Hi, basically i had a 3x7 gear mtb, i changed my chainwheel and added a bash so it was then 2x7, then i damaged my chain on a curb and while replacing it i decided i might aswell go 1x7 mainly for simplicity so i could get rid of the front derailleur and shifter, added a n-gear jump stop for added security to prevent the chain falling off (it was £8 so it was a bargain) and i never used the inner ring anyway. When adding a new chain i put the chain round the front cog and largest back cog, added 2 links then run it through the derailleur and connected it. But when on the largest cog it pulls the derailleur almost fully stretched, literally about 2cm from fully extended, do i need a longer chain? Or a smaller derailleur? Would i get a small or medium cage? My gears are 38t up front and 14-28 on the cassette if that helps

As long as the derailleur is not on the stop at the limit of its travel you should be OK. Since you only have a single chain ring and will only need to take up the 14 tooth difference on the cassette, you could have made the chain longer but it should work alright.

Yeah, if it goes into the largest gear without binding or ripping the derailleur off then it is fine. Your largest gear will never get any larger so if it works then it works.

But remember this discussion if you change your cassette in the future - if you have a 28 tooth big cog now and switch to a cassette with a 32 tooth big cog and you don't replace the chain, you may wind up walking home with a bunch of derailleur parts jangling in your pocket.

I'll make the chain length choice by first going with the longest possible and then seeing how the rear der shifts. I'll shorten the chain only if the shifting is not what i would hope for. This way the rider has a possibility to do a field repair/removal of link without running out of length. Andy.