We head over to GearFx Driveline to see how the experts assemble a rock-solid race-quality Ford 9-inch posi third member.
The unsung hero of your driveline is the rearend, specifically the third member. It takes all the power your engine is spinning out and makes it do a 90-degree turn to your wheels. It’s a brutal job that’s made even more demanding when turning left and right enters the mix. Yet, even given its importance, people often put far less thought into it than they do sexier driveline items such as the engine. If their car simply putts around town, they might get around this oversight for a long time. But the harder the car is driven, the more important the rear becomes.
And the right third member is about more than just the parts. It’s also about how those parts were put together. The best parts on the planet will howl and quickly fail if they’re assembled incorrectly. We’re building a new rear for 1969 Camaro pro-touring project called Tri Tip, and we needed something that would hold up to hard track use, so we hit up the guys at GearFx Driveline. We wanted the best third member for our pro touring build, and they had the experience to make it happen.
GearFx’s main gig is building rears and driveline parts for race cars, including NASCAR, so attention to detail is the name of the game. The good news for us is that they use the same procedures and practices when they build street third members as they do when they build the full race setups. REM Isotopic Surface Finishing, break-in dyno, and other unique steps are things you’re not going to find at typical driveline shops, but these are things that will help you have a rear that won’t let you down on the street or the track. So with that, let’s take a look at how GearFx builds a rock-solid pro-touring third member.
A good third member needs to start with a good case. Here you have two options: iron or aluminum. Iron is the ultimate for strength, but aluminum will save you weight. More important, it will save you unsprung weight. For our Tri Tip pro-touring Camaro update we wanted that weight savings, so we went with a Strange HD Pro aluminum case (PN N2300). The HD case (and pinion support) knocks off quite a bit of weight, around 8 pounds compared to an N Series Nodular case and SVO pinion support, yet it’s built to be nearly as strong as its iron cousins. The case is made from 206-T4 heat–treated aluminum and has a 32 percent higher tensile strength than the commonly used 356-T6 aluminum.
The HD Pro is fitted with these super–strong extra-large billet caps. They feature a 9/16-inch “thru bolt” design, and together they significantly reduce gear deflection, which extends gear life.
Part of the strength is in the billet aluminum pinion support, which is secured with 7/16-inch ARP bolts. There’s also a special oil channel that maximizes the oil flow to the pinion bearings. This is critical for extended hard use such as you would see in road course work.
Every single pro-touring car we’ve built has been fitted with a Detroit Trutrac posi, and there was zero reason for us to move away from this proven formula. Its planetary helical side gears and pinions make this the perfect differential for a street car that likes curvy roads and hitting the autocross or road course. Under normal conditions the Detroit Trutrac performs like a regular open differential and, when required, automatically transfers torque to the wheel with the highest traction. The 31-spline Trutrac we chose (PN 913A586) is capable of transferring up to 3.5 times more torque to the high–traction wheel. This torque transfer ratio, called the bias ratio, is accomplished by using parallel-axis planetary helical side-gears and pinions that mesh much like worm gears. Since the Trutrac uses hardened gears against hardened gears and not clutches or cones, there’s nothing to wear out or need maintenance. Also, it doesn’t require a friction modifier added to the lube.
Not all gears are created equal. According to GearFx’s general manager Jeff “Fuzzy” Horton, they’ve tested a lot of gears and the most consistent and true gears they’ve found are the Stealth Series ring and pinion parts from US Gear. US Gear uses a new gear cutting tech on these called face hobbing, which creates a continuous and more consistent cut on all teeth of the ring and pinion at the same time. We did the math and decided that 3.70 gears would be our best bet for staying in our engine’s power band, so that was the gears we picked up through Summit Racing (PN 07-890370SS). Here you can see a factory finished Stealth ring gear on the right and one on the left that has gone through GearFx’s REM ISF (Isotropic Superfinish) process. In fancy speak, the process produces a nonlinear, low Ra, finish that improves wear and reduces friction. To us laymen the gears are surface polished, but it’s way more involved than that.
The process is a two-step dance using chemical and mechanical methods to get to the desired end result. This process removes the “peaks” of a ground, cut, or honed finish while leaving the “valleys” unaffected. This improves the finish with little dimensional change to the part. In the first stage, vibratory media, along with chemicals, works over the parts to improve the parts finish on a microscopic scale.
Here’s a close–up of the vibratory media. Farther down in the tumbler the media gets finer to get in all the nooks and crannies of the various parts.
The second step is referred to as the burnish process. After the required micro finish is achieved, a mild alkaline mixture is introduced. After a relatively short period, a polished, chrome-like finish is produced. In addition to the polishing effects, this step effectively removes all traces of the film formation from the cut process. This allows for a superior surface finish at less cost compared to conventional machining/grinding operations. Life, and racing, made better through technology.
The same treatment was given to the pinion gear. The smoother the surfaces, the better the two gears will mesh together. This also has the benefit of cutting how much heat is generated when the gears are under load. Reduced vibration, reduced noise, lower lubrication temps, less wear, and lower surface friction are all benefits of GearFx’s REM IFS process. The cost to have your gears treated is just 125 bucks; to us that’s money well spent and falls into the no-brainer category.
Clean parts are happy parts, and you would be surprised how many times we’ve watched third members get assembled with parts right out of the box. Well, the guys at GearFx cut their teeth building parts for race cars where there’s zero margin for error. Given that mindset, all the parts for the build were thoroughly cleaned starting with the Strange HD Pro case. If needed, parts are also deburred prior to cleaning.
The Timken bearings, races, and other parts from Strange were also treated to a solvent bath followed by some high–pressure air. This made sure any grit and grime left over from the manufacturing process was removed from the equation.
With that done, it was time to get the assembly party started. First up was mounting the Strange HD Pro aluminum case to an assembly jig. People often ask if building a third member can be done at home. The answer is yes, but people that do it for a living have a lot of specialized tools that make it an easier proposition. Between their specialized tools, dyno, and expertise, we think the $325 labor charge is well worth it.
On top you can see the large Timken bearings from the Strange kit, and on the bottom are the bearings used in a standard Ford 9-inch rear. Bigger bearings can handle more load and will be less likely to fail under hard use.
First up was prepping the case for its new pinion gear. Here they seated the pinion straddle mount bearing. Having the right tools is key since you want to make sure to only hit the outer race of the bearing to avoid damaging it.
To make pressing the pinion into place easier, the shaft was turned down a hair on GearFx’s lathe. According to Horton, “We don’t do this on all brands. Some brands have a tighter press than others. It doesn’t take much. We only remove the phosphate coating on the pinion. We have found that the US Gear Stealth gears are just a little tighter than we like on the pinion bearing press fit.” Again, just like building an engine, the pros measure and check all the tolerances.
The pinion could then be pressed together with the Timken pinion support bearing.
Shims are used to make sure the pinion sits right in the pinion support and that the right preload is on everything once the yoke is torqued down. “We prefer not to use shims for pinion bearing preload adjustment if we can avoid it. We have seen many customer gears come in with these shims beat out. This causes a loss of pinion preload and ultimately gear or bearing failure. In our N-Series gears, we only use a single solid spacer that is surface ground to the exact size we need. If we do have to use pinion shims, we try to only use thicker ones. The Strange Pinion Support comes with shims sized for their pinion support parts, thus we have to use them,” relayed Horton.
The guys at GearFx do this all the time, so they know where to start, but due to parts variances they may need to add or remove shims to get in an acceptable range.
With the shims in place the pinion bearing is tapped into place.
And here’s how it looked with the seal in place prior to being mated with the yoke and pinion gear. We should note that the seal is only installed after we confirmed that we had the correct pinion bearing preload.
For this rear we are using a billet 1350 yoke from Strange. Normally GearFx would use their NASCAR-bred GFX 1350 yoke, but they were happy to use the Strange one we provided. If you provide them a substandard yoke, they simply won’t use it.
Yeah, you could install the yoke using a nut and an impact gun, but that’s really hard on the parts. The much better way is to use a hydraulic press to do the job.
The pinion nut, which holds on the yoke, was then torqued to spec. The use of specialized tools, jigs, and fixtures makes this job much easier.
The pinion assembly was then turned using a torque gauge. If the reading is too high (too tight) or too low (too loose), then it’s pulled back apart and the shim stack adjusted.
GearFx managed to nail the shim stack for the pinion preload on the second try and with that our pinion/yoke assembly was done.
Moving to the Detroit Trutrac posi it was time to press on the bearings. A hydraulic press makes life easier, but when you do this all day, having a button to push instead of a handle to pump is certainly the way to go.
The posi was then flipped and the ring gear and other bearing pressed into place.
The kit came with fasteners, but GearFx prefers these drilled low-profile 5/8-inch head ARP ring gear bolts. At $31.70 it was a cheap upgrade for peace of mind.
Since they are drilled, GearFx safety–wires the bolts just like they do on the rears destined for racecars. Why? Because it’s doesn’t take long, and there will be zero worries about any of the bolt vibrating loose and causing a catastrophic failure.
It‘s steps like this that separate having a third member build at a race–minded shop like GearFx compared to having one put together at your typical hot rod driveline shop.
With the ring mated to the posi and the bearings pressed in place, we could go ahead and install the pinion assembly into the Strange case and secure it using five ARP fasteners.
We then flipped the case using the jig so we could install the rest of the parts.
With the races installed over the Timken bearings the posi/ring-gear assembly was carefully set in place.
Before we put the extra-wide billet caps in place, we installed the HD adjuster nuts. Since the caps aren’t in place, they are just “finger tight” for now.
We could then put the HD billet cap into place and secure it with the fasteners.
We’re pretty sure you’ve seen this dance before. Marking paint is put on both the coast and the drive sides of the ring gear teeth. Oh, in the interest of education, the thinner inner end of the tooth is called the toe while the thicker outer end is called the heel.
As you can see, we were high (biased toward the toe) on the drive side of the teeth. Low is caused by too much clearance between the pinion and ring gears, while high is too little clearance between the two gears.
And a bit high on the coast side, as well. Our gears were a bit tight, which means we needed to adjust our pinion shims. Removing a shim moves the pinion away from the centerline of the ring gear, while adding a shim moves it toward the centerline.
The pinion depth is controlled by shims. Even if you’re really good, chances are you’ll need to adjust the shim stack to get a good pattern on the gears. That is exactly what we had to do. For this build, the perfect stack came in at 0.021 inch.
Our reward was a sweet pattern in our marking paint.
A dial indicator was then installed so we could check and adjust the backlash.
Backlash is set with the adjuster nuts. You simply loosen one side and tighten the other side to move the gear back and forth until you have a proper backlash reading. This is where the backlash dial indicator came into play. Once it was between 0.009 and 0.010 inch, we were happy.
GearFx also checks the ring gear runout, which is a great indicator of how good the quality of your gears and differential are. Ours came in at 0.002 inch, which made us happy with our choice of US Gear from Summit Racing.
The adjuster nuts have 12 holes (or sometimes notches), and the chances that your setting will line up perfectly with one is slim. So, if everything is correct you always want to advance to the next notch before installing (with red thread locker) the anti-rotation locking tabs.
Pinion runout was also checked and found to be 0.006 inch, which is very good. Although they can get it even better with their GFX 1350 Ultra-Performance yoke. It’s the same one they use for NASCAR. As Horton explained, “The tool used here is specifically designed to sit tightly in the U-joint cup pockets. This is the true spinning axis for the U-joint. If you measure this on the OD of the pinion yoke, you can get a different reading. Achieving this with some brands can be difficult. Our NASCAR pinion yoke used on our N-Series builds typically has less than .002-inch runout. If we’re using a customer’s supplied yoke, or another brand that turns out to have too much runout, we won’t use it. The yoke from Strange was good to go, but we’ve seen ones where the best we could get was 0.030-inch runout, and that’s a no-go for sure.” Click here for a better explanation.
And with that, our “race prepped” street car 9-inch GearFx rear was done and ready to head out the door.
Well, if this was any other shop it would be ready to head out the door. At GearFx all the rears are run on their dyno to check for noise levels, rolling torque, and temp. According to the paperwork, our rear came in at 77.2 dB unloaded and 79.5 dB under load, which was well within specs.
They can also do a 2-hour break–in and dyno spin (for an additional fee) that will ensure the rear is broken in and ready to race the moment it’s installed in your car. Since we were there, we splurged to get the full dyno and break–in done.
Once the rear is in the car, we’ll be filling up the case with Driven 75W-90 synthetic racing gear oil. This stuff is built for hard use and will help keep temps down and all the parts moving smoothly together.
Planning any extra hardcore road race work? Well, GearFx offers a pretty badass internal oil pump that was originally designed for NASCAR. As GearFx’s Horton explained, “We don’t like to see the gear oil over 250 degrees Fahrenheit, worse case. In the case of a pro-touring car, most that we have been involved with are fine on a full road course for 10 laps or so. If a person is planning to do more than that or their car is geared more toward track running, an oil pump and cooler should be considered. The oil pump option does require the case to be machined and the pinion end to be have a 5/16-inch hex EDMed to drive the pump. Our oil pump upgrade (PN 95-00001) is $825.” The best part is that you don’t have to mess with an external pump. Just plumb the ports to your favorite plate cooler, and you’re in like Flynn.
Advantages of GearFX Driveline SVO Nodular Iron 9-inch rear gear assembly:
- Ford SVO nodular iron (3.250-inch bearing) made in USA
- GFX 1350 Ultra Performance yoke—this is our NASCAR billet pinion yoke. They index in every pinion yoketo get the minimum U-joint runout for that pared assembly. This helps limit vibration and noise. They have a video on our YouTube that shows this.
- U-joint strap kit with 12pt bolts—included
- Genuine Ford SVOpinion seal
- Genuine Ford SVODaytona pinion support
- ARP pinion support bolts
- GFX Oil Slinger
- Timken bearings
- GFX solid pinion preload spacer—They don’t use shims on their N-Series gear builds. We have seen too many customer gears come in for rebuild where the pinion preload shims have been pounded out. This causes free play in the pinion bearings and is a quick death for the gear assembly. (see image in gallery)
- GFX Daytona pinion support “D” O-ring
- Eaton TrueTrac differential
- Premium ring and pinion—They use a couple brands of ring and pinions based on application and our experience. These are QC’ed pretty heavily to ensure quality.
- ARP ring gear bolts (safety wired)
- REM isotropic surface finishing is an available upgrade
- Dyno is used for QC and break–in purposes. They monitor temps, rolling torque. and sound
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