CAT Pumps 27339 Buna-N Crankcase Oil Seal 10FR - 2854

CAT Pumps 27339 Oil Seal: Fast Answer

The CAT Pumps 27339 is an OEM replacement crankcase oil seal made of Buna-N (NBR) 80D rubber, designed for CAT 524 and 10-frame triplex pumps. It sits in the crankcase wall at each plunger rod opening, retaining oil inside the drive end while blocking water, chemicals, and debris from entering.

The CAT 27339 oil seal keeps crankcase oil contained and contaminants out, preserving bearing and crankshaft life across CAT 524 and 10-frame pump applications where reliable oil containment is non-negotiable.

Why You'll Value the CAT 27339 Oil Seal

A worn crankcase oil seal turns a routine maintenance item into an unplanned shutdown. Replacing oil seals before they fail completely costs less time and money than rebuilding an entire drive end.

• Prevents crankcase oil from escaping along the plunger rod. Oil leaking onto floors, belts, or motors creates slip hazards and contaminates surrounding equipment. A properly seated 27339 seal eliminates that path.

• Blocks external contaminants from reaching the oil side. The seal prevents water, detergents, and debris inside the fluid end from migrating into the pump crankcase through the rod opening. Without this barrier, oil turns milky, loses its lubricating properties, and accelerates wear on bearings and the crankshaft.

• Maintains consistent lubrication for moving components. Plungers, connecting rods, and bearings depend on clean oil at the correct level. A tight crankcase oil seal holds that oil in place during thousands of hours of reciprocating motion.

• Reduces unplanned downtime. Replacing three seals during scheduled service takes a fraction of the time and cost of replacing scored connecting rods, damaged bearings, or a worn crankshaft after a seal-related oil loss goes undetected.

• Serviceable with standard hand tools. No machine shop visit is required. A technician with a seal driver (or appropriately sized flat socket), Allen wrenches, and the pump service manual can complete the job on a workbench.

• Stable under normal CAT pump crankcase conditions. NBR rubber resists petroleum-based oils and handles crankcase temperatures in the 120–140°F range typical of most 10-frame pump installations.

What Makes the 27339 Seal Different

Generic or aftermarket oil seals often use softer durometer rubber, which deforms faster under the interference fit CAT crankcases require. Some use NBR compounds with looser tolerances on outer diameter and lip thickness, leading to blow-by and early leaking. Incorrect materials, such as EPDM or silicone-based elastomers, can swell or crack in petroleum-based crankcase oil.

The CAT Pumps 27339 addresses each of these failure points:

• Buna-N (NBR) compound. The durometer is stiff enough to hold shape in the bore under repeated heat cycles while remaining flexible enough for installation and lip contact. Softer seals lose their interference fit sooner.

• Sized to CAT 10-frame crankcase bore tolerances. The outer diameter, lip geometry, and depth are matched to the crankcase seal bores on 524 and 10-frame pumps. A correct interference fit means the seal stays in place and maintains contact with the plunger rod surface.

• Lip profile engineered for reciprocating rod contact. The inner lip rides on the plunger rod to retain oil while allowing smooth back-and-forth motion. Orientation matters during installation: the lip faces the oil side of the crankcase.

• Reduced hardening over service intervals. OEM-grade NBR holds its elasticity longer than generic compounds when exposed to the 120–140°F temperature range common in pump crankcases, extending the interval between replacements.

How It Works

In a CAT 10-frame triplex pump, three plunger rods pass through the crankcase wall to connect the crankshaft's reciprocating motion to the fluid-end plungers. Each rod opening needs a seal to separate the oil-filled crankcase from the wet end of the pump.

1. Seals around the plunger rod. The 27339's inner lip wraps around the rod where it exits the crankcase. The interference fit between the seal's outer diameter and the bore holds it in position while the lip maintains constant contact with the rod surface.

2. Retains crankcase oil. As the rod reciprocates, the seal prevents oil from migrating outward along the shaft. The 10-frame crankcase holds 40 oz (~1.18 L) of ISO-68 hydraulic oil; losing even a small quantity affects lubrication of bearings and the crankshaft.

3. Blocks contaminants from entering the crankcase. On the fluid-end side, water, spray, and process chemicals are present. If low pressure seals or high pressure seals upstream wear out, pumped fluid can travel along the rod toward the crankcase. The 27339 seal acts as the last line of defense, keeping that contamination out of the oil.

Product Details

• OEM Part Number: 27339

• Component Type: Crankcase oil seal for reciprocating plunger pumps

• Material: Buna-N (Nitrile / NBR) rubber

• Approximate Weight: 0.017 lbs

• Seal Location: Crankcase, at each plunger rod opening on 10-frame pumps

• Compatibility: CAT 524 and other 10-frame pump models listing part number 27339 on their parts list (including 623, 820, 1010 piston models)

• Operating Environment: Designed for use with CAT-specified petroleum-based ISO-68 crankcase oil at typical pump room ambient temperatures

• Quantity per Pump: One oil seal per plunger rod opening; three total for a triple-plunger (triplex) drive end

Confirm 27339 against your pump's data sheet or exploded-view parts list before ordering.

Who the 27339 Oil Seal Is For

• Operators of CAT 524 and 10-frame high-pressure pumps used in industrial washdown, reverse osmosis, process cleaning, and agriculture applications.

• Maintenance technicians performing preventative crankcase service, replacing a seal kit, or rebuilding a 10-frame drive end.

• OEMs and system integrators who standardize on CAT 10-frame pumps and need to stock long-term service parts with known specifications.

• Facilities managers addressing early oil seepage or cloudy crankcase oil before it leads to bearing or crankshaft damage.

If your pump's parts list calls out 27339 for the crankcase seal, this is the correct replacement component.

Installing the CAT 27339 Crankcase Oil Seal

The following covers the general process for installing the 27339 seal. Always consult the CAT pump service manual specific to your model for torque values, assembly sequence, and model-specific notes. Proper installation practices are critical to prevent premature seal failure.

1. Preparation and Safety

Lock out power to the pump motor. Relieve system pressure and disconnect any spray or fluid line connections. Drain crankcase oil if the service manual requires it for your model.

Gather tools before starting: a 6mm Allen wrench (used to remove eight bolts securing the pump head), open-end wrenches, a precision screwdriver for prying old seals, a seal driver or flat-sided socket, clean rags, and compatible oil for lubrication. Work on a clean bench and lay out fasteners, retainer hardware, and components in order during disassembly.

2. Accessing the Crankcase Seals

Use a 6mm Allen wrench to remove eight bolts from the pump head. Slide the head off the plungers carefully to avoid damaging the ceramic plunger surfaces or any small o rings in the manifold area. Remove the plungers and connecting rods per manufacturer instructions to expose the crankcase seal bores. Keep plunger bolts, washers, and any valve or retainer components organized.

3. Removing Old 27339 Seals

A precision screwdriver helps pop out old oil seals from the crankcase side. Work around the seal's edge to create a small gap, then lever it out. Be careful not to scratch the bore surface or score the plunger rod; any damage to either surface will cause new seals to leak.

Inspect each removed seal. Hardened, cracked, or deformed rubber points to overheating or chemical exposure. If the lip has worn unevenly, the plunger rod surface may be scored and should be inspected under good lighting.

4. Inspecting the Crankcase

Before installing new seals, inspect the crankcase for debris before replacing seals. Look for metal shavings, discolored oil, or scoring on connecting rods and the crankshaft. Heavy metallic debris inside the crankcase points to deeper mechanical damage that new seals alone will not fix. Wipe all seal bores clean with a lint-free cloth. Contamination control is important during seal installation; even small particles trapped under a new seal create a leak path.

5. Installing New 27339 Oil Seals

Apply a light film of compatible crankcase oil to the inner lip. Align the seal squarely in the bore and press it in with a flat driver or socket until it seats flush at the correct depth. Do not tilt the seal during pressing; uneven seating causes blow-by.

Orient the seal with the lip facing the oil side of the crankcase. If the seal contains a spring, position the spring toward the crankcase interior. Install one seal per plunger rod opening; three CAT 27339 seals are typically required per triplex pump.

6. Reassembly and Testing

Reinstall plungers, washers, and connecting rods in the reverse order of disassembly. Reattach the pump's head using the same eight bolts and tighten them per the service manual's torque specifications.

Refill the crankcase with the correct quantity of oil (40 oz for 10-frame pumps) and ensure the level sits at the red dot on the bubble sight glass. Verify the filler cap vent is open. Run the pump and observe the crankcase face for any sign of oil seepage along the plunger rods. Check the oil level again after a short run to confirm it remains stable.

Maintenance Tips for Longer Seal Life

Extending the interval between seal changes depends on operating practices as much as the seal itself.

• Follow crankcase oil change intervals. Change oil after the first 50 operating hours, then every 500 hours. Oil change intervals for pumps should be monitored to maintain seal integrity, because degraded oil carries abrasive particles that wear the seal lip.

• Keep the pump environment clean and dry. Reduce external spray or water exposure at the crankcase face. Ensure the filler cap vent is not blocked; a blocked vent creates pressure that can force oil past seals.

• Perform routine visual inspections. Routine visual inspections can help detect early signs of seal failure, such as a thin film of oil at the crankcase wall or a drop in oil level between checks. Address seepage before it becomes a continuous leak.

• Avoid sustained bypass operation without cooling. Extended deadhead or bypass conditions raise crankcase temperatures above the 140°F range where NBR performs well. Prolonged exposure above 212°F accelerates rubber hardening and cracking.

• Replace upstream seals when they wear. Worn low pressure seals and high pressure seals allow pumped fluid to migrate along the plunger rod toward the crankcase. Oil contamination can degrade the quality of the seal material, especially when the pumped fluid contains detergents, acids, or solvents. A new 27339 seal will fail prematurely if the o ring, seal case, or fluid-end repair kits are neglected.

• Install new 27339 seals during every major drive-end rebuild. The cost of three seals is negligible compared to reopening the pump for a crankcase oil seal replacement weeks later.

Frequently Asked Questions

Which CAT pump models use the 27339 crankcase oil seal? The CAT 27339 oil seal is utilized in CAT high-pressure triplex pumps built on the 10-frame platform. Models include the 524, 623, 820, and 1010, among others. Check your pump's data sheet or exploded-view parts list document to confirm that 27339 is the specified crankcase seal for your unit.

How can I tell if my 27339 seal needs replacement? Look for oil around the crankcase face, oil trickling along the plunger rod, or a drop in crankcase oil level between service intervals. During disassembly, inspect the seal's rubber for hardening, cracking, or deformation. Cloudy or milky oil in the crankcase is another sign: it indicates water has entered past a failed seal.

Can I reuse an old crankcase seal after disassembly? No. Once removed, the seal's interference fit and lip integrity are compromised. The lip may be stretched or nicked during removal. New seals are inexpensive relative to the cost of bearing or crankshaft damage from a leaking reused seal.

What lubricant is compatible with the 27339 Buna-N seal? It is compatible with standard CAT-type crankcase oils, specifically premium-grade petroleum-based ISO-68 hydraulic oil. Avoid synthetic or specialty oils without first confirming their compatibility with NBR rubber. Buna-N is nitrile rubber commonly used for oil seals, and it performs well with mineral-based lubricants but can swell or degrade in aromatic solvents.

Does replacing the crankcase oil seal affect pump performance? The seal does not change hydraulic output directly. However, it preserves proper lubrication by keeping oil in the crankcase and contaminants out. A leaking seal leads to low oil levels, accelerated wear on bearings and the crankshaft, and eventually a drive-end failure that does affect output.

Do I need any special tools to install the 27339 seal? Most technicians complete the job with a 6mm Allen wrench, open-end wrenches, a precision screwdriver, and an appropriately sized socket or seal driver. Following the CAT service manual for your specific pump model matters more than owning specialty tools. The key is pressing the seal squarely into the bore without tilting it.

Ready to Service Your CAT 10-Frame Pump?

The 27339 crankcase oil seal is a small part that protects every moving component in the drive end of your CAT pump. Replacing it during scheduled maintenance, rather than after a visible leak, avoids the cascading damage that comes from oil loss or contamination.

• Confirm your pump model's parts list specifies 27339 as the crankcase seal.

• Plan seal replacement as part of routine drive-end service rather than waiting for leaking or bearing noise.

• Consult the official CAT pump service manual for model-specific disassembly steps, torque values, and reassembly sequence before beginning work.

Contact the Barens team today for assistance on correct part selection and compatibility.