the old screw is in the middle, flanked by 1/4-20 and 5/16-18. The diameter across the threads measures right at .250 (I suspect that the screw was a little larger when new - note the threads are flattened somewhat from use), and 1/4-20 is smaller at .245". A 5/16 nut slips right over it.

If we rethreaded the holes with 1/4-20 I'm afraid it would result in a loose fit due to the smaller diameter. 5/16 would end up taking a lot of meat out of the nut plate but would probably work...then again there are about 50 holes to tap.

Good idea on the Rivnut, I'll have to investigate that.

 

[LD1]

Oddball indeed.

Try this,

With a set of calipers, measure the root of the threads on that screw (minimum diameter...NOT on the threads, but in the valleys) and compair that to the 5/16.

Its NOT metric. 18TPI would be ~1.41mm thread pitch, and standard jumps from 1.25 to 1.50

 

[cockeyedMFer]

Oddball indeed.

Try this,

With a set of calipers, measure the root of the threads on that screw (minimum diameter...NOT on the threads, but in the valleys) and compair that to the 5/16.

Its NOT metric. 18TPI would be ~1.41mm thread pitch, and standard jumps from 1.25 to 1.50

Ok -
1/4-20 .207
1/4-18 .213
5/16-18 .263"