The Castair Replacement Industrial V-Belt, Section B, 87-inch length (SKU: 3-B87) is the correct drive belt for Cast Air compressors spec'd for a B87. It is two inches longer than the B85 — a difference that matters when the motor mounting position or sheave diameter requires that specific circumference to tension correctly. If your machine's data plate, service manual, or OEM parts list specifies B87, this is the belt. If you are not certain of the correct length, measure the old belt or call us before ordering.
Belt Specifications
- Section: B (classical B-section)
- Top Width: 17/32 inch (0.53 in)
- Belt Depth: 21/32 inch (0.66 in)
- Outside Circumference: 87 inches (B87 designation)
- Construction: Classical wrapped — fabric-wrapped outer cover over a compression rubber body with tension cords running the belt's full length
- Compound: Oil-resistant and heat-resistant rubber compound; suited for the operating temperatures and oil mist environment of a reciprocating air compressor drive
- Standard: Sized per ARPM/RMA classical belt standards; cross-references to any standard B87 designation from any manufacturer
Compatibility and Fit
- Sheave groove match: Fits B-section sheave grooves. The B-section profile is not interchangeable with A-section (narrower) or C-section (wider) sheaves. Confirm your sheave is B-section before ordering.
- Castair/Cast Air models: Fits Cast Air compressor models factory-spec'd for the B87 length. The two-inch difference between a B85 and B87 is not trivial — using the wrong length either prevents correct tensioning or results in over-tensioning the bearings. Verify the belt specification from the machine's data plate or service documentation.
- Confirming belt length: If the original belt is still on the machine, look for the belt designation molded or printed on the outer face. It will typically read B87 or 5L900 (fractional equivalent). If the belt is gone, calculate from center distance and sheave diameters, or call 866-217-0063 and we will help you work it out.
- Matched sets: If the drive runs two belts in parallel, replace both belts at the same time with matched belts. A new belt running alongside a stretched belt will carry disproportionate load and will wear faster than it should.
Installation Notes
- Create slack before seating the belt: Loosen the motor mounting bolts and move the motor toward the compressor pump until there is enough slack to place the belt into the sheave grooves by hand. Never lever or pry the belt over a sheave. That deforms the cross-section and damages the internal tension cord — damage that is invisible until the belt fails prematurely.
- Tension to spec: The standard deflection method for B-section belts is 1/2 inch of deflection per foot of free span under approximately 5 lbs of thumb pressure at the belt's mid-span. Adjust motor position and retighten mounting hardware. Do not rely on feel alone — use a tension gauge if available.
- Check alignment: A straightedge laid across both sheave faces should contact both faces flat, with the belt tracking in line. Side loading from misalignment will cause the belt to run hot, wear the edges, and fail in a fraction of its rated service life.
- Sheave inspection: Before installing the new belt, run your fingers around the sheave grooves. A groove bottom that is shiny or has a channel worn into it by the old belt means the sheave walls are no longer contacting the belt flanks properly. A new belt on a worn sheave will bottom out in the groove, lose grip, and fail early. Replace or re-groove the sheave if it is worn.
- Break-in re-tension: New belts compress and seat during the first few hours of operation. After the first 1-2 hours of run time, shut down and re-check tension. This step is frequently skipped and frequently the cause of "the new belt slips just like the old one."
- Belt dressing sprays: Do not use them. They temporarily mask slippage but do not fix its cause, and they degrade the rubber over time. If a belt is slipping after correct tension and alignment, the belt or sheaves need replacement.
When to Replace Your Compressor Drive Belt
- Visible cracking: Surface cracks on the belt back or inner groove face indicate the rubber has hardened and lost elasticity. These cracks will propagate under load. Replace the belt before it fails in service.
- Glazed surface: A hard, shiny inner face means the belt has been slipping and overheating. Replace the belt and fix the cause — tension, sheave wear, or alignment.
- Fraying or raveling at edges: The outer fabric wrap is degrading. The belt is at end of useful life.
- Squealing on startup or under load: Startup squeal that clears after a few seconds often means low tension. Squeal that persists with correct tension means the belt surface or sheave faces are glazed.
- Pump underperformance with motor running: If the motor is running at correct RPM but the pump is not building pressure at the expected rate, and there is no valve or piston issue, check the belt first. A slipping belt is a common and easy-to-miss cause of compressor underperformance.
- Annual replacement as PM: On a daily-use shop compressor, replacing the drive belt annually as a matter of scheduled maintenance is cheaper than the lost production time from an unplanned belt failure.
Why Buy From Air Compressor Factory Direct?
- Specialist since 2016: Air compressor parts are what we do. We have the product knowledge to help you get the right belt the first time.
- Free shipping: Ships free to the contiguous United States.
- Free tech support: Call 866-217-0063 with questions about belt sizing, installation, or why a drive keeps eating belts. That conversation costs you nothing.
- Full manufacturer warranty: Manufacturing defects are covered. If the belt arrives with a defect, we take care of it.
- Honest guidance: If the B87 is not the right belt for your machine, we will tell you that. Getting you the right part matters more than closing any individual sale.
Call 866-217-0063 for quick answers and help!
