How Crack Checking and REM Polishing Can Better Prepare a Transmission for Track Use

Photography by J.G. Pasterjak

[Editor's note: This article features our 2004 Chevrolet Corvette Z06 project car. Click here for more updates.]

 

What if you could stave off a transmission failure long before it happens? That’s where crack checking comes in. 

Both the constant and instant load stresses applied to a transmission’s shafts and gears can lead to cracks. “These are not issues that will ever get any better, or even stop getting worse,” explains Jeff Horton of GearFX Driveline. “Once cracks start to develop in a stressed part, it’s basically the beginning of the end. The only question is when.”

But wait, it gets worse: A part failure inside a transmission is rarely isolated to just that component. Picture all of those spinning, meshing and sliding bits inside a transmission. What do you think is going to happen when tiny, razor-sharp shards of hardened metal are introduced to that mix? Nothing good.

The best way to prevent these disasters is to preemptively replace parts that are on their way to failure. And to identify those cracked parts, we turned to GearFX Driveline.

The Magic of Crack Checking

Scattered (actually, neatly organized and cataloged) throughout the GearFX Driveline facility are transmissions, rear ends and drivetrain parts from NASCAR and IMSA racers, vintage racers, road racers—any vehicle that places lots of stress on rotating and meshing parts. GearFX specializes in prepping these parts to perform at their optimum and—this part is key—to not explode.

Not exploding was certainly a trait we wanted in the transmission of our C5 Corvette Z06 project, so while the gearbox was out of the car, we took it to the folks at GearFX to receive their magic. After disassembly, the inspection process is fairly simple.

“First we just look at everything,” Horton explains. “If you can see defects with the naked eye, you know right off that the part is no good and needs to be replaced.” The type of damage can indicate a need for further inspection in related areas, too: “Worn engagement rings can be a sign of worn synchros; damaged gear faces will warrant a close look at the shafts and bearings holding them; and so on.”

After the visual check, it’s time to get a little more high-tech with a wet magnetic particle inspection. This method of crack checking is included in the cost of a transmission rebuild from GearFX. (If you just have some loose parts you need checked, the service is available à la carte for $95 an hour—assuming the parts are clean and ready to go.)

The process is commonly referred to as Magnafluxing, a term derived from a specific brand name of magnetic particle inspection gear that’s become ubiquitous in use, even in the industry. (The Kleenex and Band-Aid people feel their pain.) Despite the sci-fi name, Magnafluxing is straightforward and excels at revealing cracks that aren’t visible to the naked eye.

1. Some cracks are easy to spot. The cracks at the root of these gear teeth can be seen by the naked eye, so the fix is easy: Just toss the defective part.

2. Other cracks are less visible but can lead to failures just as devastating. That’s where magnetic particle inspection comes in (aka Magnafluxing, as the Magnaflux brand has become genericized). Treated with magnetic dye, this otherwise invisible crack becomes apparent under UV light.

3. Crack checking is easy enough that no part should be excluded. This gear was hiding cracks not only on the edges of the teeth, but on the faces as well. It’s a failure waiting to happen.

First, the inspector magnetizes the part by passing a magnetic field through it. Then, the part is drenched in a fluid that suspends microscopic magnetic particles, which are pulled into the cracks and fluoresce dramatically when hit with UV light. The process takes only a few minutes but can prevent a world of hurt at the worst possible time. For parts made of non-magnetic materials, like aluminum or magnesium, the inspector applies a fluorescent, penetrating dye that sinks into cracks via capillary action. 

“We Magnaflux practically everything that comes through the door,” says Horton, “obviously any parts that are coming off an existing assembly that we’re going to reuse, but new stuff as well. It’s just too easy to check everything to make sure something wasn’t damaged in shipping or has the rare manufacturing defect that would mean catastrophic failure down the road.”

How well did our transmission hold up to inspection? “There were a few burrs here and there that we could deal with on a case-by-case basis,” Horton explains, “but nothing showed up during Magnafluxing that worried us from a crack perspective.” He notes that regular fluid changes and proper technique—not putting any more stress on the transmission during upshifts and downshifts than it was designed to withstand—actually go a long way toward preventing longterm damage.

Shining Those Internal Transmission Parts

While our transmission was apart, we figured that everything deserved a proper shining. The technique we used here is REM polishing: a very mildly abrasive process that imparts a silky, almost impossibly smooth surface to mechanical bits. In our case, those mechanical bits were the rotating, sliding and meshing parts of our transmission and differential. 

GearFX’s Jeff Horton explains that the results of REM polishing are far from just cosmetic, however. “Don’t think of it so much as a power adder,” he says, “but more as a power loss reducer.” Ultimately, the arrow points in the same direction no matter how you describe it.

So how does this all work? How does shining up a transmission’s innards result in measurable gains in output? According to the SAE-published research paper “Horsepower Retention by ISF (Isotropic Superfinishing) of Automotive Racing Components,” the answer lies in reducing internal frictions. Think about it this way: Reducing friction by smoothing those mating and sliding surfaces means less energy converted into heat, meaning more energy sent to the wheels. 

4. Friction leads to heat, and heat leads to failure. We polished many of our transmission’s interfacing parts to reduce friction and wear.

5. Polishing with the proper media is key to the best finish. A good tech chooses media that matches the job requirements.

6. Our transmission’s guts, ready to go back together, looked shiny and slick.

At least that’s the theory. But the writers of the SAE paper confirmed that theory by actual dyno testing with an engine hooked to a NASCAR-style T101 transmission. In the tests, the baseline, unmodified transmission consistently produced about 5 horsepower fewer than one with REM-polished internals—from 383 horsepower stock to 388 with the shiny guts. We’d be happy to see those gains from a bigger hardware change, like a cold-air intake or cat-back exhaust system.

The polishing process benefits the transmission in several other ways, too. The reduced thermal load is better for bearings, and the reduced sliding friction of interfacing surfaces makes for smoother shifts and reduced chance of fatigue-related failures in these parts.

Unlike Magnafluxing, this process requires some specialized equipment. Parts are loaded into a giant vibratory tumbler with the REM media and given a good spin. After a while, the parts come out shiny and smooth as glass. The whole polishing process for a five- or six-speed transmission costs around $500.

While it sounds simple, there’s an art to the science. “Knowing what precise media to use and for how long to get the results you’re looking for, that’s where the experience comes in,” Horton says. “I have different blends of shapes and sizes of the ceramic-based media that we can use and combine for different durations and at different speeds to get ideal results. It’s a relatively simple operation, but it takes some ‘feel’ as well. You can throw the stuff in and flip the switch and it will shine up nice, but an experienced operator can really produce some amazing results.”

A Little Transmission R&R

Of course, having the technicians at GearFX polish everything and check for cracks would be pointless if they didn’t also replace all the worn bits. Our transmission actually looked better than most, but there was still visible evidence of wear on some important synchros and gear engagement rings, like third gear. We had definitely experienced poor engagement on more than a few occasions in third, and a quick visual inspection showed that it wasn’t entirely because of our driving.

7. Step one to opening up a T-56 transmission: Hit with hammer. GearFX’s expert hammer techs did a far better job than we ever could.

8. Our transmission internals looked good but not great, displaying mostly typical wear—like the burrs on the teeth of this cog.

9. Once the entire transmission was disassembled, it was time for cleaning and crack checking.

10. These old synchro rings showed heat damage and excessive wear on the inner surface of the blocker rings.

11. These heavy-duty replacements feature more durable carbon pads on the inside of the blocker rings for longer wear.

12. Replacing the stock shift fork pads with bronze fork pads is a recommended upgrade for any Tremec that will see harsher shifting than normal daily street driving. These examples show a typical wear pattern. The lower two pads are severely worn and cracked. This can be caused by localized heat from worn blocker rings or forced shifting. Since the pads push the synchronizer, once they start to wear, it takes more fork travel to engage the synchro assembly. This becomes a compounding issue. The bronze fork pad upgrade helps guard against this and also provides a more precise shifter feel, especially in high-performance applications.

So the entire transmission was treated to an upgraded rebuild kit which dropped right into our existing case without any modification or machining. The $1599 package includes bearings, bushings, seals, synchros and upgraded carbon blocker rings. The kit also replaces the stock shift fork pads, which are prone to degradation from age and hard use, with bronze pieces. 

Thanks to all of the polishing and rebuilding, the refreshed transmission also requires no dedicated break-in period once it’s back in the car. Just fill it up with the manufacturer-recommended Dexron 3 or a good quality synthetic transmission fluid and let the excitement begin.

We didn’t just rebuild and shine our T-56 transmission, we threw some upgrades at the rear end as well. A Wavetrac diff and polished gears now reside inside the rear end housing, ready to put power to the ground. We’ll be discussing the crazy physics of torque-biasing differentials in a future installment.

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