Is there a standard for self tapping screws.

[oldcardriver]

I ordered some countersunk self tapping stainless screws from eBay and for the same identity number, they are thinner than Spax wood screws, hence is there a universal standard for screws or do different manufacturers use a different standard, or is there a different standard for different types of screws?

 

[Deleted member 174758]

There was BS.4174:1972, but the Chinese don't always seem to work to it...

Sometimes better to buy from engineer's merchants if you crave consistency, but it will probably cost more

 

[oldcardriver]

Thanks JandK, yeah, I never thought, they are bound to be chinese as they are vey reasonably priced!

 

[rsgaz]

they are thinner than Spax wood screws,

Did they say they were 4mm, 'equivalent' to No8 screws? (which is wrong BTW)
No8 screws have a nominal OD of 4.17mm, but will always read slightly less with calipers, because the edges of the threads are slightly rounded. So, a 4mm metric screw will read about 3.9mm with calipers.

 

[oldcardriver]

I didn’t have to measure the diameter, the stainless ones are about 3/4 of the diameter of the Spax!

 

[conny]

My experience of Chinese manufactured self-tappers, (small range from 3mm to 6mm), is they are rubbish.
They start to drive into the pilot hole then the heads strip out, (even at slow speed or by hand screwdriver), or they simply snap as they start to cut the thread.