Unusual Metric/Imperial screw combination

I have in my collection of screws some black 6mm cap heads which instead of being provided with the expected 5mm socket head can only be used with a 7/32" A/F key. A 5mm allen key is definitely to large and will not enter into the head of these screws whereas the 7/32 provides a good fit. Has anyone else met this unusual combination of standards? One other aspect relating to these screws is the O/D. When miked, the diameter is very close to the nominal 6mm leading to a tight fit when used with a standard nut in contrast to most commercially available screws which come out somewhat under size.

0BA?

Andrew

0BA would be same OD and pitch as M6 but I thought 0BA took a 3/16" AF key

Edited By JasonB on 28/11/2019 20:45:05

The Skeeter helicopter from the 50's was powered by a Gypsy Major 140 engine. The engine was originally a French design with all metric threads. The engines had all their fasteners with the metric threads, but the heads of nuts and bolts had Whitworth spanner sizes, as the Air Ministry was not willing to buy metric spanners.

OBA is not interchangeable with M6.

Edited By old mart on 28/11/2019 20:50:04

I have just checked out the mating allen key and it is 3/16" A/F not 7/32" as originally posted. Apologies for that. It now seems certain that my screws are in fact 0BA. Thanks for putting me right on this one.