I agree with Phill on this one with regards to pen kits having plastic inserts.
That is to stop mechanical wear in the plating, which will eventually wear off after a few times use.
I was talking to a gentleman quite a few years ago that worked for a very well known manufacturer, they used an ACME thread which was very sturdy and reliable.
The only company I know that make those type of pen kits currently are DayCom, i have used them for many years and the older kits are much better quality and they seem to be in competition with price rather than supplying quality.
Not sure if you have used them or seen the kits before, they are a little higher priced than most kits so probably negate you buying them if price is a limiter.
Hope this helps
