Thunderbird, Cougar, and Supercoupe Information

Wednesday, May 10, 2006

Eaton M90 Supercharger Rebuild and Information

The Eaton M90 Supercharger is the heart of the Ford Thunderbird's & Mercury Cougar XR7's power. It is a Roots-type positive displacement pump which significantly increases low rpm torque and horsepower in these 3.8L V6 motor.

The M90 has been tested for durability to over 150,000 miles, without maintenance. It has proven highly reliable in real world use not only in our cars, but other automotive applications. Many Thunderbird SC & Cougar XR7 owners have found these units can function for a long time under the most extreme conditions. In rare instances, they have even been found operating without lubrication!

While it has earned a reputation for its great performance and reliability, the Eaton M90 is now reaching over 16 years old on our cars. Like all machines, it eventually falls susceptible to wear and tear. Fortunately, almost every part found inside the unit can be replaced.

The Eaton blower is divided into two main sections. These are the snout assembly, and the rotor pack.

Rebuilding the Snout Assembly
The snout assembly is the most commonly rebuilt section of the supercharger. These parts are readily available, and replacing them is not especially technical. The three main components are discussed below.

Snout Seal

Many owners of original condition Supercoupes & XR7s find themselves regularly adding oil to their superchargers. When the seal in front of the snout wears out, fluid leaks through gradually. Over time, this tends to discolor the supercharger snout and surrounding areas with dirt & grime buildup.

While the stock seal is still available, a superior replacement for this application is made by Chicago Rawhide, part number 7968. It is a double lipped seal made of Viton for greater chemical and temperature resistance. The design is considered to give it greater sealing capacity under pressure and help stop dirt entering from the outside. This seal can be purchased from

Coupler Assembly

The coupler is a composite disc which engages the rotors via pins found on the drive gear and drive shaft. The stock unit is a nylon bumper and metal spring mechanism to isolate the rotors from shock and reduce noise from vibration.

When an otherwise properly functioning supercharger makes a distinct rattling sound from the front end, a worn coupler may be the culprit.

OEM (stock) and several aftermarket replacement couplers are available. However, a solid composite coupler is made available through TexasThunderbirds. This unit is considered a superior alternative in durability to the stock part by many in the SC and XR7 community.

Snout Bearings

Two main bearings are found inside the snout, one in front near the pulley and one at the rear near the coupler. These are discs containing metal ball bearings which surround the drive shaft, allowing it to spin freely.

The noise associated with worn snout bearings is often described as sounding like marbles rolling around inside the supercharger case, or simply grinding. Placing an automotive stethoscope on the snout or other locations of the supercharger will help determine the source of a noise such as this.

Replacement snout bearings are made by MRC, SKF, and NSK. These bearings are similar in design, MRC is actually owned by SKF. They can be purchased seperately from, or in value packs with other M90 parts at

Front Bearing Part Numbers:
* MRC - #203S
* NSK - # NC6203C3X28
* SKF - #6203

Rear Bearing Numbers:
* MRC - #204S
* NSK - NSKNC6204C3X28
* SKF - #6204

Helpful Instructions on Rebuilding the Snout Assembly

Various people have created instructions to provide guidance when rebuilding the snout assembly. These can be found at the following sites:

Blower Nose Seal and Pulley Replacement
How to replace an M90 snout coupler
Rebuilding your Eaton snout
Supercharger Snout Rebuilding Instructions (These can also be found here)

Repairing the Rotor Assembly

The rotor assembly of the Eaton m90 is very durable under normal conditions, and usually requires far less attention than the snout. However, special conditions can cause various parts of the rotor pack to wear out faster than normal. Increasing blower speed without other supporting modifications is one such instance.

When excessive wear to the rotor pack occurs, there are two options. Either the rotor pack can be replaced as an entire assembly, or an attempt can be made to locate for purchase and replace the individual parts which have worn out.

Rotor Pack Replacement

Replacing a worn rotor pack is easier than rebuilding it. Whole packs can often be found with less trouble than individual repair parts. Also, special tools and the effort required for rebuilding are unecessary.

Over the lifetime of the M90 design, Eaton has upgraded the rotor packs. Those found in XR7s and '89 through '93 Thunderbirds had uncoated rotors. In '94, Eaton started coating rotors with an epoxy resin. In '97, Eaton upgraded the rotor coating for the Grand Prix GTP. The exact material used is debated, but it appears to have superior durability to the epoxy coating found in '94 and '95 Thunderbirds.

The ultimate purpose of these coatings is to allow less space between rotors. Spacing the rotors closer causes them to move more air for the same RPM speed. The same levels of boost can then be achieved at a lower rpm for reduced wear. This also lowers the air temperature exiting the blower, and thus increases efficiency.

Replacement rotor packs can be purchased from various sources. HighSpeedLab offers rebuilt and coated rotor packs for 450 dollars (at the present time). A lower cost alternative is to find a used unit in good condition. Uncoated or coated Supercoupe rotors can be reliably purchased through SCCOA club members, ebay, and other sources.

Coated GTP rotors from 1997 to 2003 can be modified to fit Thunderbird M90 cases relatively easily. Used units in good condition can be purchased from Morad Parts Company. They can also be found from other members in the classifieds of ClubGP.

In addition to buying rotor packs, entire superchargers can sometimes be bought from these same sources for low prices. In this case, the rotor pack could be removed from the supercharger, and the other parts resold or otherwise reused.

Rebuilding the Rotor Pack

In some cases, rebuilding the rotor pack can be more desirable than replacing it. Cost is lower compared to a new or professionally rebuilt unit. Also, quality of the finished unit can be controlled.

These parts are usually needed for rebuilding the rotor pack:
  • Rotor seals - Chicago Rawhide Part# 8522
Located in front of the rotors, these seperate air in the rotor chamber front oil in the snout. When worn out, air passing through the seals will cause vacuum leaks and supercharger oil will be lost.

  • Front rotor bearings - NSK Part#NC6203C3X28
Replacing these bearings requires tedious realignment of the rotors to specs within thousandths of an inch. Fortunately, they also seem to wear less than other parts in the rotor pack. They are found behind the gear plate, in front of the rotors. Similar to the front snout bearing, excessive wear will result in noise, grinding, and increased drag on moving parts.

  • Rear rotor bearings - INA part #FC65477
These are needle bearings which hold the rotors in place at the rear of the rotor case.

  • Case sealant - Permatex (Loctite) part #51813
This can be used to form a seal when reassembling the blower case. When cured, it is chemical, heat, and tearing resistant. It is also easily removed if the case must be reopened.

While new bearings usually include lubrication, this grease has been discussed as an alternative by the Supercoupe community. It is a silicon grease designed for bearings in high speed applications. Additionaly, Husky grease part #00603 can be used.

These links provide more information about the steps involved to rebuild the rotor pack: