New member - help with fan belt 1976 911

[pinzy76]

Hi, Im a new Porsche owner having recently purchased a 1976 911 2.7L.
The past 2 weeks the red alternator warning light was illuminating during driving. I decided to change the fan belt looked worse for wear as you can see in the following pic. It was a 10x710 Gates belt.



The fan nut was INSANELY tight and I actually needed the help of second person to loosen it.
The shims were arranged 6 in 0 out.
And the outer pulley was cracked


I ordered a new outer pully from our host and a Conti 10x710.
Even with 6 in 0 out the belt was really tight and I could hear the fan grinding against the fan housing when cranking it by hand.
So I ordered both 9.5x710, and 9.5x725 belts just to be on the safe side - both of them were too tight as well even with 6 shims in, 0 out.
The 9.5x975 was slightly better fit but there was still slight noise of fan hitting the housing on manual cranking. When spun freely the fan makes no noise.

This is what my fan looks like.




It appears my fan has been updated by the PO to 11 blade...is this the reason these belts don't fit correctly???
I came across this thread 11 Blade Fan upgrade fan belt question
The strange thing is the old belt is 10x710 - no fan house noise when that one is put back in. But it appears over time that belt has been stretched to a length longer than the 725.
The person in the other old thread advised:
"So, if you have an engine that was modified from a 5 blade fan to an 11 blade fan without changing the crank pully, do the math and go with the larger belt to save your fan housing."

Should I try and locate a 10x735 belt like the person in the other thread advised?
- I have no idea if the crank pulley was changed or not.
Any advice would be greatly appreciated.

[OsoMoore]

My fan is 11-blade, but I'll have to wait until I get off work to go check on my part number. Does anyone else have some input?

[bassfishman]

I used a 10X735 on my 11 blade when I converted. 4 shims in 2 out.

[Breeang]

I have just been through a very similar series of (self inflicted) problems when I swapped out a five blade fan from a '77 with an eleven blade from a '74. Looking from the side. check the alignment of the crankshaft pulley at the bottom of the belt and the fan pulley at the top. If you have the wrong fan for the year of the fan shroud housing, you can end up with a 9mm misalignment between top and bottom which the belt has to twist its way around the pulleys and maybe hit the fan shroud housing.The fan and pulley,the fan housing and the alternator all have to be matched up correctly to attain this correct positioning above the crankshaft pulley.
The fan shrouds have different

[Breeang]

Power surge at work!
To continue: The mounting surface of the fan shrouds have different thicknesses where the alternator seats, the alternators are of different lengths, also the actual pulley diameters change (fan and outer pulley half as a matched pair)---all of this varied as a matched set from year to year to maintain the alignment of the two pulleys after Porsche upgraded the alternator and consequently its size. If, as I did, your PO all unsuspecting did a mix and match,you can have the belt rubbing on the shroud as well as twisting. It can be a pain to sort out. I don't have all the info. to hand at the moment. I shall search at home, but if you do some research on this site, keeping what I found out in mind you should get answers.
Another thought--have you checked that the alternator is sitting tightly in its base at the back of the fan shroud and not moving under tension from the belt?

[manbridge 74]

Stop and think about your issue for a minute or so.

If the alt was tightened all the way you might not have a problem. What do I mean? On my 74 only some of the nuts tighten down all the way to secure the alt to housing. The other nuts just keep the air deflector secured.

Investigate ala Sherlock Holmes here.

And this belt business. Your best results for cooling are not with a segmented belt (gaps on interior side). Look for a non-segmented type.

[pinzy76]

Thank you for the helpful advice.
The alignment looks ok to my untrained eye and the alternator doesn't appear to be moving under tension from the belt.
I still don't know how the PO managed to get a new 10x710 belt on there - I suspect that's how the pulley became cracked and the nut was on so tight. That belt hadnt been changed for a very long time.

I've ordered a 10x735 Conti belt which should arrive in a week or so, will update on progress.

[Jerome74911S]

My '75 engine has an 11 blade fan and uses a 10x725 belt. Three shims in, and three shims out.

Doing a crude measurement by holding a ruler across the pulleys shows that the upper pulley is around 3.75", while the crank pulley appears to be about 5".

Since mine works with a 10x725, I would imagine that your pulleys might be similar.

However, with all the variables possible your mileage may vary.

[pinzy76]

Late update to wrap up this thread.
The correct belt was indeed 10x735.
Good fit with 2 shims in 4 out.
So for those owners who have had their fan changed by PO and the regular 710 and 725 belts do not fit, I would suggest trying 10x735.

[fl763]

Not sure if anyone mentioned this but your alternator bearings could be going bad. I have done the fan blade conversion a couple times with no issue.
Thanks