Ribbed V-belt selection turns on two cross-sectional dimensions, rib pitch and rib height, that the section letter encodes. The five metric ISO 9982 profiles span 1.6 mm to 9.40 mm rib pitch, with PH handling specialty light drives and PM rated for high-power industrial machinery [S2].
On a replacement job, the three numbers stamped or measured on the old belt are section, rib count and effective length; pitch is fixed by the section letter and the matching pulley groove, not by user choice [S3]. A ribbed belt of the wrong section will ride up out of the pulley groove under load, so the section letter is a non-negotiable first filter.
Rib pitch and rib height, what the section letter actually fixes
Rib pitch is the centre-to-centre distance between adjacent longitudinal ribs on the belt's driving face, and it is the dimension that has to match the corresponding pulley groove spacing [S3]. The metric ISO 9982 family, PH, PJ, PK, PL and PM, runs 1.6 mm, 2.34 mm, 3.56 mm, 4.70 mm and 9.40 mm pitch respectively, with PK 3.56 mm being the de facto automotive profile and the one most often shipped as a V-ribbed serpentine belt [S2]. For PK specifically, TRANSCO's published geometry quotes a 3.56 mm nominal rib pitch combined with a 40° included rib angle, and that angle is what controls the contact footprint on the pulley flanks [S4].
Rib height is the second governing dimension and it sets the minimum pulley diameter the belt can run on without the ribs cracking or the cords fatiguing. Industry data puts the PH profile at 2.5 mm rib height on a 13 mm minimum pulley diameter, with rib height scaling up roughly in step with pitch across PJ, PK, PL and PM [S5]. A taller rib gives more contact area and higher torque capacity, but it also raises the bending stiffness, which is why fine-pitch PH and PJ sections are the ones that survive on small motor shafts and appliance drums where the pulley diameter is fixed by the equipment designer [S5].
How pitch and height change with each profile
The English/SAE letter system (H, J, K, L, M) is a parallel nomenclature to the metric PH/PM set and the pitches convert cleanly: H/PH 1.6 mm (1/16 in), J/PJ 2.34 mm (3/32 in), K/PK 3.56 mm (9/64 in), L/PL 4.70 mm (3/16 in), M/PM 9.40 mm (3/8 in) [S2]. Published cross-sections for the larger industrial sections list L at roughly 0.38 inch (9.65 mm) thickness on a 0.185 inch (4.70 mm) rib pitch and M at 0.53 inch (13.5 mm) thickness on a 0.467 inch (11.86 mm) pitch, with H and J proportionally thinner at 0.12 inch and 0.14 inch thickness respectively [S1].
PH is the specialty light-duty section with the shallowest rib and the smallest minimum pulley, around 13 mm, so it shows up in small appliance drives and instrument blowers where space is tight [S5]. PJ is the fractional-horsepower workhorse for consumer goods such as washing machines, dryers and power tools, and its 2.34 mm pitch is a deliberate compromise between PH's small bending radius and PK's torque capacity [S2]. PK is the automotive serpentine profile, and the rib number is the second variable the specifier has to lock in: a 6PK belt has six load-sharing ribs, a 4PK has four, but both share the same 3.56 mm pitch and the same pulley family [S4]. PL is the general industrial section at 4.70 mm pitch, and PM at 9.40 mm pitch is reserved for the heaviest industrial machinery, large conveyors and agricultural equipment where a single rib carries meaningful tangential load [S2].
Identifying a broken or unmarked belt on the shop floor
When the print on the belt is gone, the pitch is the only dimension worth measuring, and the procedure is to lay a steel rule across the ribs and read the centre-to-centre distance over several ribs then divide [S3]. A 3.54 mm read identifies a PK, 4.70 mm a PL, and so on, with manufacturers' tolerances tight enough that the measurement disambiguates the section cleanly [S3]. Belt width is unreliable as a primary identifier because it scales with rib count rather than section: a 1200 PK-8 CONTI-V MULTIRIB is 1200 mm effective length, PK section, eight ribs, and the total width is simply pitch x rib count, here 3.56 x 8 = 28.5 mm [S3].
Rib count, not pitch, is what the designer scales to change drive capacity. A 6PK compared to a 4PK on the same pulley family delivers roughly 50 percent more contact area while keeping the same 3.56 mm pitch and 40° rib angle, which is why going from four-rib to six-rib is the standard way to uprate a serpentine drive without changing the pulleys [S4]. Going the other way and adding ribs to a deeper-pitch PL or PM pulley is rarely done because the larger rib height starts to fight the bending radius on small-diameter shafts, so the section letter almost always changes with the application class, not just the load [S2].
Where the PK section sits in 2026 drive design
The PK profile at 3.56 mm rib pitch is the dominant automotive accessory drive and that position is unchanged: one PK belt can run the alternator, water pump and air-conditioning compressor in series through a serpentine path with backside idlers, provided the belt construction is rated for reverse bending [S4]. Industrial users tend to step up to PL (4.70 mm) the moment the drive is single-pulley or the load is continuous, because the deeper rib distributes contact pressure over a longer flank and tolerates slip transients better than PK [S2].
Two practical limits bound the choice. First, minimum pulley diameter: PH at 13 mm, scaling upward through PJ, PK and PL, with PM reserved for shaft-mounted industrial drives where the pulley is large enough to absorb the bending strain [S5]. Second, effective length tolerance: the standard designation 1200 PK-8 means 1200 mm effective length, PK section, 8 ribs, and the pulley centre distance has to be set so the belt runs in its designed tension window, since a belt stretched beyond its rated elongation will ride lower in the pulley groove and lose contact area on the rib flanks [S3]. For engineers cross-referencing to construction machinery and equipment drives, the rule of thumb is to pick the deepest section the smallest pulley in the train will accept, then pick the rib count to hit the torque requirement.
Comparison: PH vs PJ vs PK vs PL vs PM on four decision criteria
Choosing between the five metric sections comes down to four numbers: rib pitch, rib height, minimum pulley diameter, and the typical application class. PH/PJ live in the appliance and small-motor world with 1.6 mm and 2.34 mm pitch and sub-15 mm minimum pulleys, PK covers the 3.56 mm automotive serpentine segment, PL is the 4.70 mm general industrial section, and PM at 9.40 mm is reserved for high-torque industrial and agricultural machinery [S2][S5]. On drive efficiency all five sections cluster around 97-98 percent because the frictional engagement mechanism is the same, so the section decision is dominated by bending radius and torque density, not by efficiency [S2].
Cost and lead-time track the section: PH and PJ belts are commodity items stocked by every industrial distributor, PK is the most heavily stocked profile worldwide because of the automotive aftermarket, and PL and PM are typically made-to-order on industrial lead times of two to six weeks depending on rib count and length [S4]. For a maintenance engineer, the practical decision tree is: measure the old belt's pitch, confirm the minimum pulley diameter the drive can accept, then pick the deepest section that fits the smallest pulley, and only then pick the rib count to hit the torque requirement [S3][S5].
The next signal to watch on ribbed belt specification is any revision movement on ISO 9982 and ISO 9981, which currently anchor the PH/PM and PK automotive profile families; the 2021 and 2020 issues are the active references, and downstream OEM datasheets are still aligned to those issues [S2]. For related drive-component spec work, the HTD 3M, 5M, 8M pulley PLD values reference and the Q1 vs 3020 taper lock bush bore comparison sit on adjacent sections of the same spec library.
For component-level specifications, see height gauge.