F15A Cab Rope Lashing Points

[Tony Smith]

Originally Posted by Jacques Reed

Hi Eric,

Going by mine, they should be 1/4-20 UNC Flat head countersunk slotted screws about 3/4" long.

Have noticed often that WW2 fastener head sizes are substantially larger than those on currently made fasteners. Case in point is those screws.

Cheers,

Jacques

I have noticed that many Australian-built cabs feature Whitworth fittings, oddly often right alongside SAE threads. 1/4" UNC and 1/4" BSW are both 20tpi, but the UNC hex is 7/16" and the 1/4"W is 0.525".

Whitworth Hex sizes were oversized in relation to the thread diameter by current trends, but in 1940 an austerity measure was introduced with War Emergency B.S. 916 : 1940 to conserve steel. This had the effect of reducing bolt hex sizes by one smaller spanner size for the same thread. For 1/4" BSW, the old hex of 0.525" became 0.445", which is very close to 1/4" UNC (7/16" = 0.438"). Wartime BSW threads are very easy to mistake for UNC due to this close similarity of Hex sizes. While these measurements relate to Hex headed bolts, the same applies to countersunk screws.