Torque Converter Hub Wear
and the Pump Damage It Causes

A transmission comes back three weeks after a rebuild with fluid dripping off the bottom of the bellhousing. The tech replaced the front pump seal during the build, so the assumption is a bad seal or a bad install. Nine times out of ten the seal is fine and the real problem is the surface it is sealing against: the torque converter hub.

The hub is the machined snout on the back of the converter. It does three jobs at once. It rides in the pump bushing, which supports the whole converter. It drives the pump gear through its flats or slots. And its outside diameter is the sealing surface the front seal lip rides on. When that surface gets scored or worn undersize, no seal will hold against it, and the pump bushing that supports it wears faster and faster.

How Hub Wear Starts

• Debris from a previous failure. The single biggest cause. Clutch material and metal from the old failure get into the pump bushing, embed in the soft bushing material, and turn it into a cutting tool against the hub.
• Bellhousing or crank flange runout. Excess runout swings the converter in an orbit. The hub then loads one side of the bushing every revolution and wears an oval into both parts.
• A worn pump bushing left in place. A bushing that is already worn lets the hub drop, so the hub contacts the bushing edge instead of riding centered. Hub scoring follows quickly.
• Overheating. Hot, oxidized fluid loses film strength at the bushing interface. The result is a polished, undersized hub with no obvious scoring.
• A flexplate that is cracked or a converter that was not fully seated. Both put the hub where it does not belong under load.

Symptoms Before It Leaks

• A whine at idle in park or neutral that changes with engine speed. That is often a starving or worn pump, and a hub-driven bushing problem is a common reason for it.
• A ring of fluid mist on the inside of the bellhousing visible through the inspection cover before it ever drips on the floor.
• Fluid on the flexplate or the back of the block. Converter rotation flings hub-seal leakage outward, so the mess looks like a rear main seal complaint.
• Aluminum or bronze colored paste on the pan magnet. That is the bushing giving up.
• Line pressure that falls off when hot. A worn bushing lets pump clearances open up and internal leakage grows with temperature.

The rear main seal confusion is worth calling out. Both leaks put fluid at the bottom of the bellhousing. Engine oil is darker and thicker, ATF is red or brown and thin, and with the inspection cover off you can usually see which side of the flexplate the fluid is coming from. Clean it, drive it, and look again before you pull anything.

Measuring the Hub

Once the converter is out, this is a five-minute inspection with a micrometer.

• Run your fingernail across the seal contact band. If your nail catches on a groove, the hub is scrap. A seal lip cannot bridge a groove it can feel.
• Measure the hub outside diameter with a micrometer at four points around the circumference, in the bushing contact area and again in the seal contact area. Compare the readings.
• Out of round more than about 0.001 in means the hub has been running with runout and needs replacement.
• Compare the worn band to an unworn section of the hub. More than about 0.002 in of diameter loss in the seal or bushing band means replace it.
• Look for heat blue and for a lip at the edge of the wear band. A visible step at the edge of the contact area is the same verdict as a groove.
• Check the hub for cracks at the weld where it joins the converter cover, and check the drive flats or slots for rounding.

There are speedi-sleeve style repair sleeves for some hubs. They are a legitimate fix on a heavy truck application where a converter is expensive and the hub is otherwise sound, but they change the effective diameter and require the matching seal. On a common passenger car or light truck unit, a replacement converter is the better call.

What a Worn Hub Does to the Pump

The damage does not stop at the seal. The pump bushing is what centers the converter, and it is what centers the pump gears through the converter hub drive. Once that clearance opens up:

• The pump gears run off center, which raises gear-to-crescent and gear-to-pocket clearance and drops volume, most noticeably when hot.
• The stator support and the input shaft see side load they were never designed for, which is a common path to sealing ring groove wear.
• Bushing material circulates through the entire hydraulic system and lands in the valve body bores.
• Line pressure sags at idle when hot, which shows up on a scan tool as a pressure control solenoid working at high duty cycle to hold commanded pressure.

That is why a bellhousing leak on a high mileage unit is rarely just a seal job. By the time it leaks, the pump has usually taken damage.

The Correct Repair

When you find hub wear, the parts list is not negotiable:

• Replacement torque converter. A rebuilt converter is fine as long as the rebuilder replaced or machined the hub. Ask.
• New pump bushing, installed with a proper bushing driver, not a socket. A driver supports the bushing across its full face and installs it square. A socket cocks it and shaves material on the way in.
• New front pump seal, installed after the bushing, with the lip lubricated in ATF and the spring verified in place.
• Pump inspection. Check gear clearance against spec before the pump goes back together. If the pump has been running with a wobbling converter, its clearances have opened up.
• Cooler flush. Bushing material that stayed in the cooler will come right back to the new bushing.

On installation, spin the converter as it seats and confirm it drops fully in. You should feel two or three distinct engagements as it picks up the pump drive and the stator splines. If you only felt one, it is not home, and forcing the bolts down will destroy the pump and the hub in short order.

Preventing the Repeat

• Check bellhousing and crank flange runout on any vehicle that has eaten a converter, a flexplate or a pump bushing. Runout is a cause that survives a parts swap.
• Never install a converter that was in the vehicle during an internal failure without it being properly cleaned and inspected. Debris trapped inside comes out on the first drive.
• Keep the fluid cool. Sustained heat is what breaks the film at the bushing interface. On a towing truck, an auxiliary cooler pays for itself here.
• Service the fluid on schedule. Clean fluid keeps abrasives out of the bushing, and the bushing is what protects the hub.

FAQ

Can I just replace the front seal if the hub is scored?

No. The seal lip has to ride on a smooth round surface. A groove or a step in the hub will cut the new seal, usually within a few hundred miles.

How much hub wear is too much?

Any groove you can catch with a fingernail, more than about 0.001 in out of round, or more than about 0.002 in of diameter loss in the seal or bushing band. Verify against the service data for your unit where a figure is published.

Is a repair sleeve a real fix?

On heavy duty applications where a converter is expensive and the hub is otherwise sound, yes, provided you use the matching seal for the sleeve diameter. On common light duty units, a replacement converter is usually cheaper than the labor risk.

How do I tell a front pump leak from a rear main seal leak?

Pull the inspection cover, clean everything, and drive it. ATF is thin and red or brown, engine oil is thicker and darker. Where the fresh fluid appears relative to the flexplate tells you which side it came from.

My rebuild leaks from the bellhousing after two weeks. What went wrong?

In order of likelihood: the original converter was reused with a worn hub, the pump bushing was not replaced, the seal was installed cocked or without lubricating the lip, or the converter was not fully seated before the bolts were pulled down.