Whitlock Cobra wheel nut dimensions

[Marathon1150]

We have an American built Beneteau First 35s5 (1990) with rack and pinion steering. Because the wheel had an Edson wheel nut I assumed that Edson built the steering system. It turns out that the system is a Whitlock, probably Cobra 4/4r and that a PO installed the Edson nut. The Edson nut is 1" but has a plastic insert that downsizes it to accommodate the wheel shaft and the shaft threads are likely metric. Unfortunately the plastic insert is partially stripped and Edson does not currently sell such an insert and may never have sold one. It may have been custom made.

I believe that the required nut is metric because Whitlock literature shows metric fasteners and sizes for most things except the wheel shaft on which they are silent. Can anybody point me to a source of information regarding the size and thread type on this wheel shaft? Or a source for Whitlock wheel nuts?

I have measured the shaft diameter and it seems to be 15.66mm. But a "standard" M16 nut has threads that are too coarse. The inside diameter of the threaded plastic insert is 15.58mm but it is, as noted, somewhat stripped.

Any pointers or suggestions will be greatly appreciated.

[Marathon1150]

Just in case someone else encounters this problem, here is some additional information.

The Whitlock Cobra4/4R requires a 5/8" UNF nut to hold the wheel on to the shaft.

Many thanks to Whitlock Steering in the UK for this info.