How tight for the alternator belt?
 

Just curious - how tight should the alternator belt be? I've always heard press in on the belt with your thumb and it should deflect about 1/4". Sound about right?

3/4" for me. Max.

Yup. Sounds about right to me, depending on the span between the pulleys. On the typical lycoming alternator setup, 1/4" would be about right. Tight enough not to slip under load and not so tight as to cause undue stress on the belt or bearings. If it's too loose, you'll note glazing of the belt surfaces in relatively short order. Most auto parts stores sell a belt tensioner that will provide a more exact reference, but I think the 1/4" rule is a good one.

Torque the belt
 

Belt deflection is an unreliable method of tightening the belt.

Instead, use a torque wrench on the pulley belt and adjust the tension so that you get about 12 ft-lbs before the belt slips.

A loose belt can lead to under-voltage or over-voltage from the alternator. Overvoltage is caused by the regulator trying to force more output from the alternator while the belt is slipping. When the belt grabs, it puts a surge of voltage on the bus.

Please don't ask me how I found out!

ref Lycoming s. i. 1129A

Lycoming SI 1129A Accessory Drive Belt Tension

New 3/8? 11-13 ft/lbs 132-156 in/lbs used 3/8? 7-9 ft/lbs 84-108 in/lbs

New ?? 13-15 ft/lbs 156-180 in/lbs used ?? 9-11 ft/lbs 108-132in/lbs

Vern Little 9A

I think you meant the alternator pulley nut, correct?
This is the procedure we were taught in A&P school, to comply with the Lyco S.I.

Alternator Belt Tension
 

Great info, Vern. I would have never found this in a million years. Thanks for posting it.

Great info Vern. Thanks!

That does sound like a much more accurate method, but I'm not sure I get it. You put the torque wrench on the pully nut, hold the starter ring in place, and then adjust the tension until the belt slips with 12 ft-lbs of torque??

Yep, that's the extent of it. Obviously, you turn the pulley nut in the tightening direction so you don't risk breaking it's torque.

Right-ee Tight-ee
 

What direction is that. I think the alternator (some not sure) is opposite of "right-ee tight-ee".