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Manual Transmission

It Is a Brand-New Part; Why Do I Have to Check It?

There are many causes for comebacks in the units our industry rebuilds. Some problems result from human error, other factors outside the unit can affect proper performance, or the new parts that were installed can create issues.

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It Is a Brand-New Part; Why Do I Have to Check It?

Up to Standards

Subject: Checking new parts and units before installation
Essential Reading: Rebuilder, Shop Owner, Center Manager, Diagnostician, R & R
Author: Mike Weinberg, Rockland Standard Gear, Contributing Editor

Up to Standards

  • Subject: Checking new parts and units before installation
  • Essential Reading: Rebuilder, Shop Owner, Center Manager, Diagnostician, R & R
  • Author: Mike Weinberg, Rockland Standard Gear, Contributing Editor

There are many causes for comebacks in the units our industry rebuilds. Some problems result from human error, other factors outside the unit can affect proper performance, or the new parts that were installed can create issues.

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One thing is for certain: The vast number of different units in today’s vehicle have put a strain on the manufacturer’s capacity to produce such a wide variety of parts. On the supplier side, it is now impossible to have everything in the warehouse to fill the orders because of a vast proliferation in part numbers. 30 years ago we had about 30 different units to work on, and now there are more than 600 and climbing, including all the variations in similar models due to ratios, two- or four-wheel drive, torque capacity with different power plants and design improvements during the life of the production run.

It also seems that the vehicle manufacturers have cut the lifespan of the parts supply for various models, with less availability for past-production units. The aftermarket has played a key role in providing the parts no longer available from the car manufacturer and also in bringing the cost of parts down compared with OE.

Now we come to the place where comebacks are generated by the repair shop’s blind acceptance that rebuilt units, or parts that are brand new in the box, are always perfect. Nobody intentionally builds a bad part or a bad unit, which is the first step toward financial suicide; however, it is the responsibility of the seller AND the buyer to make sure the parts or unit is correct in all aspects before installation.

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A simple example of this is that a shop buys and receives a reman unit from its supplier. This unit, we will assume, has gone through various quality-control checks by the rebuilder but has been moved through the freight carrier to get it to the shop. It is entirely possible that despite the best efforts of the rebuilder, the freight carrier has been a lot less than kind to the package in the delivery system. Too many shops have called with various problems created by rough handling after installing the manual transmission or transfer case without ever shifting it when it came out of the box.

Sounds simple, right? You would be surprised how many shops never try all the gears or shift positions prior to installation, and it now becomes an expense issue. If you do not shift the unit before installing it and after installation cannot get into first gear, you will never know whether the unit is capable of making the shift and you have a clutch-release problem or the flaw is inside the gearbox. Confirm that the unit bench-shifts correctly before installation and go about making money.

The following issues are the result of years of tech-line problem solving and real-world experience. OEM parts are made from prints with specific design and manufacturing specifications. They go through an inordinate and expensive series of pre-production testing and prototype research. Aftermarket parts are generally reverse-engineered from OEM parts because the prints are owned by the OE manufacturer and not available. Some manufacturers have expensive profiling machines that can measure every aspect of a gear and produce a print from the OEM model, but there may be changes in the alloy used or the way the gears are manufactured or finished to bring down the price of each part. This in no way means they are bad or defective, but it does affect their performance in some instances.

There are several ways to manufacture gears. One is soft-finished gears and the other is hard-finished gears. For this article we will define a soft-finished gear as one that is cut, hobbed and ground and then, after the machining process is complete, is sent out for heat treating to acquire the proper metallurgy and hardness. Hard-finished gears are heat-treated prior to finish machining; when the proper temper is achieved in the alloy, they are finish machined. Hard-finished gears are more expensive to produce, as the hardened gear must be machined by using diamond-coated tooling to cut through the hardened alloy, and that tooling is expensive and perishable, adding to the manufacturing costs.

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During heat treat the alloy may shrink or expand, changing tolerances of the finished product, which is very important on soft-finished gears. It is up to the user to check the new part against the part being replaced to make sure the tolerances are correct. Where we see this most frequently is on the thickness of the gear (Figure 1). The gear bore that will ride on the shaft is almost always correct, but if the heat treat made the alloy grow in thickness, the endplay on the stacked gears will change and could seize the new gear to the shaft in short order, or make it impossible to get a retaining snap ring of the same thickness to seat on the shaft. This happens most often on parts where the OEM product no longer is available because of the age of the unit, such as Muncie, Saginaw, T4/T5 and T170 series, to name a few. You cannot in many instances buy OEM for past model units, so you need to really compare what came out with what goes in.

Clutches have similar issues, particularly with aftermarket replacement sets. Almost all clutches manufactured for late-model vehicles will have a hydraulic release system using a master and slave cylinder for clutch release. Measuring the finger height on the pressure plate against the OEM clutch when the pressure plate is installed on the flywheel is critical. There must be a sufficient air gap between the disc and the flywheel/pressure plate on release, and there must be an air gap usually in the range of 0.160-0.220 inch between the release bearing and the fingers after installation. To measure this you want to measure from the face of the engine block to the fingers (pressure plate installed). Then measure from the face of the bellhousing to the release-bearing contact surface with the slave cylinder fully released.

Example: If the measurement from the face of the block to the fingers is 3.00 inches and from the face of the bell to the release position on the release bearing is 2.80 inches, you have an air gap of 0.200 inch for a good clutch release. Do not confuse this with the air gap between the clutch disc and flywheel when the clutch is released. An incorrect air gap will result in a clutch that does not release fully. This will create transmission shift problems or damage to the synchronizers and speed gears and is usually due to differences in the manufacture of the pressure-plate fingers or the slave cylinder.

Another common problem with new parts purchased occurs in ring-and-pinion replacement. New technology in ring-and-pinion manufacture has two types of finished gears in the marketplace. The older technology was the five-cut gear, or a gear set that is face milled, and the newer technology is the two-cut gear set, which is face-hobbed (Figure 2). You may receive ring-and-pinion sets of either type even from the same manufacturer. You need to be able to distinguish between the two types of manufacture, as they have very different backlash requirements and very different patterns when you roll the gears after setup to read a pattern.

This means that if you are replacing an original gear set that was face milled (five-cut) with a face-hobbed (two-cut) set, the backlash and the patterns will be different although either gear set will function correctly if set up to the proper specs. Looking at the tooth profiles will tell you which you are working with. With the face-milled (five-cut) gear, the face of the tooth will be tapered with the heel (outer part of the tooth) being wider than the toe (inner part of the tooth). The face-hobbed gear (two-cut) will be uniform from heel to toe.

The rule of thumb when setting backlash on the gear set on a face-hobbed (two-cut) gear set is 0.004 inch; face-milled (five-cut) gear sets usually will have a backlash spec of 0.006-0.008 inch or more depending on the diameter of the ring gear. Also, the face-hobbed set will have a tapered pattern, but the five-cut face-milled set will have a more-rectangular pattern. It is obvious that without this information the uninformed builder will knock himself out trying to understand why his backlash is so different between the two types of gear sets. For more-in-depth information see my article in the May 2007 Transmission Digest.

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Internal parts are not the only areas that have created wasted time and comebacks for shops. There is a large market in replacement electric motors/encoders on transfer cases. You can buy OEM motors, which usually are more expensive than aftermarket clones, and many builders opt to save money to make a job more profitable.

That savings can be a double-edged sword in any parts purchase. You buy parts to replace whatever is damaged, worn or broken. You are selling this to the customer, not buying it for personal enjoyment. Therefore, it stands to reason that if you mark up the parts correctly – 40% or more to attain the desired profit level – you should sell the parts that will offer the best chance for long-term, trouble-free use by the car owner to protect your reputation and your cash.

We sell only OEM replacement motor/encoders at very competitive prices (cheaper than the dealer), as we have too many experiences in the field with aftermarket replacements of dubious quality. I know that I will get phone calls from aftermarket manufacturers condemning my position here, but our experience shows too many unhappy experiences with the aftermarket product.

The factory encoder motor is built with a needle bearing to support the transfer-case selector shaft. Many aftermarket parts use a bushing. Remember that these motors in many instances carry a large torque load because the are shifting the transfer case and applying an internal clutch pack. This increased torque load will side-load the bushing, creating a flat spot that eventually will bind the motor.

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Another issue seems to be the grease that is used to lubricate the motor and planetary drive gears, which seize up in temperatures below 20° in the aftermarket product.

We have found aftermarket product with smaller motors than OEM, and we have found many aftermarket replacement motors that have inferior electronics packages for the encoder that give up prematurely, setting codes and failing to perform like the OEM. The few dollars saved will have little meaning when the job comes back.

There is an awful lot of excellent aftermarket product available. It is important to realize that manufacturing differences do occur, and a few minutes making measurements and comparisons can save you from having to work on the same unit again for free. The downside is not only lost time and effort but also lost opportunity costs when your lift and bench are tied up with a comeback and you cannot complete a paying job.

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