Tech Tips For Measuring Lengths For V- and Serpentine Accessory Drive Belts

10 min read

Tech Tips For Measuring Lengths For V- and Serpentine Accessory Drive Belts

10 min read

The primary car guy imperative states that it’s a struggle to leave stock stuff alone. In our case, we had near-stock SS396 big-block Chevelle whose steel accessory drive pulleys did not fit the engine compartment vibe. So we opted for a set of Trans-Dapt polished aluminum pulleys. This demanded a custom belt length for both the alternator and power steering v-belts. The range of adjustment for the alternator was limited by valve cover height so we had to come up with a specific test for length. Along the way, we learned a few technical tidbits about both v and serpentine belts.

We contacted our buddy Jay Buckley, the Director of Product Management at Dayco, who manufacture belts, hoses, water pumps, and other high-quality replacement parts. We learned for example that v-belts are made from neoprene rubber that tends to stretch slightly immediately after first use. Serpentine belts however, are constructed of an EPDM rubber that does not stretch. The point is that v-belts will need a readjustment after the initial installation to ensure they remain tight while with serpentine belts you just install and move on.

Belt Length V-belt string system

We installed a new v-belt accessory drive on this big-block Chevy and needed the proper belt length for the alternator and power steering. We used a length of yellow 12-gauge wire to measure the inside circumference after placing the alternator in the middle of its adjustment travel. We marked the wire with a black Sharpie and laid the wire alongside a measuring tape indicating exactly 56 inches. Remember this is the inside circumference of all three pulley grooves. We added one inch to the length to represent the outside circumference.

We decided to employ the classic string method to measure belt length on our big-block Chevelle project. We used a length of 12-gauge wire to measure the inside circumference of our three-pulley alternator belt system. But that requires a second step beyond just the length. The measurement is the inside circumference of the belt and not the actual designated v-belt length which is the outside circumference.

In our particular case, we placed our alternator in the middle of its adjustment and measured the wire to come up with 56 inches. Because most automotive v-belts are roughly 3/16-inch in depth, this must be compensated for in order to come up with an outside circumference. The rule of thumb is to add 1½ to 2 inches to the inside circumference measurement. Our setup demanded a belt between 57½ and 58 inches in length. The total range of adjustment offered just less than 3 inches of possible lengths but ours was limited by hitting an adjustment bolt head if the belt was too long and the alternator hitting the valve cover if the belt was too short. Our power steering pulley measurement came out to 41½ inches. We bopped over to the auto parts store and picked up new belts for both and our measurement technique produced a close fit. We’ll admit we estimated short at first and had to go back twice!

Gates V-belt code

This tight shot of a Gates belt reveals a part number 7545. The last three digits represent the belt length in inches – in this case 54.5 inches. If there is an X in the part number, this refers to a cogged style belt. The 1385 number on the far right is the length in millimeters which equals 54.5 inches.

Among the things we learned while working on this is to pay attention to what the parts counter guy lays on the table. We asked for a v-belt that was 57½ inches and he handed us a 9-series Gates belt that is substantially wider and therefore didn’t fit the pulleys by riding much too high in the grooves. In the Gates part number vernacular, we needed a 7 series belt instead of a 9 series. Once that was amended, the belts fit properly.

After installing the belts, this brought up the question of how tight to make the final install. There are specific recommendations for belt tension based on the type of belt. In our case, the spec for a new belt is 150 lbs. and 120 lbs. after a three-minute break-in period. One affordable technique is to use a Dayco tool called the Krikit that uses finger force to measure the belt tension. Pres down on the tool in a open span until the tools emits an audible click and then read the force on the scale. If you’re into fun little tools, this thing is available through eBay and Amazon for under $30.

V-belt installation

After measuring both belt lengths, we purchased new belts from the local auto parts store and quickly installed them. We added 1 ½ inches to our belt length on the alternator belt to ensure the alternator would not come close to our nicely painted valve cover. For the power steering belt, we added one inch to the measurement and it worked perfectly.

If you just want to get close, a Dayco engineer passed along this anecdotal approach. Press your index finger down on the belt and if your fingernail turns white when the belt stops deflecting, the tension is right. While hardly a calibrated tension, it sounded fun which is why we passed it along.

Our friends at Powermaster sell high output alternators that often use v-belt drives and these higher capacity alternators demand more power to turn. Powermaster’s recommendation for belt tension is that the belt is not tight enough unless you can begin to turn the engine with a socket and breaker bar turning clockwise on the alternator fan nut. That seems a bit excessive, but then a 140 amp alternator at full output will place a much greater load on the belt than the old 60-amp alternators from back in the day.

To put this in perspective, let’s convert 120 amps at 14 volts of output to 1680 watts. The conversion from 1680 watts to horsepower is equal to 2.2 hp. So that little v-belt has to drive over two horsepower of continuous load which does not include the load from the water pump which could easily be another 2-4 hp at high speed. That original 60 amps equal 840 watts or almost exactly half the load at 1.1 horsepower.

Because of these increasing loads place on v-belts, the OE’s upgraded in the 1980s to serpentine systems. Serpentine belts offer a much greater surface area to allow higher accessory drive loads. This offered the luxury of running all the components such as the water pump, power steering pump, alternator, and sometimes the A/C compressor all on the same belt. Adding belt width from 4, 6, 8, and even 10 ribs merely increases the belt’s load capacity which is evidenced by serpentine belts driving centrifugal supercharges that demand 50 horsepower and more.

belts Powermaster test

Powermaster’s Technical Bulletin 100 states that if the v-belt tension is set correctly, turning a socket and breaker bar clockwise on the alternator pulley nut should not slip the belt but instead attempt to turn the engine. Dayco and others recommend using an actual tension gauge.

Holley offers a very nicely engineered serpentine accessory drive system for the LS and LT engines that rivals the factory stuff and at a reasonable price. We’ve also employed variations on the factory theme by adapting a late ‘80s Chevy 4.3L V6 accessory drive that is literally a bolt-on to a small-block Chevy but does require a reverse rotation water pump.

If you need a belt length for a mystery serpentine system, the measuring technique is somewhat similar with one notable difference. We pulled this technique straight from Holley’s serpentine system instruction manual.

Because serpentine systems use a spring-loaded tensioner, the best way to determine the belt length is to measure the system with the tensioner in the relaxed position. Make sure you weave the string or wire in the correct belt orientation. With the measurement made, subtract 5/8-inch (16mm) from the length to compensate for the position of the loaded tensioner and that’s your belt length. To convert the inch measurement to millimeters, just multiply the length in inches by 25.4. As an example, if the belt measures 66 inches: 66 x 25.4 = 1676mm.

Deciphering the numbers on a serpentine belt is not difficult. As with v-belts, width and length are the two parameters. Most OE replacement serpentine belts are 4 to 6 rib belts and the length is referred to in inches and/or millimeters. A Dayco serpentine belt for a 2002 LS1 Camaro is listed as a PN 5060790 is a six-rib belt that is 79 inches in effective length represented by the last three digits in the part number. As with v-belts, if its necessary to convert from inches to millimeters, just multiply the inch length by 25.4. At the auto parts store, ask for a 4PK000 belt with the 000 equal to the length in millimeters.

Belts serpentine tensioner pulley gauge

The spring-loaded tensioner on most factory serpentine systems employs a scale to indicate the proper belt length. If the indicator is on either side of the range, the belt length is incorrect and should be altered.

This explanation turned out to be quite involved once we began our investigation, but as with all things, they can become detailed when exposing the nuances. If you learned a few things about accessory driver belts and systems, then we’ve done our job!

Parts List

ComponentPart Number
Mr. Gasket Pulley Holder8012MRG
Mr. Gasket Water Pump Pulley Shim Kit 6129
Quick Time Harmonic Balance Spacer KitRM-715


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