Well, the AX has an aluminum backplate so that isn't a factor I guess. I have not heard of any needle valve problems on the 2 AX engines on my field. They are very happy with them. No head leaks either. This is not the LOW end LA engine, this IS a good engine. I would assume that OS had a reason for switching to 4 bolts on the head and I doubt that it was to save money. An engine that leaks and gets bad mouthed won't sell and THAT doesn't save money. They say they have redesigned the needle valve for easier tuning also.
Guys on my field don't need to fiddle with theirs at all, it seems