chain tension

Has anyone ever used an old rear mech to tension their single speed chain? Saves spending a fortune on a chain tensioner providing you have an old rear mech knocking around (and I bet we all have garages/sheds full of bike bits) and a spare gear cable. I've road tested it and not only does it...

Since my newest bicycle I'm plagued with serious chain tension variation. First a Sugino now also a Stronglight. Other causes have been eliminated, I replaced chainring/chain/rear cog with new, still directly a tension variation of 2 cm up and down, which is said to result from a 0.5 mm...