How to Measure Upper Control Arms for Dirt Cars

How to Measure Upper Control Arm Lengths for Dirt Late Models, Modifieds & Street Stocks

Getting the correct upper control arm length is one of the most important measurements when replacing or ordering arms for dirt late models, dirt modifieds, and street stocks. An incorrect length throws off camber, caster, and overall front-end geometry. Here’s a clean, practical way to measure them correctly and the key details you need to note by car type.

Primary Measurement: Control Arm Length

Measure from the center of the cross shaft to the center of the ball joint hole.

This is the standard industry measurement used by most manufacturers and shops. Use a quality tape or calipers and measure carefully — even 1/8" can matter on these cars.

Tips for accuracy:

• Make sure the arm is clean so you can clearly see the center of the shaft and the center of the ball joint bore.
• Measure both left and right sides of the car independently. They are often different lengths.

Important Notes by Car Type

Dirt Late Models It is very common for the right-side (shorter) upper control arm to use a ball joint that is angled 13° or 15°. The left side is typically straight. Always note whether the ball joint is straight or angled when measuring and ordering.

Dirt Modifieds Most modified upper control arms that accept a screw-in ball joint use the same style as late models. Measure the same way (center of shaft to center of ball joint hole) and confirm the ball joint type.

Street Stocks Many street stock upper control arms are metric offset designs. These arms are typically offset 1.25". Be sure to note the offset when measuring and ordering so the arm sits correctly in the chassis.

Common Ball Joint Cross-References (Moog Numbers)

Knowing the correct ball joint helps when ordering complete arms or just the joints:

• Dirt Late Models: Most common screw-in upper ball joint is Moog K772.
• Dirt Modifieds: Arms that accept a screw-in ball joint commonly use the same Moog K772.
• Street Stocks:
  • Small ball joint pattern → Moog K5208
  • Big GM ball joint pattern → Moog K6204

Always verify the ball joint pattern on your specific arm, as some chassis can vary.

Critical Secondary Measurement: Cross Bolt Hole Spacing

In addition to overall length, measure the cross bolt hole spacing. This is measured from the center of one bolt hole to the center of the other bolt hole on the cross shaft.

Common spacings:

• Dirt Late Models & Dirt Modifieds: Most use 6" center-to-center.
• Street Stocks: Most use 6-7/8" center-to-center.

This measurement is just as important as the arm length when ordering replacements. Getting the wrong spacing means the arm will not bolt up correctly to the chassis mounts.

Quick Measurement Checklist

1. Measure length: center of shaft → center of ball joint hole (both left and right).
2. Note if the right-side late model arm has a 13° or 15° angled ball joint.
3. Confirm offset on street stock arms (commonly 1.25").
4. Identify the ball joint style and match the correct Moog number.
5. Measure cross bolt hole spacing (center-to-center).
6. Write everything down before ordering.

Accurate measurements keep your front end geometry where it belongs and make ordering the right arms much smoother. If you’re measuring for replacements or custom arms and need help matching lengths, offsets, or ball joints, we’ve got the parts and the experience to get you the correct pieces.