Author: American Powertrain

Pedal Pusher: How to measure and adjust your Clutch Pedal Ratio

So you just converted your clanky old manual clutch to a hydraulic release bearing setup and you thought the pedal would be easier to operate than before. While that should be the case, it isn’t always, and that is due to geometry. Much like a brake pedal, there is a specific ratio that you need to achieve in order for the pedal to be easy to operate. For hydraulic clutch systems, that ratio is 6:1.

Manual clutches are different from hydraulics, they use a bell-crank system that increases the effectiveness of your clutch pedal. With a hydraulic system, it is all in the placement of the master cylinder push rod on the clutch pedal. The higher the pickup point is on the pedal (closer to the fulcrum of the lever), the easier it is to push the pedal. There are three key factors to this equation: pedal length, pick up point to fulcrum length, and master cylinder bore.

The bore of the clutch master cylinder is the smallest part of the equation because most clutch masters are in the 7/8” range. The bigger you go, the more effort is required to operate the master cylinder. Provided you have a 7/8” bore or smaller, the 6:1 standard is used.

To determine the ratio of the pedal, you need to do some measuring. There are two measurements you need to take: the length from the pivot (fulcrum) of the pedal to the pushrod hole (Y), and from the fulcrum to the center of the brake pedal (X). The formula is X/Y=Ratio. For example, your stock clutch pedal is 14 inches long (X), with a pickup point measuring 4 inches from the center of the fulcrum (Y). 14/4=3.5, which is 3.5:1. While this may have worked for the factory manual linkage, it is about half of what it needs to be for a hydraulic set up. In this situation, 100 pounds of foot pressure yields 350 pounds of pressure to the master cylinder. Move the pickup point up 1.75 inches, (Y measurement of 2.25”), and that same 100 pounds of foot pressure yields 600 pounds at the master cylinder.

For most applications, you can raise the pickup point by simply drilling a new hole in the pedal arm, but some cars, like GM A-bodies, use a convolute pedal design that requires welding if you want to raise the pickup point. This can be done with a ¼” tab of steel welded to the pedal in the location you want it. This process can become more complicated when you are trying to use a factory firewall hole for the master cylinder.

The sharper the angle is on the master cylinder, you run the risk of creating a bind, which no amount of pedal ratio can overcome. There are a couple of solutions for this scenario. The first is to move the master cylinder up on the firewall, decreasing the angle. This is not always possible, as the brake booster or other items that cannot be easily moved are in the way. The other option is the American Powertrain adjustable firewall mount.

The patented firewall mount allows the master cylinder to accommodate the clutch pushrod angle while being mounted above or below the pickup point on the pedal itself. Because the master cylinder is a sealed system, the angle itself does not affect the operation of the master cylinder. If you have this mount and the angle is still too great, then you have to move the master cylinder.

Here is the diagram for measuring pedal ratio. Always measure to the center of the fulcrum, pickup point, and to the center of the pedal.

This is a brake pedal, but the math is the same. On this pedal, the pickup point is 5 5/8” from the fulcrum.

The pedal measures 14 inches from the fulcrum to the center of the pedal. The math reveals this is a 2.48:1 ratio, which is ludicrously low.

To make this pedal match the 6:1 (which is also the optimum ratio for brake pedals), we drilled a new pickup point. All better.

On this 1965 Mustang, the factory clutch pedal is very close to the fulcrum, yielding a suitable ratio for the master cylinder. We were even able to use the original firewall hole.

GM A-body cars (Chevelle, Skylark/GS, 442, etc) have funky clutch pedal as shown here. If you have to change the ratio, fabrication and welding is required.


Written By: Jefferson Bryant


Red Dirt Rodz
4518 Braxton Ln
Stillwater, OK 74074
405-880- 5343

American Powertrain
2199 Summerfield Rd
Cookeville, TN 38501
1-931- 646-4836

Heil Hydra(Max)! Setting up your hydraulic release bearing

High-performance engine installed- Check. Close-ratio Tremec T56 installed- check. Clutch fully operational- nope. Well that isn’t good, it is kind of hard to put 650hp to the ground when your clutch won’t engage or disengage when you operate the clutch pedal. Unlike a mechanically-operated clutch, most modern manual transmissions require hydraulic release bearings, and if you are not close enough, you may as well be a mile off. Installing the American Powertrain Hydramax hydraulic release is not difficult, in fact it is pretty easy, but it does require some math.

The relationship between the clutch diaphragm and the release bearing is the key to a properly adjusted hydraulic set up. The release bearing should ride the diaphragm without putting pressure on the fingers, but also should not be too far away. It is pretty easy to install the bearing where it is either too close (clutch is always disengaged) or too far (clutch won’t disengage). Because these installations are not stock, you have to figure out this relationship on your own. Don’t worry, it isn’t difficult.

What you need:

  • Pen
  • Scratch paper
  • Straight edge
  • Calipers or ruler that reads to .001
  • Partially-assembled drivetrain.

To start the process, the engine must be assembled with the flywheel and pressure plate. The clutch does not have to be installed at this point, because it doesn’t matter; but if you want to do everything at once, install the clutch with an alignment tool in place. Additionally, the bellhousing must be installed on the engine as well. Begin by placing the straight edge across the center of the opening in the bell housing. Use a ruler or measuring tape to determine the resting depth of the diaphragm, this measurement is taken off of the fingers themselves where the release bearing rides. Take this measurement in three places and write the results down on the scratch paper.

Moving on to the transmission, the center alignment dowel must be installed, along with the Hydramax bearing with the notch locked onto the dowel in the fully seated position. Use a straight edge on the transmission, running across mounting surface for the bellhousing. Measure from the transmission mating surface to the face of the bearing where it rides on the diaphragm. Measure in three places and record the results.

The two sets of measurements provide the variables needed for the final calculation using the following formula. This equation will tell you how many shims are needed to achieve the proper air gap between the bearing and the diaphragm. The recommended air gap is .150-.200”, you can have as little as .100” (absolute minimum), but try to stay inside the recommended range. It is possible on some GM applications to not have enough room to get to even the .100”. If this is the case, you may have to shim the actual transmission from the bell housing using washers or spacer. This is due to the shallow nature of the GM bell housing design. There are 3 shim thicknesses for the big 3. Mopar uses .063, GM uses .090 and Ford uses .057. The conical shape of the GM shims creates the .090 thickness, even though the metal used to create the shim is not that thick.


Bell facing to clutch diaphragm – minus Bearing Face to Trans bell face – minus .150 / .090 = Number of shims 


For example: Bell to diaphragm= 2.450, bearing to trans mount face= 2.125

2.450 – 2.125 – .150 / .090 = 1.94

This means your application needs 2 shims, which yields an air gap of .180”, which is perfectly acceptable.

For GM T56, Tremec Magnum, and Viper 6-speeds, American Powertrain recommends the following procedure for measurements:

Install the clutch and pressure plate to the flywheel on the engine. Place the straight edge across the fingers of the diaphragm and measure from the inside edge to the block mating surface. Record this measurement as A.

Next, install the Hydramax bearing retainer base with guide pin to the transmission and load the bearing onto the retainer, with the guide pin in position on the bearing. Install the bellhousing to the transmission. Place a straight edge across the bellhousing and measure to the bearing face. Record this measurement as B.

Using the formula below, calculate the number of shims. A block plate for a scattershield can be factored in by adding the thickness to measurement B.


B-A-.150 / .090 = # of shims


Once you have determined the number of shims, count them up and stack them behind the release bearing on the transmission shaft, install the bearing, and mate the engine to the transmission. It is that simple. Before you mate the trans to the engine, lube the o-ring on the inside of the bearing base with a little DOT 3 brake fluid.

To demonstrate the process, we installed a HydraMax hydraulic release bearing system on a GM LS- series engine with a Muncie 4-speed (mechanical linkage with an LS is not a simple proposition) which was being installed into a 1969 Chevelle. Setting up the bearing was simple, and the results yielded a smooth clutch without any hiccups.

The process begins by mounting the flywheel to the engine. We torqued the bolts to spec.

The clutch does not have to be installed if this is a mockup, but if it is the final install, make sure it is in there. Be sure to put the clutch in using the correct orientation.

Use an alignment tool to keep the clutch in position.

Next, the diaphragm is installed to the flywheel. There may be multiple bolt patterns on the flywheel, so you might have to find the right holes and line it up.

Use a couple of bolts to secure the bellhousing and then use a straight edge and ruler or caliper to measure the depth of the transmission mounting pad to the diaphragm fingers. Measure in 3 different places and note each measurement. This is measurement A.

The hydraulic release bearing mounts on a stud in the transmission. The American Powertrain kit comes with several studs to match the threads from the front bearing cover. Remove one bolt and match the threads.

Then the stud is installed into the transmission. Don’t forget to use medium threadlocker on the threads and bend the retaining tabs back over the hex on the stud.

The Hydramax bearing slides over the input shaft and locks in place on the stud, this is a free-floating unit, it does get bolted down.

Now measure the height of the release bearing to the transmission mounting flange. Make sure
that you measure to the top of the bearing. This is measurement B.

Using the formula, we determined we need 2 shims: (A – B)-.150”/.090 will determine the number of shims required. For example, 2.45-2.125- .150/.090 = 1.9 shims, so you use 2 shims. You can run as little gap as .100”, but .150 to .200 is optimum. Stack up the shims and install them behind the bearing.

At this point, the bellhousing is removed from the engine and bolted to the transmission using new grade 8 bolts.

Each hydraulic line threads onto the bearing and the line ran out of the bellhousing through the clutch fork hole. The lines are secured with a wire clamp.

Finally, the engine and trans are mated together and ready for installation in the vehicle.

Written By: Jefferson Bryant


Red Dirt Rodz
4518 Braxton Ln
Stillwater, OK 74074
405-880- 5343

American Powertrain
350 Transport Dr
Cookeville, TN 38506
1-931- 646-4836

American Powertrain Expands Distribution Adds Original Parts Group, Inc. (OPGI) to Dealer Network

Now offers American Powertrain 5 and 6 speed transmission conversion kits for Chevrolet A Body Vehicles

American Powertrain, the world’s largest TREMEC dealer and manufacturer of muscle car transmission systems and related components, is pleased to announce that the company’s products are now available at Original Parts Group Inc. (OPGI).  OPGI is the industry’s leading supplier and manufacturer of restoration and high performance parts for classic GM vehicles offering over 75,000 classic GM restoration and performance parts and accessories including transmission replacement parts.

Tony Giroux, OPGI New Products/High Performance Product Manager, says, “We are widely known for our comprehensive selection of GM restoration parts. In actuality, OPGI now offers many high-performance replacement products; though until now, never a transmission conversion kit. We have long noticed that 5-speed and 6-speed swaps are growing in popularity, especially for Chevelle, so the timing was right to investigate a conversion kit supplier that had the quality and depth of product we are looking for all our A and G Body vehicles. I’ve long been familiar with American Powertrain and this company fits our standards in product quality, ease of instruction and customer service.”

OPGI is now offering both 5 and 6-speed conversion kits for popular classics including the GTO, LeMans, Chevelle, El Camino, Cutlass, 4-4-2, Skylark, Monte Carlo, Regal and GS.

Currently, the 5 speed ProFit TKO transmission conversion kits for the GTO are featured in the company’s GTO free print catalog as well as being available online. Additional American Powertrain product offerings include the Hydramax T56 and Muncie/TKO hydraulic clutch kits and GM Bellhousing spacer.

Matt Graves, American Powertrain’s Marketing Manager, says, “OPGI is highly respected as the authority for restoration GM parts and is swiftly gearing up their high-performance product line. We are very excited to add OPGI to our growing dealer network and look forward to growing our sales with them.”

Giroux adds, “American Powertrain is now in the GTO catalog because our largest catalog, the Chevrolet Chevelle, was already out to the public. This has been an excellent decision based on the feedback we have received to date from our customers.”

All OPGI catalogs are completely free and available by mail, phone (1-800-243-8355) or through the OPGI website at

For more information visit To speak with one of American Powertrain’s highly qualified technicians, call 931-646-4836. For weekend and holiday calls, a call back service is available at 423-773-9789.

American Powertrain Announces New Pro-Fit 6-Speed System for 1968-82 C3 Corvettes

System includes TREMEC® MAGNUM transmission 

American Powertrain, the world’s largest TREMEC dealer, is now introducing the next generation Pro-Fit 6-speed system for 1968-82 C3 Corvettes.  The system looks like the original 4-speed transmission with the correct shifter position in the factory console.

Designed as a complete kit, it features an American Powertrain customized TREMEC® MAGNUM 6-speed transmission that will shift smooth as silk, a new crossmember, driveshaft, trans mount, offset shifter mechanism, calibrated speedo drive, speedo cable, reverse light harness, 6-speed shift pattern plate, and mounting hardware.  The transmission holds 700 lb./ft. and will shift at 7,000 RPM!

American Powertrain’s DOM steel driveshaft, built with an exclusive billet face flange yoke, will make driveshaft installation a five-minute job, even in the narrow tunnel of a classic Vette. Options are available to upgrade to an aluminum, chromoly or carbon fiber driveshaft.

The price for the 6-speed Pro-Fit Kit for C3 Corvettes starts at $4,395, transmission included. Bellhousing is sold separately. All Pro-Fit kits are covered by American Powertrain’s two-year warranty and includes free extended hours tech support.

For more information visit To speak with one of American Powertrain’s highly qualified technicians, call 931-646-4836. During weekends and holidays, call 423-773-9789.

American Powertrain Announces Tunnel Hump For 60-72 Chevy C-10

Getting frustrated when installing your TKO or MAGNUM Pro-Fit kit for 60-72 C10, and unable to find a factory hump? They are nearly impossible to find and depending on how you are building your truck, transmission clearance can also be an issue with the factory steel humps. American Powertrain announces their new Tunnel Hump as the final missing piece. The fiberglass Tunnel Hump offers more room for fitment and is a cleaner install option than factory steel humps because the factory hole (that normally has to be blocked) was not incorporated into the American Powertrain design. With this piece, there will only be a hole where you place it for your shifter to fit through. Made in the USA, 100% gel coated fiberglass. Simply install with screws or bonding adhesive.