1978 911 SC Alternator Belt Help!

brooklyn34 · brooklyn34's Garage

Recently replaced alternator belt, noticed only 3 shims. How many shims should I have and in what order should they go.

James Brown · James Brown's Garage

as long as it's tight your ok, now, you didn't say the magic words first.........

80-911SC · 80-911SC's Garage

sorry to conflict you james but the gentleman needs 6 total whether in between or on the outside the total needs to be six , with out the proper thickness the nut on the alternator will not seat right on the alt shaft and will allow premature bearing failure, ie catastrophic meaning money, and many issues easily resolved by ordering a few dollars worth of shims

Formerly Steve Wilkinson · 10-24-2009, 05:03 AM

I've always heard that too--that you need all six shims. But for the life of me I can't figure out why that's so. Whether you have three (for example) shims outside the pulley halves or simply tighten the nut down another millimeter or so would make absolutely no difference to the position of the outer pulley half.

Can somebody explain the thinking behind this? I'm really curious why "the nut on the alternator will not seat right." Are there not enough threads to allow it to seat properly without the full complement of shims in place? (I have all six shims, so it's just a matter of curiosity on my part.)

Scott Douglas · Scott Douglas's Garage

Stephan
Yes, I think that is the case [not enough threads...]. You can check this by removing all the shims and pulley, install the nut and measuring.

Formerly Steve Wilkinson · 10-24-2009, 06:31 AM

Okay, that makes sense.

brooklyn34 · brooklyn34's Garage

Thanks fellas

brooklyn34 · brooklyn34's Garage

One more thing how do you know if the bearing fails in alternator? Will car not start? Thanks again.

SCOTITUDE · 10-24-2009, 01:46 PM

a god awful " holy s**t I blew something up" noise comes out of the engine bay. Its the fan hitting the housing. It was the first thing that failed on me when I first got the car 14 years ago. i had no idea what it was but i turned it off right quick and called a porsche buddy because it does not sound good. Luckily I did no damage and had the alternator replaced.

James Brown · James Brown's Garage

Ok here me out, the alternator has a tapered shaft that the fan fit on to. It is held in place by the outer V belt and the nut. the shims are placed between the inner and outer V halves to compensate for variances in mfg. belts and old ones that stretch so that when the nut is tightened, it won't crush the V belt and the tension is correct (more or less shims installed to make it right). When you snug down the nut (no torque value for this nut) the fan taper fits tightly to the alt. taper (like a chuck in a drill press). it won't go too far in and if the nut is tight, it won't come off.
now, the extra washers are "kept" on the outer side so there there if and when you need them on the next belt change. the thickness of the shims will not "bottom out" on the alt. shaft even if no shims are present. as long as the correct amount of shims are on the inside-in my small mind-that should be ok. 3 shims on the outside will only decrease the amount of the nut rotation by maybe 30-50 degrees. There are plenty of thread left on the alt. shaft (on mine) so it did not bottom out. BTW mine has 2 shims on the inside and 2 on the outside.
So as far as should there be 6 shims present? yes, but will it blow up or damage things? my thoughts are no. That's based on how I understand how this thing works.
The more critical part in this system is the alt mounting to the fan housing that's where the precision is if not mounted properly.

did find the torque value-40Nm

theiceman · 10-24-2009, 06:04 PM

the shim thing is a myth .. at least it is on my car ... I could have no shims on the inside and it would tighten right up. it is threaded WAY in and this is not an issue . i do however have about 5 shims on my 78.

Failing bearings will begin to squeel like Ned beaty in Deliverance and is usually caused by the belt being too tight stressing the bearings.
Sounds like you are in good shape..

Formerly Steve Wilkinson · 10-24-2009, 06:13 PM

There most definitely IS a torque value for the alternator nut,at least on an SC. It's in my little technical specifications booklet--the one about the size of a large matchbook, about 105 pages long--and I look it up every time I change the belt or work on the alternator (which, it being a 911, is way too often). It's raining hard right now in the Hudson River Valley and the car is under its cover, so I'm not gonna go out and look it up, but trust me, there is a torque specified.

theiceman · 10-24-2009, 06:29 PM

30 foot lbs

80-911SC · 80-911SC's Garage

if you don't have the correct amount of shims on the shaft the nut will not "seat down at the correct depth on the shaft , didn't make sense to me at first but the guy who's name is on the back of most of these cars said to put all the shims in so I'm not going to argue with that . I had 4 shims on my car when i got it and the PO said the alt may be going bad , i got the missing shims and a new belt and ...........

no issue with the alt / bat light no noise no issues , and thats about a year now so thats all i am saying , i don't know it all but this i have first hand with so that is that ...

for those of you that want to run low on shims good luck , post and let us know when you fix it what you do, the shims are like 2 bucks or less each so 6 dollars now your call alternators are a lot more than that , not saying you'll never change an alt but to save 6 bucks ? I'd spend the six bucks.

Steve

theiceman · 10-24-2009, 06:48 PM

well I am not going to turn this into a huge debate . Manuals are for when you are not sure what to do, so most should stick to the manual and protect themselves. They must have changed the alternator over the years and as most companies do , lagged behind on the documenation or just left it to play safe. On my car even with no shims I still have about a half an inch of thread after it is tight .. I am just sayin ...
But you are correct .. stick to your manual and 6 shims .. than you can't go wrong ..

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by theiceman

the shim thing is a myth .. at least it is on my car ... I could have no shims on the inside and it would tighten right up. it is threaded WAY in and this is not an issue . i do however have about 5 shims on my 78.

Failing bearings will begin to squeel like Ned beaty in Deliverance and is usually caused by the belt being too tight stressing the bearings.
Sounds like you are in good shape..

Stephan Wilkinson,

Theiceman just given you the answer!!! The number of shims between the alternator face and the half pulley determines the alternator belt tension (provided the inside shims are tightly compressed). The extra shims have nothing to do with the belt tension!!!! You could toss them away, keep them, or used them, the belt tension is still determined by the 'spacers installed' or shims as we call them.

If you used 4 shims to get the desired belt tension, it does not matter whether you have 1 or 2 extra spacers left!!!! The extra shims are there for adjustment purposes in case needed. The trick is using the 'special' curved alternator washer to compress the half pulley against the 'installed shims'.

Tony

Formerly Steve Wilkinson · 10-25-2009, 05:30 AM

Well, I never for an instant thought the external shims had anything to do with belt tension, even an English major can figure that out. The question is, do the external shims have anything to do with NUT tension? Not sure anybody has given "the answer" yet. Lot of good guesses and theories, but the only "answer" seems to be in the manual: use all the shims, and if you don't, god will smite your muffler bearings, ream out your kanibbling pins and cut off lubrication to the avocado joints.

theiceman · 11:14 AM

actually the theory is that the thread only goes so far down the shaft and without the shims on the outside of the pulley half you will " bottom" the nut against the end of the threads before it tightens against the back of the pulley. If you torqued it up to 30 lbs it would be the nut tightening against the end of the threads not the back of the pulley half ... but as I said i will never bottom out the threads on my alternator unless I am missing the entire pulley half

Don't get me wrong I put the spacers there in case I am stuck and have to buy a slightly off size belt.

I am just the type of guy who questions and needs to understand .. and I understand this one completely . Other alternators may be different so as stated stick to the manual .... did i waste enough bandwidth on this ?

BTW i have the original Marchal- Rhone alternator in my 78 with the external regulator.

K Sykes · K Sykes's Garage

I really shouldn't have read this thread. My alternator bearing blew out today. Awful sound. I only had 5 shims.....

More interestingly, the belt was pretty loose, I was thinking of tightening it up a bit. I could turn the fan by hand.

Kinsley

theiceman · 10-25-2009, 06:16 PM

Well Kinsley thats karma for ya..
at least you will be well informed when you got to hook up your new alterantor ..actually having said that .. take yours out , go to a bearing shop and have them sell you the right bearings, take them to an alternator shop and have them press them in for you . I did this and it cost me 40.00 total including bearings .. and that was 5 years ago ..