Originally Posted by supershaft
Small issue? If the smaller cush drive 17.5 PFA gear is NLA that would require buying a new input shaft and cush drive coupler. ...
Is the 991 gear really NLA? I'll check this coming week. If it is, then if you want to consider '86 another break point then go for it.
I was thinking about a different coupler change ('87-on) and forgetting about the shaft splines. The '87-on 39.5mm coupler is the fat one. Not sure if I have any of the '85-'86 37mm ones around but I think you are talking about the pre-'85 ones breaking.