Tensioning a poly v belt?

[Ian_Edwards]

I've just fitted a new Balmar alternator, with a poly V belt, and I'm looking for a simple way to tension it correctly.
I've looked online and it's all about buying a special tensioning guage.
Do I really need this?
Or is there a simple rule of thumb?

 

[iveasmalta]

Don't know if this helps you but for Mercruiser serpentine belts they say between the longest distance between pulleys with a moderate force of thump pressure a 6mm deflection is good.

 

[V1701]

Rule of thumb 1/2" or so, really don't overdo it but engine manuals usually give some indication. If such a tensioning gauge exists you don't need one...

 

[PetiteFleur]

6mm sounds about right, but check regularly. I've just fitted a new poly vee belt on my Beta having to do it for various reasons, the original had done over 1000 hrs and the belt had very distinct 'vees' whereas the new belt only had shallow 'vees'.

 

[NickRobinson]

Just swapped out a screeching (on cold start up) poly belt. New belt had flats on the vees while the old one were pointed.
No noise now, used thumb to try tension.