The topic of engine crankshaft thrust bearing failure creeps up from time. I’ve written about it twice, once in 1998 and again in 2015. Unfortunately, these articles aren’t in the current database, so it’s not simple for people to find them online. This gives us an opportunity to index it in the latest Gears database and add a few updates. The core content of this article is from the 2015 issue.
Since that first article, several organizations like PERA, MOTOR, Engine Rebuilder Magazine, and Danny’s Engine Portal have moved away from the converter-ballooning idea, even citing ATRA and that first article as a source.
But that doesn’t mean our work is done. Plenty of “experts” hang onto things like converter ballooning, using the wrong converter, using the wrong flexplate or bolts, missing, or even installing the transmission pump gears backward.
So let’s get into it. The scenario generally goes something like this: Someone decides to rebuild the engine in their motorhome or truck. And while they’re at it, they get the transmission rebuilt, too. From then on, they can’t keep a crankshaft in the engine.
The engine rebuilder trots out some dusty old article that cites converter ballooning as the cause. The bickering between the engine rebuilder and transmission shop begins, with the customer in the middle. We also see this failure if the transmission hasn’t been worked on. This is the most tragic because the customer may be pressed to have the transmission replaced, all because of this converter ballooning meme or some other “transmission-related” problem. Oddly though, we don’t see a scenario where only the transmission was rebuilt. The key here is that it always follows an engine rebuild.
So let’s look at the common thoughts about torque-converter-caused crankshaft failure.
TORQUE CONVERTER BALLOONING
Torque converter ballooning occurs when the pressure inside the converter gets so high that it expands, or stretches, the converter, just like you’d think of when blowing up a balloon (hence the name).
As much as this seems like a conceivable culprit, there’s just one problem: For the ballooning converter to press on the crankshaft, it’d have to press on the transmission too. There’s nothing except the pump for the converter to press against. Pumps are not designed to sustain a load like that and would fail immediately. So, if there’s no damage to the pump, converter ballooning is pretty much out of the question. But it may not be out as a clue.
You see, if the torque converter is exposed to pressure high enough pressure for it to balloon, then it’s this “high pressure” to look into, not the ballooned torque converter. Unfortunately, if there’s any evidence of converter ballooning, the converter gets replaced, and off we go again (to another failure).
TORQUE CONVERTER PRESSURE
Converter pressure is the pressure inside the torque converter during operation. It’s the byproduct of the difference between the amount of fluid flowing into the converter and what can get through the cooler and lube circuit.
Most transmissions don’t have a pressure tap to measure converter pressure, so the next best way to check it is to “tee” into the outlet cooler line. The pressure here will be as close as you get to what’s inside the converter. For clarity, you’re not attaching a gauge to the outgoing cooler line. You’re using a “T” fitting that allows the ATF to flow naturally; you’re just sampling what’s flowing through the line. Normal converter pressure varies by transmission type and load. In addition, most transmissions (like the Ford C6 we’re using in our test) have a regulating circuit to limit converter pressure. The normal pressure here is about 25 psi. The most common transmission involved with crankshaft thrust bearing failure is GM’s THM 400. It doesn’t have a converter regulating circuit and relies on cooler flow to keep the converter pressure under control. Adding an external cooler will increase torque converter pressure, but it’s usually not a problem. Under a load, you may see 50 to 80 psi of cooler line pressure. This is normal. But considering its pressure is higher than most transmissions, poor crankshaft machining won’t work on a GM engine in front of a THM 400 where it may get by on other applications.
Okay, let’s get into it and look at how the converter exerts pressure on the crankshaft. For this, we’ve set up a jig (a hydraulic press and scale) to measure converter pressure’s effects (Figure 1). The concept is straightforward: The converter operates like a hydraulic ram, no different than what you’d find on a hydraulic lift or jack.
The ram portion of the converter is the converter hub. You might even think of it like a servo piston. The force a servo piston applies is simply the surface area of the piston multiplied by the pressure working on the piston.
In the case of the converter, it’s the area of the converter hub. The diameter of the converter hub in our example is 2 inches, so the area of this converter hub is 3.142 square inches (Figure 2).
The jig allows us to pressurize the torque converter and measure its force as if pressing against the crankshaft. We’ve sealed off passages in the pump to contain the pressure and added a pressure source and gauge to the converter drain tap. Notice, too, that we’ve included the flexplate; I’ll explain that later.
Here’s how the test works: We place the pump and converter assembly on a scale within a hydraulic press. We use the press as a containing fixture, not to apply any force to the converter. When we pressurize the converter, we can measure the force it exerts toward the scale, which is just what it would exert against the end of the crankshaft.
Figure one shows our scale, pump, and converter in the press with zero PSI applied to the converter. We’ve zeroed the scale and are ready to start the test.
Applying about 40 PSI to the converter transfers about 83 pounds of force to the scale (Figure 3). With about 50 pounds of converter pressure, we get about 118 pounds of force (Figure 4). As you can see, we could get into trouble if the converter pressure were to get too high. Now imagine what pressure it takes to balloon a converter, and you begin to understand that it’s the pressure inside the converter that we have to consider.
This is where adding coolers and making lube modifications can cause problems. The good news is that engines can withstand a good amount of force before they go south. That is, until someone rebuilds the engine incorrectly.
So what about that flexplate? Some discussions surrounding this problem claim that the flexplate absorbs the converter’s force and reduces its effect on the crankshaft. Let’s test that hypothesis:
We’ll place a hollow adapter on the flexplate for this test (Figure 5). Now the force from the converter presses on the flexplate and then to the press.
As we pressurize the converter with the same 40 PSI, notice the weight on the scale: It’s roughly the same as our previous test (Figure 6). The flexplate offered no reduction in the force exerted against the crankshaft.
EXCESSIVE CONVERTER CLEARANCE
This is an interesting aspect of converter operation, and it took me a while to wrap my mind around it. If you look at a side view of a converter, you’ll notice the space between the turbine and converter housing (figure 7).
Under high-torque conditions, a pressure head develops in this region. This pressure works to push the front cover of the converter housing and the turbine apart and can be intense. So much so that, given the right conditions, it can force the turbine into the impeller (figure 8). It’s also a contributor toward ballooning.
Under normal conditions, the turbine moves backward, takes up the clearance, and bottoms out, first against the stator and then against the rear bearing surface of the converter, neutralizing the effects of the pressure head. But in rare instances, look out! If the turbine splines lock on the input shaft splines, this force will continue to press on the crankshaft until the turbine touches the stator, and the clearance between them goes to zero. It is an amazing condition and extremely rare. It’s generally limited to high-performance applications.
Three things have to happen for this scenario to play out:
- High torque (to achieve the pressure head between the turbine and converter housing)
- Spline lock on the turbine
- Excessive endplay
Here’s the scenario: During a high-torque condition, it’s normal for some degree of turbine spline lock to occur. You can see this by measuring flexplate movement while raising and lowering line pressure.
Try it on a dyno: Measure the amount of flexplate movement while varying line pressure when there’s no load on the converter (transmission in park or neutral). Since the turbine splines are free to slide, the only limitation is the pressure in the converter. Then do the same test with a load on the converter. With the turbine splines bound, the converter can only move toward the crankshaft until the clearance in the converter goes to zero.
Now introduce a lot of clearance caused by misassembling the converter. Under high load, the pressure head between the turbine and converter cover increases.
If the turbine can’t slide back on its splines and bottom out on the stator, it’ll push the converter toward the engine until it takes up all the clearance in the converter. If that clearance is excessive, there’s a chance it can bottom out the converter against the crankshaft. If that happens, it’s over for the crankshaft thrust bearing.
This is such a rare condition that it’s almost not worth mentioning, but if a discussion on the topic gets heated, someone’s bound to mention it, so it’s worth knowing.
Finally, ancillary things like the wrong torque converter, bolts, or flexplate. Since the converter is free to move in and out of the transmission with pressure changes, just eliminate the transmission from the discussion. Imagine only the engine, flexplate, and torque converter fully assembled as a visual. Is there any scenario where you can see these components adding force to the crankshaft? You might bend the flexplate or force the converter into the crankshaft bolts, so there’re marks on the converter, but these are all interference-related topics. None of it moves or presses on the crankshaft. Forget all of these as silly conjures.
Always remember it is normal for a torque converter to press on the crankshaft. In some cases, it can reach close to 200 lbs of force. Design engineers expect this and design the crankshaft thrust bearing to withstand the force.
That’s about it when it comes to the topic of crankshaft failure. So, when someone comes to you with crankshaft thrust washer failure that’s “obviously been caused by converter ballooning,” ask to see the corresponding pump damage. If it’s not there, the crankshaft damage probably wasn’t caused by the converter: Look elsewhere for the source of the problem.
I want to thank PERA, MOTOR, Engine Rebuilder Magazine, and Danny’s Engine Portal for helping share this problem’s root cause. They all share terrific insights on crankshaft thrust surfacing.