Pneumatic Cylinder Seal Kit or New Cylinder? A Repair-or-Replace Checklist

When an air cylinder starts leaking, loses force or begins to stick-slip, maintenance has to make a call: order a seal kit and rebuild it, or fit a new cylinder? Most advice stops at “if the rod and bore are fine, rebuild it”. That skips an earlier question that matters just as much: can this cylinder be rebuilt at all? This checklist works through three steps (can it be repaired, is it worth repairing, and which seal kit do you need) using SMC and Festo’s own documentation.

The short answer

First check whether the cylinder can be taken apart. Some series cannot. SMC’s manual for the CJ2-Z states that the cover and tube are joined by rolling crimping, the seals cannot be replaced, and a leaking unit should be replaced with a new cylinder. If the cylinder can be disassembled, inspect the rod and bore. If only the seals have worn and there is no scoring or bending, a seal kit is usually the right fix. If the rod or bore is damaged, new seals will soon fail again, and replacing the cylinder usually makes more sense.

Once you decide to rebuild, find the seal kit. SMC usually numbers kits as series + bore + “-PS”, and the kit does not include grease, which is ordered separately. Festo’s official spare parts catalogue finds the wearing parts kit from the order code, type code or a photo of the type plate.

Step 1: Can this cylinder be repaired?

Not every cylinder is designed for service. Some small, compact or low-cost designs use crimped or rolled joints between the end covers and the tube, and are never meant to be opened.

The maintenance section of SMC’s CJ2-Z manual is explicit:

“For the CJ2-*Z series, the cover and cylinder tube are joined by rolling crimping, and cannot be disassembled. Seals of CJ2-*Z series cannot be replaced.”

The same manual’s troubleshooting table lists reduced force caused by air leaking past the piston seal or rod seal, and gives the countermeasure as “Replace the cylinder with a new one.” The same action is given for a deformed piston rod.

Source: SMC CJ2-Z operation manual (CJ2*-OM0067P).

So for small-bore or compact cylinders, read the maintenance or disassembly section of that series’ manual before ordering anything. If the manual says it cannot be disassembled, do not force it. Even if you get it apart, restoring the original seal and joint strength is unlikely, and you may create a safety risk.

The reverse also helps: if SMC’s maintenance parts listings include a seal kit for a series, that series is generally designed to be serviced. The entry point is SMC’s maintenance parts page.

Confirm the cylinder is really the leak

From general experience, check that the leak really is in the cylinder before you strip it. Fittings, push-in connectors, badly cut tube ends and valve exhaust ports can all sound like a cylinder leak. Park the cylinder at one end under pressure and listen or feel around the rod end, end covers and fittings. Continuous leakage at the rod usually means the rod seal. Air steadily escaping from the opposite port suggests the piston seal is bypassing. A leaking fitting just needs a new fitting or a fresh tube cut, with no need to open the cylinder.

Step 2: Is it worth repairing?

Repairable is not the same as worth repairing. The table below summarises general engineering practice, not manufacturer data. Your own conditions decide the final answer.

The quickest way to judge the rod and bore is to look and feel once the cylinder is apart: scratches, pits or flaking plating on the rod, and lengthwise score marks inside the tube. Leakage at the rod end combined with visible rod damage points firmly at the rod.

Step 3: Finding an SMC seal kit

SMC usually numbers seal kits as series + bore + “-PS”, with extra suffixes for special versions. The MGP guided cylinder shows how SMC’s maintenance parts list works:

Source: SMC MGP maintenance parts list. Replace □ with the bore size; each bore is ordered separately.

Details that are easy to miss:

• Grease is not in the kit. SMC’s list states the grease pack is not included and must be ordered separately, for example GR-S-010 (10 g).
• Versions within a series use different kits. Air-cushion, end-lock and standard kits are not interchangeable, so read the full model code from the nameplate.
• Check new versus previous model. SMC notes that MGP production has been discontinued, with MGP-Z as the new series (MGP-AZ for the air-cushion version), and explains how to tell them apart. An old MGP needs the old-model kit.

SMC seal kit lookup, step by step

1. Photograph the nameplate and record the full model code, including bore, stroke and all suffixes.
2. Split the code into series, bore and options such as cushion or lock.
3. Use SMC’s maintenance parts list to confirm new versus previous model and find the kit number.
4. Note the grease pack number and order it with the kit.
5. Follow the replacement procedure given in SMC’s documentation.

For a worked example of a specific kit, see our article on the SMC CG5BN40SV-DCM687CM-PS seal kit.

Finding a Festo wearing parts kit

Festo calls its seal kits “sets of wearing parts”. The official Festo spare parts catalogue lets you:

• search by part number, order code, type code, product key or serial number;
• upload a photo of the type plate to identify the product;
• use the “Spare parts usage” view to see which products a spare part fits;
• open repair and installation instructions.

For example, Festo part 753090 is listed as “Set of wearing parts DSBC/G-50–“, the wearing parts kit for DSBC/DSBG cylinders with a 50 mm bore. For other bores and kit contents, rely on Festo’s site and catalogue rather than third-party cross-reference lists. Festo’s catalogue also explains the manufacturing date code on the type plate (for example, E4 means April 2014), which helps you estimate how long a cylinder has been in service.

Common mistakes when replacing seals

General workshop experience:

• Exhaust and isolate first. Trapped air in the cylinder or lines can move the rod unexpectedly.
• Follow the manufacturer’s procedure and tools. Both SMC and Festo publish replacement instructions.
• Watch the lip direction. A reversed seal leaks in one direction. Protect lips from threads and groove edges during fitting.
• Use the specified grease. The wrong grease can swell or harden seal materials.
• Still leaking after a rebuild? Recheck rod and bore for damage you missed, check cushion seals, and check fittings and tubing.

Discontinued cylinders: rebuild or move to the new series?

SMC’s list shows the original MGP as discontinued, with MGP-Z as the successor, yet it still lists kits for the previous model. Rebuild an old MGP with the old-model kit. If you are thinking of replacing the whole cylinder with the new series, confirm mounting dimensions, stroke and accessories against SMC’s new-versus-old comparison material. Our overview of discontinued Festo products covers the same issue on the Festo side, and the ISO 6432 mini cylinder cross-reference helps when a standard-dimension cylinder is the better path.

If you do end up choosing a new cylinder, our cylinder selection guide walks through bore sizing and force calculation, and the SMC components FAQ answers common sourcing questions.

Send us the model and we will check it

Photograph the nameplate, write down the full model code, and describe where it leaks (rod end, end cover, or loss of force). Email us and we will help you confirm the seal kit number and the repair-or-replace call.

FAQ

Can every cylinder be resealed?

No. Some series cannot be disassembled and their seals cannot be replaced, SMC’s CJ2-Z being one example. Check the series manual.

Does an SMC seal kit include grease?

For MGP, SMC’s maintenance parts list states the grease pack is not included; order GR-S-010 (10 g) separately.

MGP is discontinued. Are seal kits still listed?

SMC’s maintenance parts list still shows kits for the previous MGP and asks you to confirm whether your cylinder is the new MGP-Z or the previous model. Whether a kit can still be ordered is for SMC to confirm.

Do cushioned and end-lock cylinders use the same kit?

No. For MGP, the air-cushion version uses MGP□-A-PS and the end-lock version uses MGP□-B-PS.

Which Festo kit fits a DSBC with a 50 mm bore?

Festo lists 753090 as “Set of wearing parts DSBC/G-50–“. Confirm the contents in Festo’s spare parts catalogue.

What if it still leaks after new seals?

From general experience, look for rod or bore damage, reversed or nicked seals, and leaking fittings or tubing.