Presently I have a 2 Neil Pyrde X60 masts. One is 400 and the other 460cm. What does everyone think about using say the bottom half of the 460 and the top half of the 400 to give me a 430 mast?
Some brands like Ezzy are designed to do that ......... but I doubt the NP is.
The main thing is the join needs to be snug - not tight, not loose. If too loose it will damage the mast, if tighter than usual it will probably not want to come apart afterwards.
If the join fits correctly, why not try it?
The bend curve is different for different length masts. Your two halves will bend differently. It may work or it may not, depending on the luff curve of the sail.
Being bored one day, I tried this with the Ezzy RDMs I have. Ezzy only recommend that when doing this that the tip is the longer length, and the base the shortest.
I rigged my sails using the normal masts, then the longer tip/shorter base combo, and then the shorter tip with longer base combination.
The shorter tip with longer base did not work very well at all. The shapes of the sails were all wrong, and the cammed sails would not rotate. The longer tip with shorter base looked the same as the correct/equal length masts.
Even though you are going to use NP masts, (and it sounds like they are meant to match ferrule sizes, based on Bender's link) I think you will run into the same problem, where the longest section needs to be for the top and the shortest for the bottom. I expect the reverse will not work at all.
After snapping the bottom of an x3 430 and a top of a 460 (or was it the other way around?) I used them together for my smaller sails for a couple of years without an issue. As mentioned - the fit was loose so I cut up an empty silicone tube to use as a shim! Worked fine.
Thanks for all the tips everyone. The 400 and 460 don't really fit together very well, but it is interesting that I could pair a 400 with a 430 tip and that would enable me to purchase a new NP 6.2m without having to fork out for a full mast. I had decent prang on my North spectro 6.0m and blew the top panel out, the sail is finished. 
(Was 22 years old however).
fibrespar tidalwave tops will also fit onto the sdm pryde bottoms, i have for a few years ran a 400 fibrespar top over a 430 X3 cut down 20 bottom, have used this on my old 4.7 and 5.0 ezzy waves and all the tine on my 07 KA Koncept for speed, works well only rotation is a bit stiff when wind on lighter side but swapped to pryde cams and was then all good, latelyhave run this on an 08 Koncept and its works fantastic witn no changes and actually looks better than my bigger sails with the correct KA masts, have had good results with this combo over the years and done over 42 with this mast combo last week.
Edit - I have a X3 430 top sitting in the shed doing nothing if you want it sailhack.
Thanks Jamie, but I've gone away from cams & now only run RDMs - although the buzz of a cammed sail on a SDM still lingers. I'm sure that there are a few out there that will take you up on your offer.
