Dual V belt pulley machining, matched belt desires.

Oct 16, 2010
Last December, I replaced an inoperable A/C compressor, with a second alternator, on my LA-318.

In the process of aligning the alternator's pulleys, I noticed that the newly mounted second alternator ( a Nippondenso) has 2mm more space between the V Belt grooves.

I was not intending to immediately employ the alternator to make juice, so My straight edge clamped to pulleys was only measuring to the inside V belt, and I got alignment to under 1mm of parallel misalignment and zero measurable angular alignment.

IN doing this, I also found the original alternator was 6mm out of alignment with crank pulley, and had to shave 6mm off its mounting ears.

Anyway that 6mm imperfection, still allowed one single V belt to attain its 120 amp rating, in stationary testing, though cold belts and pulleys and quick rpm changes or damp conditions , caused belt squealing starting at 75 to ~ 90+ amps output requiring me to back off field current, until squealing stops.

I'm still using the same worn stretched V belt, but soon, two new V belts need to be employed.

Only recently have I attached an adjustable external voltage regulator to the mounted aligned ND alternator, and 2 AWG to the battery from it, and it feeds its own AGM battery bank.

So I want to run two V belts, as I have enough battery capacity, that is capable when well depleted, of maxing out both alternators at idle, and 2K+ engine rpm.

This 2mm extra gap on ND alternator pulley bothers me.

I stopped running dual belts long ago, as the AC compressor was INOP, and I rarely needed more than 90 amps, and one 3/8" v belt was adequate most of the time, and when I did use 2 belts, the 6mm parallel misalignment combined with the angular misalignment of AC compressor pulley, always had one belt too tight or too loose and would create weird annoying harmonics I could hear and feel from drivers seat.

Ideally, I want the alternator pulleys perfectly aligned with the crank pulley, and a pair of matched V belts and all 3 pulleys to have the same gap between belts. I've got the inner belt groove aligned quite well,


This 2mm extra gap between belts on the ND alternator, kind of defeats the purpose of attempting to achieve ideal from this point forward.

Yet this same ND alternator, with this same extra gap, that neither matches AC compressor or Crank pulley, was sold by Chrysler for many many model years. I've queried many of their owners and those with ND alternators all have this extra 2mm gap between the dual V belts, and does not match the AC or crank pulleys.

My ignorance of the 6mm of parallel misalignment that existed, for so long as I never put a straight edge on it, points toward perfect V belt alignment, not mattering, or being of much consequence.

But the OCD in me wants to achieve as close to ideal as I can, as I will have the ability/ desire and occasional need, to max out both alternators, and I'd prefer to not have to dial back the voltage regulators field current, when the squealing occurs.

So that leads me to a machine shop, and a custom pulley, AND also trying to find a matched set of V belts, the latter of which is much easier said than done these days.

Screenshot_20210206-212037.jpg Screenshot_20210206-212101.jpg

The above pics were taken from alternatorparts.com

I have an extra of the pulley on the right, but would need a 1mm spacer, even if there was enough remaining metal to machine it to accommodate the bearing snout of teh ND alternator on the left.

To those with lathe/ machine shop experience, how much skill does the operator need to have in order to machine a pulley, like on the left, but with the belt gap of the pulley on the right?

Any idea of the cost of a custom pulley?
I'd not want to pay a lot, if the precision and accuracy is not achieved.

Interestingly, the ND pulley, while 1mm smaller at 74mm diameter, turns 100 rpm slower than the chrysler alternator 75mm pulley on the right, according to my optical tachometer, but I've not actually since busted out the digital calipers to measure actual pulley diameter, or checked belt depth within the pulleys, only tension, but that 100 rpm is a huge difference ( well, 12-15 amps or so) in output at hot idle, in drive, foot on the brake.

I could, and just might, get the closest thing to matched V belts I can find, and run the existing pulleys, but I need to figure out if I can get a new pulley precisely machined for an acceptable cost, first.
Serpentine belts and pulleys are certainly not in my budget.

This is where I hope for input by some Bitoger, with actual experience machining pulleys, can help me make a better informed decision.

Getting a matched pair of belts itself, is likely going to require a bunch of trial and error with compliant and patient parts store employees. I got severely frustrated during my last attempt doing this, for a Boat, last year.
Trying to find belts from the same lot number, and manufacturing date, near to each other on the drum, was impossible, even when the more experienced counterperson took over from the ignorant teen, and was able to understand what I sought.
I walked away with as close as possible, and when I put them on their slightly different lengths on perfectly aligned pulleys was readily apparent, and I planned to neuter the capability of the alternator, by the regulators 'belt saver feature' to prevent belt squeal, when so much other crap went wrong on the boat, i was not able to actually program the regulator.

So getting even semi matched V belts and then breaking them in on unmatched pulleys, and then finding them inadequate when demanding max output of both or either alternators, is a process I'd very much like to avoid, along with the weird harmonics experienced in the past with poorly matched belts on poorly aligned pulleys.
Aug 14, 2013
As a machinist I can tell you that this is easily doable, but it's not cheap. The shop I run is all cnc lathes and mills. That means programming, tooling, fixturing and set up. One pulley would run around $700 or so. 10 pulleys a 100 each 100 pulleys about 30 each. One off parts are very expensive from a modern machine shop. If you can find someone with an old manual lathe you would probably get it done much cheaper. Hope this helps.
Jul 13, 2020
San Diego County
Consider using a link belt. Used extensively in metal and woodworking with spectacular results, including me. The reduction in noise, vibration, and smoothness is uncanny. Not sure about auto or dual pulleys, but some say they have done it.
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Thread starter
Oct 16, 2010
Thanks for input. My Uncle has a lathe, but was not feeling well, and that round tuit escaped us.

I've not needed to generate anywhere near the 240 amp potential of these two alternators, and have done nothing further regarding the belt(s) or mimatched pulley.

I did drive cross country twice with old belt. I carry a backup used alternator belt, and I know I could drive a few hundred miles on my existing battery capacity, if I had to.
Jun 5, 2003
Apple Valley, California
Reserecting an old thread. I'd go look through picapart for a car or truck that had a dual pulling alternator.any years ago I used a pulley from a Fairmont 6 cyl on the gm alt on my scout. It bolted right on.


Nov 6, 2017
A little more necro action and tangential experience with the topic with manual machining:

A buddy once asked me to take a dual V-belt pulley from a York and turn it into a serpentine pulley. You obviously need the dual V-belt setup to begin to have enough width to accommodate your average, say, six groove serp.

Anyway, it went surprisingly well. I found that a 60* threading tool seemed perfect for cutting the grooves and the interior tapered side "walls" at either edge. I took an existing serp pulley from the same vehicle and simply gauged by eye the spacing from peak to peak and measured the depth of grooves.

This was 2011 and he's still running that pulley on his XJ.....

edit: for the side "walls" you wind up with mucho tool contact so you want LOW speed-- I ran 30 RPM. CNC would shine here because you could use a basic turning or profiling tool to cut the tapered sides and have minimal tool contact