I have a cast aluminium timing case cover which is the same shape / form as the later WNG steel covers. It is totally different to the normal civilian / WNG alloy covers. I saw another post that stated that toward the back end of 1944 Ariel (who went to steel primaries/timing covers in about 1943) started listing the alluminium primary covers again. So did they also revert to alloy timing covers, and if so why did they make them in the 'steel' form rather than just return to the more rounded standard alloy form used on the earlier bikes?

The cover is definitely cast as the rough casting markings are visible on the inside. The outside may have been origionalyl rough cast but has been 'smoothed over'.

Another factor is that I have tried it on my 1939 WNG inner timing case and there is no way that it will fit unless the rear screw boss is ground down a small amount, the rear mounting point on the case has a triangular section that will not fir over the case boss.







Cheers
Alan

I am not an Ariel WNG expert but i think the cast alloy cover you have has been cast using an original WD steel cover as a pattern. No proper provision has been made for shrinkage of the new alloy casting and this is why it does not fit and also why some of the holes have been slotted to fit.