Cheapest option may actually be replacement of the oilseal holder and attention to the slider thread. An old neglected holder can be cut to make a crude die, but from experience these fine threads are easily deformed. New holders come with the risk of poor thread form, even from established cottage industry niche suppliers, and a good used one is a better bet, at least you know it fitted once.
Unless you can find someone with the tooling to clean them up as a favour, investing in the tap and die or proper workshop time will be prohibitive from a cost point of view. All the suggested options for cleaning up the existing threads are well worth a go, the secret is to engage the threads correctly and ease the tight spots bit by bit. The holder will screw down all the way relatively easily when all is in order.
Swarfy.
