Look along the side of an older tractor or industrial diesel. If you see a long, narrow pump with a row of fuel lines marching out the top, you’re looking at an inline pump, and plenty of those are Bosch A-size.
The A-size is one of Bosch’s smaller inline families, sold as the PES-A in four- and six-cylinder versions (PES4A, PES6A) for farm, construction and industrial engines. The Case IH MX110 tractor is one confirmed home. We test it, rebuild it, calibrate it and ship it back.
Also Known As
Same pump, many names:
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Bosch A pump, A-size pump
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PES-A, PE-A, PES4A, PES6A
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Bosch inline injection pump
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Cousins built to the same Bosch pattern, such as the Zexel PE(S)-A and the Simms SPE-A
Did You Know? Bosch names its inline pump sizes with letters. The A-size sits toward the small end of the range, well below the P-size pumps on big Cummins sixes.
How the Bosch A Pump Works
Think of a row of grease guns with a camshaft doing the squeezing, one gun per cylinder, over and over in firing order.
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A supply pump, often bolted to the pump body, fills a fuel gallery that runs along the plungers.
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The pump camshaft lifts each plunger through a roller tappet.
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A slanted groove on each plunger, the helix, sets how much fuel each stroke delivers. A control rack rotates all the plungers together to change it.
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A delivery valve atop each barrel closes after each shot, keeping the line primed.
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The governor moves the rack in response to throttle and rpm.
Shop Talk: PES6A packs a lot into five characters: PE is an inline pump with its own camshaft, S marks the mounting style, 6 is the cylinder count and A is the size. The digits that follow describe plunger size and design.
What It Fits
| Make and model | Engine | Years | Notes |
|---|---|---|---|
| Case IH MX110 tractor | PES6A | ||
| Other ag, construction and industrial diesels | Four- and six-cylinder | Various |
Not on the list? Send a tag photo and we’ll identify it.
Part Numbers We See
Start with the type code, which begins PE or PES, then the cylinder count, then the letter A. The Bosch part number sits on the same tag, and engine makers often add their own number.
Why A Pumps Fail and the Symptoms
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Age: O-rings and gaskets harden and weep.
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Water and dirt: they wear plungers, barrels and delivery valves.
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Long storage: seasonal machines grow varnish that sticks the rack and governor.
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High hours: worn camshaft lobes, tappets and governor parts.
Symptoms:
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Hard starting or long cranking
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Black or white smoke
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Low power under load
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Hunting or surging rpm
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A miss on one cylinder
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Fuel leaks at the pump
Owner’s Tip: Drain the water separator, change the fuel filters and check supply pressure with a gauge. Air and water cause plenty of symptoms that look like a bad pump.
What We Do to Your Bosch A Pump
| Service option | Typical evaluation or service scope |
|---|---|
| Test and report | Bench test as received, per-cylinder delivery check and a written report |
| Rebuild and calibrate | Teardown, cleaning, new seals, worn elements, valves and tappets replaced, governor serviced, calibration and a test sheet |
| Exchange (when available) | Ask whether a compatible exchange unit is available and how the core process works |
Bench Notes: Calibration is where an inline pump is won or lost. Every element must deliver the same fuel at the same moment, or one cylinder works harder than the rest. That’s why every A pump goes on the stand after assembly.
What Model-Specific Verification Can Document
Evidence for Bosch A Pump (PES-A) Repair and Rebuild: When an approved procedure exists for this exact component, useful documentation can distinguish identification, visible condition, observed wear or contamination and supported functional test results. Record the assembly number and available engine data before deciding which checks will answer the reported complaint. Bench findings cannot replace the engine-side diagnosis. See how pump testing is evaluated.
Repair Options and Technical Considerations
Repair decisions depend on the exact pump, its condition, the components required and the service approved after evaluation. Ask about any available exchange options and review the service scope before authorization. The service scope depends on the condition of elements, valves and governor components and on parts support for older pumps.
How to Ship Your Bosch A Pump
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Photograph every tag on the pump and the engine.
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Note the machine, engine, symptoms and any adjustments.
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Drain the fuel and cap every port.
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Ask before removing the drive gear.
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Bag it and box it with 2 inches of padding all around.
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Fill out the mail-in repair form and ship to the address provided during intake.
Ship It Right: Don’t let the pump rest on its governor or outlet fittings. Fill the gaps so nothing rattles.
Related Pumps and Parts
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Zexel PE(S)-A
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Bosch P3000
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Bosch MW
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Simms Minimec
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All Bosch pumps
FAQ
Is a Zexel PE-A the same as a Bosch A pump?
They’re close relatives. Diesel Kiki built Bosch-licensed pumps, so the designs share a pattern, but parts and calibrations vary by application. Send the tag and we’ll sort it out.
My pump isn’t on your list. Can you still help?
Probably. Inline pumps went on more engines than any list can hold. Send a tag photo or request a repair evaluation before you ship.
Should I rebuild it or buy new?
New can be hard to find for older pumps, and a rebuild keeps the original calibration. Our rebuild vs reman vs new guide lays out the trade-offs.
How long will a rebuilt A pump last?
That depends mostly on fuel and filters, so we can’t promise a number. Clean, dry fuel is the best insurance. Review the written terms applicable to your repair before work begins.
Your A pump was built to be rebuilt. Send it to a shop with the stand and the patience to do it right. Start a Mail-In Repair or request a repair evaluation.