Discerning type of thread without using a gauge 1

[Yaojun Han]

I'm looking for a replacement for a spacer that I only have a sample for. It's a male/female type of spacer, and male end is 4-40 thread. Problem is, the female side can either be 6-40, or M3.5x0.6 thread. Is there a way to tell without using a gauge or felt (I don't have one)?

On another note, is 6-40 thread compatible with M3.5x0.6 ones? Could there be a list where cross standard thread compatibility is documented?

 

[moon161]

25.4 mm/in /.6 mm/thread = 42.3 TPI, so no, not the same/interchangeable.

3.5 mm is .02" bigger that the major dia of a 6-40 screw.

It's unusual to make a part with two thread types, usually all SAE or all metric.

You'll probably find a M3.5 x .6 just wont fit and then it's easy, isn't it?

 

[BrianE22]

Buy a #6-40 screw and an M3.5x.6 screw and see how each fits.

 

[IRstuff]

This is why I have both SAE and metric wrenches and nut drivers in my toolbox. Although, there is a lug nut size that's very close, but there's still some risk of damaging the nut

TTFN (ta ta for now)
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[TugboatEng]

What are the markings on the head of the screw? A metric fastener may be marked with an 8.8, 10.9 or A2, A4.

 

[Yaojun Han]

What are the markings on the head of the screw?

The spacer did not have markings, perhaps due to it being a non-standard part.

 

[Ron]

You would need a microscope to see markings on a 4-40 screw.