Alternator belt length

[Moseley]

Some time ago, I purchased a couple of belts from one of the usual suspects to suit the alternator fitted to our van. When I went to fit one, I was surprised to find that it was so long that I could actually loop it over both pulleys whilst they were all bolted up. At the time, I didn’t do too much investigation and just purchased the same as what was fitted previously, and that has worked fine.

Anyway, fast forward to now and I thought I’d have another look. The original that fits is a Contitech 10x900 (albeit I pretty much have to move to just one or no shims as it breaks in, suggesting it is a little shorter than ideal). The new ones are branded JP Group 11.3x912. When I measured the outer length of both, I’ve found the Contitech is slightly undersized at 891mm and the JP is oversized at 922. This might explain why the fitment is so different.

So, does anyone have experience with similar, or could recommend a brand where the length is a bit closer to what it should be? I’d prefer to run an 11.3mm belt but don’t want to start a process of trial and error purchasing of different ones.

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[sparkywig]

I wouldn't use a toothed belt, they wear so much quicker, a V belt was originally fitted.
After much mucking about with poor quality "standard" alternator belts I now use wedge drive belts. A SPA 900 fits my application perfectly, and at 12.7mm wide it sits right in the centre of the Berg crank pulley.

I think you'll find that both your belts are 912mm long, but one is measured using the outer circumference and the other using the inner length.
Drive belts are normally measured using the inner length.

 

[Moseley]

It’s certainly a bit odd, you’d have thought there’d be some consistency. I’d at least expect the ‘branded’ Contitech one to be accurate - if it is normal to measure the inner length, then it is even further from the 900mm model number. I’ll try measuring the inners of both belts, and also the diameters just to see what’s up, but I guess I’m probably just going to have to buy another and see how that fits. Interestingly, it seems that the main VW retailers are now selling Optibelt V belts which aren’t grooved, so maybe that is the answer.

 

[Moseley]

Another thought, I wonder if the bottom pulley is for a dynamo setup? It is an aftermarket aluminium one, but what is the actual difference between a dynamo and alternator bottom pulley?

 

[sparkywig]

These engines were designed originally with dynamos, but there are no specific dynamo or alternator bottom pulleys.
Because I'd had some many issues with currently available alternator belts, I measured the inner length between the pulleys using some wire, and then searched for a drive belt to suit.

 

[Moseley]

Makes you wonder why all the retailers seem to be quite specific about alternator conversions, stating that if converting from a dynamo setup to an alternator, then an early 9.5x905 should be used, but if it was of an age where an alternator was fitted as standard, then use an 11.3x912. Can’t understand that one unless I’m missing something obvious?

 

[mike202]

I have had the Optibelt 11.3 x 912 V belt on my alternator converted engine its always been ok. (since 2014)

I still have the same type of belt, but the Crankshaft pulley and alternator pulley wheels are both new, and it fits still, but now there's only one shim between the alternator pulley halves. I suppose this is ok as the belt will only stretch with age not get tighter.

edit: Just realised what I have said there, of course if the belt stretches I will need to remove shims, and as there is only one I won't have much scope !
However if it ever stretches that much I will just put a new one on.

 

[sparkywig]

The issue I had was that to fit a new belt correctly it only had one shim fitted, after 100 or so miles when the belt wore in it was too loose.

 

[mike202]

My belt is not a new one, and has done 3000 miles on the new engine and pulley wheels so far without stretching any more. So I'll see how it goes. Interesting about seeing whats available though, as maybe a shorter belt may be the answer.

 

[rlepecha]

I went throught the process of trial and error using off the shelf SPA belts a few years ago, when I noticed the same problem you guys are.
The current optibelts and similar a available from the usual suppliers appear to be too long, not sure if it's a quality control issue or what.

Tested a few belt sizes, and ended up with an SPA907, they are available in best quality on eBay for around £5 if I remember correctly. With the correct D marked lower pulley (dual port) and alternator up top I have approx. 1/2 of the shims inside and 1/2 of them outside.


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[mike202]

I went throught the process of trial and error using off the shelf SPA belts a few years ago, when I noticed the same problem you guys are.
The current optibelts and similar a available from the usual suppliers appear to be too long, not sure if it's a quality control issue or what.

Tested a few belt sizes, and ended up with an SPA907, they are available in best quality on eBay for around £5 if I remember correctly. With the correct D marked lower pulley (dual port) and alternator up top I have approx. 1/2 of the shims inside and 1/2 of them outside.


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Is it like this one? 13mm is quite thick, compared to the 11.3x912mm I have on at the moment.

https://www.ebay.co.uk/itm/SPA-SECTION-V-BELT-SIZES-SPA632-SPA1032-V-BELT-13MM-X-10MM-VEE-BELT/114082134246?hash=item1a8fd338e6:m:modeYfn1wlhuHDO0cgE06Iw&var=414355617582

 

[rlepecha]

I went throught the process of trial and error using off the shelf SPA belts a few years ago, when I noticed the same problem you guys are.
The current optibelts and similar a available from the usual suppliers appear to be too long, not sure if it's a quality control issue or what.

Tested a few belt sizes, and ended up with an SPA907, they are available in best quality on eBay for around £5 if I remember correctly. With the correct D marked lower pulley (dual port) and alternator up top I have approx. 1/2 of the shims inside and 1/2 of them outside.


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Is it like this one? 13mm is quite thick, compared to the 11.3x912mm I have on at the moment.

https://www.ebay.co.uk/itm/SPA-SECTION-V-BELT-SIZES-SPA632-SPA1032-V-BELT-13MM-X-10MM-VEE-BELT/114082134246?hash=item1a8fd338e6:m:modeYfn1wlhuHDO0cgE06Iw&var=414355617582

That's the one yeah. It is thicker than the standard, but fits in a the standard upper and lower pulley well.
It's 12.7mm wide by the way, that advert is wrong. The specs for the belts are an industry standard.

I guess depending on how accurate the alternator stands are machined and tolerances on the pulleys there is a chance you would need a slightly different size. But, my experience suggests these the SPA 907 are a good fit. I have used them on two different engine builds and belts from a few different manufacturers with no problems.

I would be tempted to pay a little more for a distributor on eBay that specifies a manufacturer however, so you don't get some cheap quality part.

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[sparkywig]

I did the same, but after trying different sizes mines's a SPA 900.
Definitely go for branded belts.

 

[mike202]

That's great, so it looks as though either the Optibelt 900mm or 907mm will be a good bet but I'll have to try and find out which one is best for my engine. Both are 12.7mm wide.

Sorry to OP for thread hijack btw!

This place sells them I've ordered the 900mm first. Could be a bit a short but we'll see. I might need all my shims.
https://uk.rs-online.com/web/p/drive-belts-v-belts/1367239/

A question though, how many shims is the most you would want to have in the alternator pulley wheel halves?

 

[sparkywig]

You can use any combination of the 10 shims between the pulley halves and under the pulley nut/washer, as long as all 10 shims are fitted.

 

[Moseley]

No problems for the hi-jacking, my question has been answered and all the other information being posted is really useful. Good to see I’m not the only one who has been scratching their head.

Out of interest, I have purchased a ‘standard’ 11.3x912 Optibelt branded belt to see how that fits compared to the original JP Group ones that I was unable to fit. I will have a measure for comparison and give it a trial fit once my alternator is mated to the engine once again.

It’d be useful if you could post up some advertised length vs actual length numbers and what the fit was like (i.e. how many shims were required to tension) so that we can get an idea of which would be a good one to purchase if needing something a bit shorter / longer. Might have to measure the outside length, which I appreciate isn’t correct, but at least would serve as a reference point as it is easier to measure.


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[Moseley]

Just done some measuring, and test fitting will follow in due course...

Contitech 10x900, actual dimensions 10.2x891
JP Group 11.3x912, actual dimensions 11.3x922
Optibelt 11.3x912, actual dimensions 10.9x901

Appreciate that I’ve measured the external length of the belts, but that is certainly some variation there. From my experience, the Contitech fits, but isn’t the ideal width and is too short. The JP Group is far too long. I will find out soon if the Optibelt is ok and report back.


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[atafonso]

These engines were designed originally with dynamos, but there are no specific dynamo or alternator bottom pulleys.
Because I'd had some many issues with currently available alternator belts, I measured the inner length between the pulleys using some wire, and then searched for a drive belt to suit.

Sparky, i also thought for a long time that the pulleys were the same, but there’s actually two sizes of stock crank pulley.
I have had both with me and could compare the size difference. With the bigger one, the 912 belt fits perfectly and does not require the removal of all the shims to make the belt tight.

Abel

 

[cunning plan]

I tried a couple of belts available from the usual suppliers for my Bosch alternator, but neither seemed to fit correctly.

I ended up guessing a few mm less to try and get a better tension and ended up with a 10 x 888mm V Belt from EuroCarParts.

 

[sparkywig]

These engines were designed originally with dynamos, but there are no specific dynamo or alternator bottom pulleys.
Because I'd had some many issues with currently available alternator belts, I measured the inner length between the pulleys using some wire, and then searched for a drive belt to suit.

Sparky, i also thought for a long time that the pulleys were the same, but there’s actually two sizes of stock crank pulley.
I have had both with me and could compare the size difference. With the bigger one, the 912 belt fits perfectly and does not require the removal of all the shims to make the belt tight.

Abel

OK, every day's a school day. I measured the ones I have lying around and they are all the same.
I'm running a Berg Equalizer, so out of interest what are the sizes?