All the threaded kits I know of have a nylon insert in the cap to mate with a metal thread on the coupler - this to overcome metal on metal abrasion, or more accurately, metal on plating abrasion, and also to provide a cost effective method of creating a "stop". (generally the wide part of the coupler behind the tread butts up to the narrower nylon insert, thereby coming to a stop). Even the most expensive kits out there do it that way, although I'm prepared to be corrected.
Having said that, there are one or two kits around that have a push fit cap, which is achieved via a nylon insert in the cap that locates on a small cove on the nib section. Axminster's Artisan is one such example, although a brief trawl on ebay will yield the same kit at half the price.
