Having what seems to be a cold weather belt stretching problem, replaced the oe tensioner as it 'creaked' with movement so I assumed it was ratcheting the belt longer, it was replaced with a Gates unit of similar design. I Also replaced alt bearings (cs144) and ac pulley bearing assembly, water pump is good. After 2x Goodyears and 1x Dayco- I'm scratching my head. Quality belts aren't cheap so I don't want to keep throwing them away if it's another problem. One thing I have noticed with other vehicles here including v belts that anything made in Mexico just doesn't stand up to -25 to -36C. The new tensioner seems to be free moving but of course it is very stiff. I've heard of 6.5L diesal guys actually reclocking the tensioner to relieve the stress on the belt, this would be a last resort for me. I do have a Gates belt to try (Gates does not have a HD green stripe version for this) and I haven't tried GM yet ( I assume it would be a Gates). Or would a GM tensioner be that much better, I wouldn't buy a jobber but thought Gates or Goodyear would be a good choice. No air pump/ no delete idler.

how much is it stretching? could it be that your tensioner, though new, isnt working properly? or perhaps they sold you the wrong one?

Well I suppose the part could be wrong- TPI was prety consistant through 87-90 for the tensioner. rate of stretch goes from difficult tight to install the belt to the indicator showing on the left scale in about 2 months.
I thought Gates tensioner was quality- I wonder ifshould try the GM one. Seems to be a common thread that alot of off brands do stretch- and I understand they all will stretch when new. I do have a Gates belt I didn`t want to waste.

I'd say switch to the GM part if you are going to replace it. Can't blame you on not wanting to trash a good belt, stuff ain't none too cheap. You might also try another parts house to see if they carry a different brand.

For the price of the tensioner- it's almost the price of a belt, will give it a shot.