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Quarter twist on the longest open run at the tightest.  Half twist is usually fine if belts and pulleys are good shape.   

 

About as much twist as you can do with just thumb and finger.  

 

Not too tight, you'll bung the bushings in the water pump and generator

Edited by Sharps40
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I find twist to be a lot more accurate with a modern belt.

I typically use a tension guage.

Manual calls for 1/4" deflection with finger pressure on the longest span but belts are more heavily reinforced now, they flex a lot less.

 

If you look at truck manuals from the 80s you'll see that the factories had started recommending a certain amount of flex at a certain pressure. For a 7/16 belt we're now looking at about 90lbs of pressure to make it flex 1/4" along a 16" span to give the same amount of static preload that used to be achieved with finger pressure according to Gates.

 

The end result of tension at the pulleys is the same when installed it just takes a lot more to flex a modern belt.

Problem is that makes it hard to use the old school deflection method without a spring scale or gauge and get anywhere close to consistent results. The deflection a modern belt gives at light pressure and proper preload is just too small, the window is really narrow.

 

Twist isn't nearly as affected by the lengthwise reinforcement so its roughly the same as it's always been.

Edited by 50mech
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The 3/4" belts don't twist too well.

I leave them a little loose...1" deflection to save the water pump...as those wide belts have plenty of grip surface and won't slip...

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The 24489 Napa (Gates) belt is a cog belt. So, it's a little more flexible wrapping around the pulleys. I did the twist method and could only twist it about a 1/4. With that, I can turn the fan and it will slip the belt. I don't have a tension guage. I haven't started the engine yet. 

The belt that was on it before was the wrong belt and was too long and the generator couldn't adjust out any farther. That belt was too loose and flopped a little, but the pump and generator worked fine. 

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21 minutes ago, Dennis Detweiler said:

The 24489 Napa (Gates) belt is a cog belt. So, it's a little more flexible wrapping around the pulleys. I did the twist method and could only twist it about a 1/4. With that, I can turn the fan and it will slip the belt. I don't have a tension guage. I haven't started the engine yet. 

The belt that was on it before was the wrong belt and was too long and the generator couldn't adjust out any farther. That belt was too loose and flopped a little, but the pump and generator worked fine. 

That type of belt is really good on smaller pulleys as it conforms to the small diameter better than a standard configuration.  I use them a lot on woodworking equipment as they vibrate or pulsate less than normal belts.  But, that comes as a cost as they have less area in contact with the pulleys due to the notches.  That characteristic is probably the cause of you fan slippage.  After it has run awhile, the surfaces may become stickier as the cut surface wears a little and you can add some more tension.

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2 hours ago, Dennis Detweiler said:

The 24489 Napa (Gates) belt is a cog belt. So, it's a little more flexible wrapping around the pulleys. I did the twist method and could only twist it about a 1/4. With that, I can turn the fan and it will slip the belt. I don't have a tension guage. I haven't started the engine yet. 

The belt that was on it before was the wrong belt and was too long and the generator couldn't adjust out any farther. That belt was too loose and flopped a little, but the pump and generator worked fine. 

Better to be a little loose than too tight. I can slip the fan without a lot of effort on mine too, not with a finger or two but any more effort than that yes. Single ,narrow v belt ,cogged ,that much leverage. Just how it is.

 

I'd check it again after just 5-10 min run time though...they can loosen up quick at first.

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Ah, the German method, gutentite.  Learned that one from an old boss of mine, born, raised and educated in Germany.

 

His Dad was in the Wehrmacht and wore out three horses going into Russia, spent ten year in Russia after the war ended before they let him go.

 

He had an interesting outlook on things.  Good engineer and a car nut, though his preferences ran to Mercedes.

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