I purchased my TJ wagon a couple of weeks ago with a stuffed trans.
I have since replaced the trans and now the car is on the road . Before l purchased the car it had been sitting for over 6 weeks . The car was well serviced and a timing kit was fitted to the car 45 ks ago. Once l got it mobile again it soon got the dreaded tensioner rattle. The mechanic that did the RWC pick it and l confirmed it in my mind buy use of this forum .
When l took the harmonic balancer and timing cover of the cold motor I found ...
The timing tensioner was fully extended , the hole on the shaft that the pin goes into was past the outer casing by a good 3-4 mm and the belt was not up to tension . The belt pulley adjusting nut was also tight .
To me it looked like the belt my have stretched . The tensioner looked original and l think still serviceable . I replaced it with a new genuine tensioner anyway and readjusted the tensioner pulley .
The car now starts and runs fine , very happy .
So my question is .. Do some brands of belts stretch a little ??

Zero stretch

Belts do not stretch, the break

I have seen this on cars where belts have been fitted only weeks prior, some people simply dont know how to fit a belt in the first place and I think they do not hold the pin back, they just remove the old belt, put the new one on with some pressure on the already extended pin

Zero stretch

Just to extend on this . . . . if the belt actually stretched, the belt teeth would not line up correctly in use, and some slippage of the teeth on the cog would occur at initial contact. If you have ever looked at a well used belt, you will see no slippage wear on the teeth.

As above, most likely someone didn't follow the correct procedure, had a bad day, hated the car, or was overconfident . . . . or all of those at the same time.

A toothed belt if it did stretch wouldn't match the teeth of the "cogs" of the wheels and I would imagine quickly be destroyed by violent skipping over the metal teeth.

It is normally recommended to replace the Cam belt idle pulleys at the same time as the belt and tensioner (most kits have them included), one of these has an offset axle and two radial holes that a tool is used to hold the "cam" wheel to adjust the amount of tension on the belt and tensioner - sounds like someone loosened this and/or did refit the new one properly.

A toothed belt if it did stretch wouldn't match the teeth of the "cogs" of the wheels and I would imagine quickly be destroyed by violent skipping over the metal teeth.

It is normally recommended to replace the Cam belt idle pulleys at the same time as the belt and tensioner (most kits have them included), one of these has an offset axle and two radial holes that a tool is used to hold the "cam" wheel to adjust the amount of tension on the belt and tensioner - sounds like someone loosened this and/or did refit the new one properly.