In reply to timjones:
I think i read that about the threads being the same strength,this is great if the ice is rock hard all the way to the surface but unfortunately this is not always the case.
I used to carry an assortment mainly 16cm a couple of 13cm and a couple of 22cm 360's for belays.(i think the best screw for belays as you can get them into smaller gaps between chandelier ice and bumps/cauliflower ice etc,but a pain to rack/place on the lead).
Experience has tought me that the ice is nearly always harder and more solid/less aerated the deeper you go hence when i replaced my bd'd for helix screws which again i learned place better/easier/faster at least for me.. i went 50/50 on 15's and 20's (they used to be 22cm).
Also it seems the more i climb harder ice,wi5+/6 pillars etc the more i come across chandelier/aerated ice where the screw goes in too easily at first then bites into more solid/stable ice as the screw goes in.
Clearing ice like this would be a pointless and energy sapping pastime and climbing with nomics negates this option anyway
Ive yet to replace my shorter screws but of course its still good to have a couple when you dont have a choice because the ice is too thin.
this guy seems to think longer screws make stronger anchors...
http://hmga.gr/storehouse/word-acrobat/Ice%20Climbing%20Anchor%20Strength%2...
Whether it does or does not,i know i'll feel safer on those hard brittle routes,im not about to get into the typical ukc argument just simply explaining my reasons for this experience led choice....
The longer screws might be a bit of a pain to carry yet only time will tell,but really its only 6 screws that are 4cm longer,and this year im going to try the grivel end caps(black hole)to save my salopettes,i reckon i can remove them easily with my teeth before placing but again time will tell....
cheers cliff....