Part 2 of 4: Cylinder Drift Test on 10 Year-Old 660SJ Boom Lift

Part 2 of 4: Cylinder Drift on Your 10-Year-Old 660SJ Boom Lift – Why It Matters and How to Test It

Last week in Part 1 we covered buying that 10-year-old 660SJ at auction and the major upfront costs. If you’re a roofer (or any contractor) who just bought one of these machines, one of the very first things any good mechanic will check is cylinder drift. On a 10-year-old unit this test is critical — even with low hours, worn seals or valves can create a real safety risk when your crew is 60 feet in the air.

Cylinder drift is when the boom rod slowly moves (extends or retracts) even though the control valve is in neutral. It’s one of the most common issues on older AWPs, and catching it early keeps the machine safe and prevents bigger repair bills later.

What Causes Cylinder Drift?

  • Load-holding valves leaking or malfunctioning – These valves are supposed to lock the oil in place. Normal minor leakage can occur, but worn valves let oil slip past.
  • Damaged or worn piston seals – Seals inside the cylinders wear out over time, allowing oil to leak internally from one side of the piston to the other (or externally).
  • Thermal expansion/contraction of the hydraulic oil – This is completely normal. Oil expands when hot and contracts as it cools toward ambient temperature. It’s not leakage — just physics.

How to Run the Cylinder Leakage Test

The test must be done with the hydraulic oil at stabilized ambient (shop) temperature. Here’s the simple procedure:

  1. Position the boom in an open area with no obstructions.
  2. Apply the rated load and position the cylinder so the rod is easy to watch.
  3. Place a calibrated dial indicator against the cylinder rod.
  4. Record total movement over exactly 10 minutes.

If the drift stays within the limits shown in the table below, the leakage is acceptable. Anything over the limit means it’s time to inspect (and likely replace) the piston seals and/or load-holding valves.

Table: Maximum Acceptable Cylinder Drift in 10 Minutes

Cylinder Bore Diameter Max. Acceptable Drift
inches mm inches mm
3 76.2 0.026 0.66
3.5 89 0.019 0.48
4 101.6 0.015 0.38
5 127 0.009 0.22
6 152.4 0.006 0.15
7 177.8 0.005 0.13
8 203.2 0.004 0.10
9 228.6 0.003 0.08

Note: These limits are based on a normal leakage rate of about 6 drops per minute.

Thermal Drift vs. Real Leakage

All hydraulic oil expands and contracts with temperature changes. On a 10-year-old 660SJ that’s been sitting, you may see the boom creep slightly as the oil moves toward outside air temperature. This is normal and more noticeable on longer booms.

What If the Drift Test Fails?

If the numbers are over the limit, you’ll most likely need new piston seals. On the 660SJ there are two typical cylinders to check:

Both are genuine OEM parts available directly at www.gciron.com. Replacing one or both kits plus labor usually adds $1,500–$3,000 depending on which cylinder is leaking.

Bottom Line for Roofers

Have the cylinder drift test done as part of the first annual inspection (already included in the $6,900–$7,200 first-30-days budget from Part 1). If it passes, you’re good to go. If it fails, the seal kits above are the most common fix and are straightforward to order from gciron.com.

Next week in Part 3: We’ll cover safety decals, paint, and appearance — the small but important details that keep you compliant with OSHA and your insurance company.