How to Measure an Implement Wheel Bolt Pattern and Center Bore

Before ordering a replacement implement wheel, record three details from the wheel you have: the number of mounting holes, the bolt-circle diameter and the center-bore diameter. These describe the mounting pattern and center opening. A lug count by itself is not a complete bolt pattern.

Keep those measurements together when comparing Ag Wheels. If a measurement is uncertain, confirm it before selecting a replacement rather than choosing the closest-looking wheel.

Key takeaway

• Count the mounting holes. Record the lug count separately from the diameter.
• Measure the bolt circle the right way. Straight across opposite holes on 4, 6 and 8-lug wheels; adjacent holes (3-1/4 in.) on a 5-lug wheel.
• Measure the center bore separately. It tells a 5 on 5-1/2 in. implement wheel (4 in.) from a trailer wheel (3.19 in.).

1. Count the wheel's mounting holes

Count the mounting holes around the wheel's center opening and write down that number. Southwest Wheel's implement wheel listings include four-, five-, six- and eight-lug patterns. The large center hole is a separate opening, not another lug hole.

A notation such as 6-6 describes two things: six mounting holes and a 6 in. bolt circle. It does not describe the wheel's overall diameter or its center bore.

2. Measure the bolt-circle diameter

The bolt circle is the imaginary circle that passes through the center of each bolt hole. Its diameter is the second part of the bolt pattern. How you measure it depends on whether the wheel has an even or odd number of holes.

Four-, six- and eight-lug wheels

Measure from the center of one stud hole straight across the center bore to the center of the opposite stud hole. That reading is the bolt-circle diameter. A four-lug implement wheel will read 5 in., a six-lug 6 in. and an eight-lug 8 in.

Five-lug wheels

A five-lug wheel has no hole directly across from another, so you cannot measure straight across. Instead, measure from the center of one stud hole to the center of the next adjacent stud hole. On a 5 on 5-1/2 in. implement wheel, that measurement should be 3-1/4 in. The 3-1/4 in. reading is not the bolt-circle diameter; it is how you confirm a 5-1/2 in. bolt circle.

3. Measure the center-bore diameter

The center bore is the diameter of the large center hole that fits over the hub. Implement wheel catalogs often call it the pilot. Measure straight across the center hole, edge to edge, and record it as a separate measurement, not as the second number in the bolt pattern.

Standard implement wheel bolt circles and center bores

Implement wheels use these bolt circle and center bore combinations:

Implement wheel or trailer wheel? Check the center bore

The only implement bolt circle that can be confused with a truck or trailer wheel is the 5 on 5-1/2 in. pattern. The center bore is the giveaway: a 5 on 5-1/2 in. implement wheel has a 4 in. center bore, while a normal 5 on 5-1/2 in. trailer wheel has a 3.19 in. center bore. If you measure 5 on 5-1/2 in. and a 4 in. center bore, you have an implement wheel.

The 6 on 6 in. pattern is an implement-wheel pattern, not a trailer-wheel pattern. Keep the complete measurement when shopping, not just the lug count.

Hub listings use the same pairs. For example, implement hub AG-H3066600-2ZHA is titled with a 6 in. bolt circle and a 4-5/8 in. pilot. Match the wheel's bolt circle and center bore to the hub before ordering.

Why implement wheels are rated at 20 mph

An implement wheel's capacity is a load rating set at a maximum speed. Implement wheel and hub package capacities in the manufacturer's catalog are based on 20 mph maximum speeds, and the primary product summaries for parts 106533, 107174 and 106870 state SPEED RATING 20MPH. The rated load applies only up to that speed, so do not exceed 20 mph.

Excessive speed is treated as severe service, along with hillside operation, a high center of gravity, fixed tandem axles, walking beams and rough terrain. Severe service reduces the load a wheel and hub package can carry. Wheel and hub capacities are also given for a single-wheel application; dual, tandem or walking-beam loading is not included in the rating.

An assembly is only as strong as its lowest-rated part. If you pair a wheel rated lower than the hub package, the assembly's capacity drops to the wheel's rating. Check the selected wheel's own speed rating and capacity before ordering, and do not infer either one from wheel size or lug count.

Keep a measurement record before ordering

• Mounting-hole count: confirmed number of holes.
• Bolt-circle diameter: confirmed measurement in inches.
• Complete bolt pattern: hole count plus bolt-circle diameter.
• Center-bore diameter: separate confirmed measurement in inches.
• Any readable wheel part number or markings, to compare with the selected listing.
• The selected wheel listing's stated speed rating and capacity, checked separately from the mounting measurements.
• Any measurement that remains unconfirmed, so it can be resolved before ordering.

Wheel diameter and width are additional checks; they do not replace these mounting measurements. Rim diameter is measured at the bead seat, and the tire must match it exactly: mount a 16.5 in. tire only on a 16.5 in. wheel, never a 16 in. tire on a 16.5 in. wheel. A mismatched tire and wheel can separate explosively during mounting and cause serious injury.

Once the measurements are established, see How to Choose Implement Wheels, Hubs, and Spindles for the next step in choosing the wheel, hub and spindle combination.

Southwest Wheel - Your Trailers Best Friend Since 1919.