Flathead Six Oil Pump Refresh: Bring Back Oil Pressure (and Keep That Old Mopar Alive)

If your Mopar flathead six has started greeting you with a cold-start tick, a lazy oil-pressure needle, or a “sounds fine… until it doesn’t” vibe, the oil pump is one of the smartest places to investigate before the engine starts paying you back in bearing noise. A fresh pump (or a properly rebuilt one) restores pressure, stabilizes hot-idle readings, and gives the entire lubrication system a fighting chance—especially after years of sitting, sludge, or fuel contamination. The key is doing it like a gearhead: verify pressure with a real gauge, rebuild the pump on the bench with measured clearances (not vibes), seal it like your engine’s life depends on it (because it does), then prime and test before you let it fire. (TM 9-1840A, 1952). 

Symptoms and baseline oil-pressure test

Start with the basics: confirm you actually have a pressure problem, not a bad dash gauge or sending unit. The factory approach (and the smart approach) is: install a known-good mechanical gauge and compare readings cold vs. hot at idle and at a steady “cruise” rpm.

On the Dodge T-245 / 230.2 cu in flathead six used in M37-era applications, the manual notes the oil pressure gauge should show 15 psi or higher at idle and approximately 50 psi at higher speeds. It also notes oil pressure is regulated by a relief valve, with the relief spring controlling maximum oil pressure to 50 psi. (TM 9-1840A, 1952). 

How to test like you mean it

1. Check oil level first. Low oil is the fastest way to “diagnose” an engine that’s actually just thirsty.

2. Warm the engine fully. Hot oil tells the truth; cold oil can flatter worn parts.
3. Install a mechanical gauge at the sender location. The manual notes the oil-pressure sending unit is on the left side of the cylinder block (TM 9-1840A, 1952). 
4. Record readings: cold idle, hot idle, and a steady higher speed (hold 1,500–2,000 rpm in the driveway if you can’t safely road-test).
5. Interpret the pattern:
   • Low at all speeds can be pump wear, suction leaks, a stuck relief valve, or severely diluted oil.
   • OK cold but falls off hard when hot often points to clearance issues (pump and/or bearings). The manual explicitly calls out low oil pressure causes including excessive bearing clearance, worn pump, or badly diluted lubricating oil. (TM 9-1840A, 1952). 

Note on specs: Flathead six pressures vary by application, gauge, and year. If you’re not working with a T-245/M37-type setup, treat the numbers above as a strong reference point, then verify with your factory manual.

Tools, parts, and rebuild-kit picks

A flathead oil pump refresh is one of those jobs where the right measuring tools make the difference between “freshened” and “fooling yourself.”

OldMoParts currently lists an Oil Pump Rebuild Kit – 6 Cylinder (SKU L-461) and a complete Oil Pump Assembly – 6 Cylinder (SKU L-260) for a wide range of Chrysler/Dodge/DeSoto/Plymouth applications. The rebuild kit is shown with the core hard parts (rotor/gear components), gasket, O-ring, and pin, while the full assembly is sold as a complete pump and notes it includes a gasket.

If your oil pressure issues smell like relief-valve trouble, OldMoParts also offers an Oil Pressure Relief Valve Spring Package – 6 Cylinder (SKU L-425) that includes two length springs and a gasket.

Parts and tools checklist

Parts and tools below are typical for a bench rebuild plus reinstallation. Verify exact fitment with your year/engine family.

Bench teardown and inspection

Once you’ve confirmed pressure is actually low (or suspicious), pull the pump and rebuild it on the bench where you can keep everything clean and measured. The factory manual describes the system as a pressure-type lubricating system with oil drawn from the pan by a rotor type oil pump and forced through drilled passages to bearings and other engine components. (TM 9-1840A, 1952). 

Bench disassembly checklist

Use a “clean-room mentality.” Dirt you introduce here goes straight to bearings later.

• ☐ Drain oil and remove pump (keep track of distributor/pump indexing—more on that below).
• ☐ On the bench, disassemble carefully and retain small parts (pins, screws, gasket).
• ☐ Drive out the distributor drive gear pin (if applicable) and remove the shaft/inner rotor assembly per the manual’s procedure. (TM 9-1840A, 1952). 
• ☐ Wash components in a suitable solvent and dry with compressed air as described in the manual. (TM 9-1840A, 1952). 
• ☐ Inspect the oil strainer/pickup. On the T-245 system, the oil strainer is a floating type attached to the suction pipe and pivoted so it can float, drawing cleaner oil while sediment settles. (TM 9-1840A, 1952). 

Reassembly and sealing details

Reassembly is where a lot of “rebuilt pumps” fail—not because the parts were wrong, but because sealing and orientation were sloppy.

Rotor/shaft installation: The manual calls out correct installation of a new inner rotor: press it on until the shaft end is flush with the rotor face and keep it square during pressing. (TM 9-1840A, 1952). 

Distributor drive gear orientation (critical): If your pump drives the distributor, pay attention to the manual’s note: when a new distributor drive gear is used, it must be located correctly on the shaft, and the drive gear pin should end up at right angles to the slot in the upper end of the shaft. (TM 9-1840A, 1952). 

Gaskets and O-rings:
OldMoParts’ L-461 rebuild kit is pictured with a gasket and O-ring—use them. One small air leak on the suction side, or one weeping gasket at the block, can turn a “fresh pump” into a no-pressure nightmare.

Torque specs: TM 9-1840A gives broad torque specs for major engine fasteners but does not list a specific torque for the oil pump cover screws or oil pump mounting fasteners; it instructs tightening cover screws evenly to ensure uniform gasket contact. Use your factory manual for your exact year/engine. (TM 9-1840A, 1952). 

Reinstall, index, prime, and pressure test

If you pull the pump on a flathead six that uses the pump/distributor drive relationship, you can’t just slap it back in and hope timing lands where it used to. TM 9-1840A provides a clear indexing procedure: set #1 piston to TDC firing, align the offset slot correctly, rotate to the specified tooth, and install the pump with a new oil pump gasket. It also notes using the distributor rotor position around 7 o’clock with points just about to open, and making sure the cork oil seal washer is in place when installing the distributor. (TM 9-1840A, 1952). 

Priming methods and why it matters

Priming isn’t superstition—it’s about filling galleries and preventing a dry start. A Melling instruction manual notes priming is a crucial step and can be done by filling the pump inlet with clean engine oil or by cranking the engine without ignition until oil pressure registers. (Melling/Manuals+, 2023). 

Practical flathead-friendly options:

• Crank with no spark: Disable ignition (and ideally fuel), then crank in short bursts until the gauge shows pressure.
• Pre-fill the pump inlet: If accessible during install, wet the inlet with clean oil before bolting up.

Post-refresh startup test (what “good” looks like)

After refilling, start and immediately verify pressure. For the T-245 reference engine, the manual suggests 15 psi or higher at idle and around 50 psi at higher speeds, with relief limiting maximum pressure to 50 psi. (TM 9-1840A, 1952). 

If pressure is still low, don’t start stretching springs. The manual is blunt: do not attempt to stretch the relief valve spring if oil pressure is low—remove the valve and check for dirt or lint between valve and seat, then continue diagnosis (bearings, pump wear, diluted oil). (TM 9-1840A, 1952). 

Maintenance schedule (simple, realistic, and flathead-friendly)

Use the manual’s oil-capacity guidance to keep refills honest: the T-245 data lists 5 quarts refill (without changing the filter element) and 6 quarts if the oil filter element is replaced/drained; the oil pan capacity is listed similarly (5 qt without oil filter, 6 qt with oil filter). (TM 9-1840A, 1952). 

A practical routine:

• Check oil level regularly (these engines can seep).
• Change oil and filter element on schedule, and immediately if fuel dilution is suspected. (TM 9-1840A, 1952; TotalEnergies, 2017). 
• If the car sits, treat the fuel system like a storage system—ethanol blends can create problems in older rubber components and storage conditions.

When your flathead has steady pressure, clean oil, and a pump with tight clearances, it’s back in its comfort zone—quiet on startup, happy at cruise, and far less likely to throw a starvation surprise. If you’re ready to get your oil pressure back where it belongs, OldMoParts’ Oil Pump Rebuild Kit (L-461), Oil Pump Assembly (L-260), and Relief Valve Spring Package (L-425) are solid places to start the parts hunt.