Blow Off Valve (BOSCH) Modification for the Supra GA70 (1G-GTE)

By Phil Strong - Auckland, New Zealand


The 1GGTEU equipped Mark-III Supra (as produced in Japan between 1986 and 1992) as stock has no blow-off valve.  Several members of the Supra Club of New Zealand have modified the common Bosch BOV design for the 7M to work on the 1G.  This article describes the process for installing the BOV.

Click on photographs for higher resolution pictures.

Parts Required:


  2. BOV Inlet Adapter (Steel or Stainless)

  • 40mm Square Bar x 90mm Long

  • 3/4" BSP Threaded Pipe x 25mm Long

  3. BOV Outlet Adapter (Any Material)

  • 25mm Dia Pipe x 36 Long

  • 16mm Dia Pipe x 75 mm Long

  4. Rubber Tubing

  • 1 metre of 4mm (bore)

  • 0.3 metre of 25mm (bore)

  5. Miscellaneous

  • 2 for 4 mm tubing

  • 4 for 25mm hosing

  • 2 for 16mm hosing (throttle hose)

  • Gasket material for replacement throttle pipe gasket.

  • Thick gasket material for BOV adapter to throttle pipe.

BOV Inlet Adapter:

  1. Grip 40mm Square in 4 Jaw chuck & M/C end face
  2. Reverse and m/c to length (approx 90 mm)
  3. Mark out position for 3/4 BSP tapped hole
  4. Reset work in 4 jaw chuck and drill 24.25mm hole.
  5. Tap 3/4 BSP x 14mm
  6. Rest in 4 Jaw chuck - centre drill end - support work using centre piece and turn down to lip diameter and 25mm dia to shoulder.
  7. Remove centre and drill 20mm dia hole to break into tapped hole.
  8. Assemble threaded pipe to adapter (may be silver soldered if required)
  9. Cut thick material gasket washer to locate between adapter and throttle pipe.
  10. Clean and dress adapter to remove flux excess weld and filings etc (blast with compressed air)

BOV Outlet Adapter

  1. Cut pipes to Length and dress ends.
  2. Drill and elongate aperture in 16mm pipe
  3. File concave to end of 25mm pipe until it fits snugly over the 16mm pipe.
  4. Silver solder or braze pipes together
  5. Clean and dress adapter to remove flux excess weld and filings


Throttle Pipe
  1. Carefully remove pipe from car
  2. Remove old gasket from both faces
  3. Punch out plug from pipe
  4. Set pipe in level position using machine vice
  5. Locate hole and drill out to suit 3/4" BSP (24.25mm)
  6. Tap 3/4 BSP thread.
  7. Mill face square to tapped hole then countersink to depth of 1mm
  8. Remove ALL filings and debris from pipe using compressed air.


  1. Fit thick gasket over screw thread of inlet adapter and screw into throttle pipe so that the outlet is inline with the pipe (see above).
  2. Cut (or fit) new gasket for fixing to throttle, and refit throttle pipe to engine.
  3. Cut a 55mm length of 25mm pipe and fit to the outlet adapter.
  4. Cut out a 10-20mm section of the idle air bypass hose (see right), and insert the outlet adapter.
  5. Cut a 50mm length of 25mm pipe and fit to the BOV inlet.
  6. Now assemble the remaining sections as shown above and in the picture at the top of this page.
  7. Cut the 4mm vacuum hose and fix between the BOV and the left hand outlet on the Plenum Chamber. (see lower right)
  8. Fix all hoses with appropriate clamps.