Tech Article

B&M HoleShot


B&M HoleShot Converter Construction

Let's take a look at a B&M HoleShot converter starting with the engine side and working our way back toward the transmission..

Four (or Five) Main Parts of the Torque Converter


Attaches converter to the flexplate


Spins input shaft in transmission


Multiplies torque by redirecting fluid flow in converter and incorporates one way clutch assembly


Pumps fluid through converter and drives oil pump in transmission.

Lock-up Assembly

Included in most B&M GM TH-700R4 Converters

Cover Assembly

The cover assembly is essentially the stock cover with the addition of B&M designed drive lugs or modified stock drive lugs. The cover OD is machined to provide a smooth surface for the new body-weld. The pilot is inspected and re-machined as needed to provide a good fit into the crankshaft of the engine. This is extremely important as the pilot on the converter centers the converter behind the crankshaft. Proper alignment of the converter will eliminate most vibration problems related to the Converter and Transmission assembly

Turbine Assembly
The turbine assembly is completely furnace brazed (copper colored material visible on turbine fins). Furnace brazing is a method used to strengthen converter components. It is similar to welding or brazing, however it's method is a little different. Braze material is applied to all of the fin areas, then the entire turbine is placed into a furnace, which heats up the braze material causing it to melt and fill the voids. Not only does this process strengthen the entire assembly it also makes the turbine more efficient by eliminating any of the cross leakage between the fins. The turbine also includes a steel billet turbine hub with broached splines. This stronger turbine hub will eliminate the possibility of stripped splines in the center hub. The raised center section is designed to be a bearing and race pilot for the needle thrust bearing assembly.

Lock-up Clutch Assembly
GM Style - 4L60 and TH-700 R4
This is located between the turbine and the cover. The pressure plate and damper assembly is essentially the same as what is found in a street clutch in front of a manual transmission however it is included in the torque converter. The pressure plate is manufactured from stamped steel and includes a machined location where clutch material is bonded to the plate. A smooth surface is critical for smooth lock-up operation. A wavy surface at this area will decrease the lock-up clutch capacity and result in slippage.

B&M bonds new friction material to the plate in all lock-up converters. This material is very similar to the friction material used on the clutches in the transmission. To achieve lock-up, fluid pressure is released between the cover and the pressure plate allowing the pressure plate to move forward and lock to the machined surface on the cover.

Ford AOD - Split Path Style
This style requires a combination of converter and transmission components to provide the method for lock-up operation. The Ford AOD transmission has a unique turbine shaft and direct drive shaft combination. Torque is transmitted from the converter via the turbine shaft into the forward clutch cylinder in the transmission, much like a typical automatic transmission. To obtain lock-up or direct drive in the AOD the direct drive shaft is splined into a damper at the center of the converter cover assembly. The other end of the shaft splines into the direct clutch assembly. When hydraulic fluid pressure is applied to the direct clutches, direct drive or lock-up occurs. During lock up operation the converter is essentially by-passed. Lock up operation occurs in third and overdrive.

In the B&M HoleShot series converters for the AOD transmission, B&M eliminates the lock up operation with a specially designed turbine hub. This turbine hub contains two splined areas, one for the turbine shaft and the other for the direct drive shaft. This unique style turbine hub eliminates one of the weak links in the AOD transmission, the direct drive shaft. By combining the direct drive shaft and the turbine shaft, third gear and overdrive functions normally, however there is no direct lock up from engine to transmission. This results in only about 4% slippage in the AOD, typical for a Ford C4 or C6 converter. No special input shaft assembly is required when installing a B&M HoleShot, as it will easily install with all of the factory components.

Stator Assembly
The stator has a very important function inside the converter as it multiplies torque by redirecting fluid flow in converter and it incorporates a one-way clutch assembly. The one-way clutch assembly allows the stator to hold, when the converter is multiplying torque and to spin in the direction of the oil flow when not holding.

The removable thrust face rides on the front bearing assembly (two thin races and needle bearing), while the rear thrust surface of the stator pilots the huge 3-1/4" impeller thrust bearing.

The one-way clutch assembly (sprag) is designed to hold the stator when the converter is in the multiplication stage or when the converter is at stall. This is extremely critical at the starting line when the engine is up against the transbrake or is footbrake stalled. The stator is capable of multiplying the torque the engine produces which will help to launch the vehicle. If the one-way clutch slips the vehicle will have a very lazy launch, poor reaction times and lousy 60-foot times. The 22-element sprag found in the B&M Quick 60 converters is the best one-way clutch in the industry.

Impeller and Performance Characteristics
The impeller assembly is the second key area (stator is the other) in the converter when considering the converters performance characteristics. The impeller can be modified to raise or lower the stall speed as well as affect the top end performance of the converter (MPH). By performing a fin bend change on the outside fins you can change the stall and change the top end MPH by a measurable amount.

The impeller also includes an anti-balloon plate and thrust plate for strength and durability in the Nitrous HoleShot models. A new precision ground and heat treated impeller hub is also installed into the assembly. This hub serves two functions, it pilots the converter into the transmission and it drives the fluid pump in the transmission. The bearingized surface of the hub rides on the bushing in the transmission pump and is also the surface the pump seal contacts.

Final inspection of the converter includes concentricity and perpendicularity checks, pressure testing and spin balance to insure the converter will fit and function as designed.




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