Understanding the B&M 700R4 Transmission

The B&M 700R4 is a four-speed automatic transmission that was originally developed by General Motors in the early 1980s as a successor to the TH350. B&M Racing & Performance Products builds on this proven design with heavy-duty internal components, upgraded clutches, and a high-performance valve body. For Tri-Five Chevy owners (1955, 1956, and 1957 models), swapping from the original two-speed Powerglide or even a three-speed TH350 into a 700R4 represents a major step forward in both highway cruising and stoplight acceleration.

What makes the 700R4 a standout choice is its 0.70:1 overdrive fourth gear. With a typical 3.55:1 rear axle ratio, the engine RPM drops from roughly 3,200 at 70 mph to about 2,400 RPM—a huge improvement for fuel economy, noise levels, and engine longevity. The transmission also features a deep 3.06:1 first gear, giving you off-the-line punch that feels far livelier than a Powerglide’s 1.76:1 first gear.

In stock form the 700R4 can handle engines up to about 350–400 hp. B&M beefs up the planetaries, applies a stronger input drum, and uses a high-capacity torque converter clutch to handle up to 500 ft-lb of torque, making it suitable for mild small blocks and even big blocks in a Tri-Five.

Compatibility with Your Tri-Five Chevy

Before buying a B&M 700R4, you need to understand what else must change. The Tri-Five chassis and drivetrain were not engineered for overdrive transmissions, so several adaptations are required:

  • Engine length: The 700R4 is slightly longer than a Powerglide or TH350. You may need to shorten the driveshaft or use a slip-yoke with the correct spline count (usually 30 spline for 700R4).
  • Crossmember: The factory transmission crossmember will not align with the 700R4 mount pad. Aftermarket crossmembers (e.g., from CPP or Speedway) are available that bolt to the factory frame rails and move the mount rearward.
  • Transmission cooler: A 700R4 requires a dedicated cooler—do not rely on the radiator tank alone. Use a 20,000–25,000 GVWR plate cooler mounted in front of the radiator.
  • Speedometer: The 700R4 uses an electric speedometer sensor, not a mechanical cable. You’ll need a reluctor ring and a VSS signal converter (Dakota Digital makes a good unit) to drive your factory dash speedometer, or switch to a GPS speedometer.
  • TV cable: The throttle valve (TV) cable is the most critical adjustment. It controls line pressure and shift timing. Hook it to the carburetor throttle linkage using a proper bracket (e.g., from Bowler Transmissions or Lokar). An incorrect TV cable setup will burn up the transmission in minutes.
  • Flexplate: The 700R4 uses a 168-tooth flexplate with 3 bolt holes for the torque converter. Ensure your engine has the correct 168t flexplate (small block V8s typically do).
  • Starter position: Some Tri-Fives with aftermarket headers may have starter clearance issues. The 700R4 bellhousing shape is similar to the TH350, so this is rarely a problem.

If you are swapping from a Powerglide, you’ll also need to change the shift linkage because the Powerglide has a different shift pattern. Aftermarket floor shifters like Hurst Quarter Stick or a column-shift adapter kit solve this.

Cost Breakdown of the Installation

Costs vary widely depending on the level of upgrade and where you source parts. Here is a realistic breakdown for a DIY installation in a typical 1957 Chevy Bel Air with a small block V8:

  • B&M 700R4 transmission (new, street/strip version): $1,999 – $2,599
  • Torque converter (2500–2800 stall, lockup): $250 – $400
  • Transmission fluid pump priming tool and fluid: $50 – $100
  • Heavy-duty transmission cooler (plate style): $80 – $150
  • Cooler lines (pre-bent or stainless hose): $50 – $100
  • Aftermarket crossmember: $100 – $200
  • Driveshaft shortening (if needed): $150 – $250
  • TV cable kit with bracket: $80 – $150
  • Speedometer converter (Dakota Digital SGI-5E): $150
  • Flexplate (if not already 168t): $30 – $60
  • Gaskets, sealant, consumables: $30 – $60
  • Transmission jack rental (if not owned): $40 – $60
  • Labor (if hiring a shop): $800 – $1,500

Total DIY: $2,900 – $4,500. With professional installation, expect $3,800 – $6,000.

The largest variable is the transmission itself: B&M also sells a “700R4 Super Street” with upgraded steel planetaries, race-proven clutches, and a billet intermediate servo for about $2,900. For most street-driven Tri-Fives, the standard 700R4 is adequate.

Preparation for Installation

Preparation saves time and prevents mistakes. Gather these items before you lift the car:

  • Vehicle safety stands (4-ton capacity minimum) and wheel chocks. The Tri-Five has a solid axle—support both the front and rear.
  • Transmission jack or a floor jack with a transmission cradle adapter.
  • Complete socket set (metric and SAE), torque wrench, PB Blaster or penetrating oil.
  • New transmission fluid: Dexron III or the newer Dexron VI (12 quarts for full fill).
  • Prefill the torque converter with 1–2 quarts of fluid before installation to avoid destroying the pump on initial startup.
  • Check the flexplate runout—should be less than 0.010 inch. If not, shim the crankshaft flange.
  • Bench test the transmission shift solenoids and lockup clutch (apply 12V to the EPC solenoid and TCC solenoid) to ensure they function before installation.

Safety note: The Tri-Five frame is low to the ground. Use tall jack stands that allow clearance to slide the transmission out from under the car.

Steps to Install the B&M 700R4 Transmission

Follow these steps carefully. The process takes a full weekend for an experienced amateur mechanic.

Step 1: Disconnect Battery and Drain Fluids

Remove the negative battery cable. Drain the engine coolant if you are replacing a transmission with a radiator cooler line in the bottom of the radiator. Drain the old transmission fluid (if any remains) and remove the transmission fill tube.

Step 2: Remove Existing Transmission

  • Place a drain pan under the transmission. Disconnect the driveshaft (mark the yoke orientation). Tri-Fives have a simple two-piece driveshaft with a carrier bearing—unbolt it from the differential and slide it out from the transmission tail housing.
  • Remove the starter (two bolts). The starter can interfere with transmission removal; it’s easier to take it out completely.
  • Disconnect the shift linkage and TV cable (if any). For a stock column shift, unbolt the linkage at the transmission lever.
  • Disconnect the neutral safety switch and backup light wires.
  • Remove the transmission cooler lines at the radiator.
  • Unbolt the transmission mount from the crossmember.
  • Place a transmission jack under the transmission. Remove the bellhousing bolts (four on a 700R4). Carefully pull the transmission rearward until the torque converter is clear of the flexplate. Make sure the converter stays seated on the pump—if it slips out, fluid will spill and the pump may be damaged.
  • Lower the transmission and remove it from under the car.

Step 3: Prepare the New B&M 700R4

Before installing, do these checks:

  • Install the torque converter fully onto the transmission input shaft. You should hear three distinct clicks as three splined surfaces engage (pump drive, stator, input shaft). The converter should sit about 1/8” away from the flexplate face when fully seated.
  • Apply a thin coat of anti-seize to the torque converter pilot hub and the flexplate bolts.
  • Install the transmission mount onto the tail housing (use B&M’s supplied mount, or a universal Energy Suspension mount).
  • Attach the TV cable bracket to the transmission case (passenger side). Tighten the cable anchor set screw loosely—you’ll adjust later.
  • Install the speedometer sensor or VSS adapter if you are using an aftermarket speedometer converter.

Step 4: Install the New Transmission

  • If the car has an aftermarket crossmember, bolt it in place now. If using a stock crossmember, you may need to drill new holes or slot the existing ones—measure carefully.
  • Raise the transmission on a jack. Align the torque converter with the flexplate. Rotate the engine slightly with a breaker bar so the converter bolt holes line up. Trial fit the bolts—do not tighten completely yet.
  • Push the transmission forward until the bellhousing is flush with the engine block. Hand-start all four bellhousing bolts. Tighten to 35 ft-lb (specification for 5/16″ bolts).
  • Tighten the torque converter bolts to 40 ft-lb in a star pattern.
  • Attach the converter-to-flexplate covers (if your version has them).
  • Replace the starter and its wiring.

Step 5: Reconnect Drivetrain

  • Install the driveshaft. If the driveshaft was shortened, ensure the slip yoke slides freely into the tail housing. Apply a small amount of synthetic grease to the splines.
  • Connect the transmission cooler lines. Use a flare nut wrench to avoid rounding. If using aftermarket hoses with AN fittings, torque to spec.
  • Mount the transmission cooler in front of the radiator. Use zip ties or supplied brackets. Connect the lines: flow from transmission OUT to cooler IN, then from cooler OUT to transmission IN.
  • Connect the shift linkage. For column shift, adjust the linkage so that the shift indicator matches the transmission detents. Typically: Park (detent #1), Reverse (#2), Neutral (#3), Drive (#4), Second (#5), First (#6).
  • Connect the backup light wires and neutral safety switch (if using floor shift, adjust to allow starting only in Park or Neutral).

Step 6: Fill with Fluid and Adjust TV Cable

  • Lower the car to the ground. With the engine off, add 5 quarts of Dexron III through the dipstick tube.
  • Start the engine and run at idle for 30 seconds. Check for leaks. Shift through all positions (P,R,N,D,2,1) with your foot on the brake. Add more fluid gradually—total capacity with a cooler is about 11–12 quarts.
  • Check the fluid level on the dipstick. The 700R4 should be checked when hot, with the engine running, transmission in Park. Idle at normal operating temperature (160–200°F).
  • Adjust the TV cable: With the throttle closed (idle), pull the cable until it clicks (zero slack). Then press the throttle to full-open and confirm the cable moves freely. A misadjusted TV cable is the #1 cause of 700R4 failure—if it’s too tight, line pressure drops and the transmission burns clutches; too loose and it shifts too early and slips. Use a TV cable adjustment tool from B&M or follow their procedure exactly.

Step 7: Test Drive and Fine-Tuning

Take a cautious test drive. Accelerate gently from a stop—the transmission should shift through first, second, third, then to overdrive at about 45 mph under light throttle. Under hard throttle, you should feel firm upshifts. If the transmission hesitates or flares between shifts, stop immediately and recheck the TV cable and fluid level.

Adjust the shift speed and firmness by changing the TV cable geometry: moving the cable connection point farther from the throttle pivot increases line pressure and makes shifts sharper. For a mild street car, stock settings work fine.

Real-World Performance of the B&M 700R4

Tri-Five owners who have made this swap report dramatic improvements. On the highway, the overdrive gear reduces engine RPM by about 700–900 rpm at 70 mph compared to a TH350. That translates to nearly 20–25% better fuel economy on long trips—from 14 mpg to 17 mpg in a typical 283 cu-in engine, and over 20 mpg with a 350 and a 3.08 rear gear.

Acceleration also benefits. The 3.06 first gear, combined with a 2500–2800 rpm stall converter, gives strong launches even with a mild cam. At the drag strip, a 1957 Bel Air with a 350 hp small block and B&M 700R4 can knock 0.3–0.5 seconds off its quarter-mile time compared to a Powerglide, while still cruising at 2,200 rpm on the interstate. Owners praise the “two transmissions in one” feel—it’s a stump-puller off the line and an overdrive cruiser on the freeway.

One common note: the lockup torque converter function provides a minor rpm drop (about 200 rpm) when engaged in 4th gear, which further improves fuel economy and reduces transmission fluid temperature. Some owners wire the lockup to a manual switch to keep it off during city driving for smoother operation.

Maintenance Tips

  • Change the transmission fluid and filter every 30,000 miles if used primarily on the highway, or 15,000 miles if raced or used in stop-and-go traffic. Use Dexron III or VI.
  • Inspect the TV cable adjustment annually—cable stretch can change shift timing.
  • Keep the transmission cooler fins clean of debris. A clogged cooler causes overheating.
  • Consider installing a temperature gauge with a sensor in the transmission pan. Ideal operating temp is 160–200°F. At 250°F, fluid life halves rapidly.
  • If you hear a whine or harsh engagement, check the transmission mount bolts and the driveshaft balance.

Conclusion

Installing a B&M 700R4 transmission in your Tri-Five Chevy is one of the most effective upgrades for improving daily drivability without sacrificing performance. With careful planning, proper TV cable setup, and attention to the supporting modifications, you can have a transmission that shifts smoothly, accelerates hard, and lets you cruise the highway with ease. While the initial cost may be higher than a simple rebuild of a Powerglide, the long-term benefits—fuel savings, lower engine wear, and a more enjoyable driving experience—make it a wise investment for any classic Chevy enthusiast.

For further reading, check out B&M’s official transmission page, or visit Summit Racing’s B&M 700R4 selection. The TriFive.com forum also has detailed installation threads and troubleshooting tips.