JS Alternators — Built in the USA
Quick Belt Finder
Find the correct Dayco serpentine belt for your JS high-output alternator. GM truck & SUV fitments below — all other makes use the Dayco guide.
GM Belt Lookup
Popular GM Truck & SUV · Dayco Belts
GM vehicles only. This tool covers popular GM truck & SUV fitments. For Ford, Jeep, Ram, and all other makes — use the Dayco guide on the right.
Recommended Belt — Dayco
Belt Look-up Instructions
JS high-output alternators with the smaller 1.75" (45mm) pulley typically need a belt ½" shorter than factory. The factory tensioner marks are your best confirmation.
1
Open Dayco's Belt Catalog
Access Dayco's Belt Catalog for a complete guide to belts, fitments, and specifications for all vehicles.
2
Enter Your Vehicle Info
Enter your Make / Model / Year / Engine or use your license plate number.
3
Select the Alternator Belt
Many vehicles list 2 belts — select the one driving the alternator, not the A/C belt.
4
Note the Effective Length
Find the field labeled:
EFFECTIVE LENGTH (in):
EFFECTIVE LENGTH (in):
5
Subtract ½ Inch
Subtract 0.5" from that number — this is your new target belt length.
6
Find a Matching Part Number
Use Dayco’s belt cross-reference tools or lookup guides to match your belt’s part number to compatible replacements or equivalents.
7
Confirm with Tensioner Marks
After install, the tensioner should sit in the same position as with your OEM belt — not too loose, not too tight.
Popular GM Truck & SUV Belt Reference Chart
Complete GM Belt Reference
Verified Dayco part numbers · All known fitments
| Engine | Year Range | Brand | 250A / 320A / 370A / 400A Smaller Pulley |
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