The hydraulic cylinder is the longest-lived major component on a Rotary 2-post lift and, when it does finally act up, one of the more misdiagnosed. This is a technician’s guide to the Rotary rotary hydraulic cylinder families, when to reseal, when to replace, install effort, and how to order the right one on the first try. Auto Lift Services is an Iowa-based Rotary dealer, installer, and service company, and we ship rotary hydraulic cylinder assemblies, seal kits, and complete power unit packages nationwide. Same-day shipping on stocked items when you order by 2pm Central, and a live parts desk at 800-674-9302 if you want us to look it up while you’re on the phone.
The Rotary cylinder family tree
There are a handful of core cylinder families across Rotary’s 2-post history. N346 shipped on the SPO10 and SPOA10 10,000 lb frames for many years. N342 shipped on the 12K SPO12/SPOA12. N380 covered several heavier commercial frames. Current-generation replacements use updated seal packages and slightly revised rod diameters — mostly transparent to install, but not always cross-compatible. Older asymmetric SPOA frames used different rod lengths than symmetric SPO frames of the same capacity. The serial-number range on the data tag is the tiebreaker on which revision your lift takes.
Reseal vs full replacement
Most ‘bad cylinder’ calls are actually seal jobs. A tired seal weeps externally, or a worn packing lets fluid bypass internally and the plunger drifts down under load. Both are reseal territory. What pushes you to a full rotary hydraulic cylinder is physical damage: scored plunger, pitted bore, bent rod, or a chrome plunger where the finish has begun to lift. If you can catch a fingernail on the plunger, don’t waste seal kit money — put a new cylinder in. On a lift more than 15 years old, if you’re already inside it for seals and the plunger has any surface pitting, the labor math favors a whole assembly.
Diagnosing which cylinder is at fault
A 2-post has two cylinders, and only one is usually the problem. Fully raise the lift, set the locks, and watch each column for external weep — the leaker is obvious. If neither column weeps but the lift drifts, it’s internal bypass. Bleed the system, drop the lift a foot and lock it, then watch the low column for drift over 10 minutes with a load on. Whichever side drops is the bad one. Cross-contamination from a burned power unit sometimes chews both cylinders — inspect both plungers when you’re in. If the fluid comes out black, milky, or full of shavings, replace it and change the filter screen.
Install effort — reseal
A seal kit on a Rotary cylinder is a 2-3 hour job for one tech who’s done it before, 4-5 hours the first time. Steps: fully raise and lock, bleed the affected cylinder, remove the hydraulic line and cap it, unbolt the cylinder from the carriage and lower mount, pull the cylinder, disassemble the gland, replace the seal stack, reassemble, reinstall, bleed, cycle 10 times, and inspect. Torque the gland nut to spec — over-tight scores the rod and you’ll be back in there. Use fresh AW-32 or AW-46 fluid, not what came out. The whole job runs cheap if you have the seal kit on the shelf and expensive if you don’t.
Install effort — full cylinder
A full rotary hydraulic cylinder swap is about the same hours as a reseal — the disassembly is identical, you just don’t tear down the gland. New assembly, new fittings if the old ones look tired, new fluid, ten cycles, done. If the lift has been in service 15+ years, replace the hydraulic hose on that side while you’re in there; the labor is trivial and the hose is cheap compared to another trip out. Full assemblies come pre-charged with test oil and ship in a factory box that doubles as the return crate if the cylinder is ever warranty-flagged.
Seal kits, cylinders, and hoses we stock
We stock seal kits for the N346, N342, and N380 families plus current-generation replacements, and we stock complete cylinder assemblies for every current Rotary 2-post frame. Hydraulic hoses, JIC fittings, breather caps, and check-valve assemblies are all on the shelf. If you want to see what’s available without a specific part number, browse the Rotary aftermarket parts collection. If you know your number, search it in parts lookup and we’ll confirm fit against your serial before we ship.
Power unit and hydraulic system health
A rotary hydraulic cylinder rarely fails in isolation — the power unit that feeds it usually shows symptoms first. If the motor draws hard, runs long, or pulls low pressure, put the whole power unit in when you replace or reseal the cylinder. New units come with a fresh reservoir, solenoids, and check valves for the price of a rebuild on the old one. If you have two cylinders and one is done, plan for the other’s seal kit within the next couple of years. Buy them together, ship them together, install them at once.
Order the right rotary hydraulic cylinder
The best order comes in with a photo of the data tag, the serial number, and a photo of the cylinder itself with the hose fitting visible. That triangulates the family, the revision, and the fitting size. Call our parts desk at 800-674-9302 or search directly in Rotary parts. If you’re weighing a full lift replacement instead of a rebuild, our 2-post lift buying guide covers what’s currently available. We install and service Rotary equipment every week — same shop, same techs, same phone number, no call center between you and the parts you actually need on the truck.
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