Triumph · 1970–1977The Stag Archive

How-To & Repairs / Transmission

Converting a Stag from automatic to manual

A scope-of-work briefing on swapping the Borg-Warner 35 automatic in a Triumph Stag for a 4-speed manual with Laycock overdrive. Covers the parts required, the trim and pedal-box changes, propshaft and driveshaft implications, and the impact on originality and market value.

Tools needed

  • Trolley jack, axle stands and gearbox jack
  • Full metric and imperial socket sets
  • Torque wrench calibrated for the ranges specified in the workshop manual
  • Drill, rivet gun and basic bodywork tools for pedal-box fitment
  • Multimeter and soldering iron for the overdrive wiring

Parts needed

  • Correct 4-speed manual gearbox (single-rail Stag/TR6-type) with a working Laycock A-type overdrive
  • Manual bellhousing, clutch fork, release bearing and slave cylinder
  • Flywheel to suit the manual conversion, plus starter ring gear as needed
  • New clutch kit — cover, driven plate and release bearing as a set
  • Clutch master cylinder, pipework and pedal-box assembly for a manual car
  • Manual gear lever, gaiter, knob and correct centre-console/tunnel trim
  • Propshaft of the correct length for the manual gearbox output flange
  • Overdrive switch, wiring loom, relay and inhibitor switches
  • Speedometer drive/cable of the correct ratio for the manual box
  • New rear gearbox mount, bellhousing bolts and gasket set

Warnings

  • Keep every automatic part — bellhousing, torque converter, gearbox, cooler lines, pedal box and trim — labelled and stored. A future owner may want the car returned to its original specification.
  • A converted car is not the same as a factory manual-overdrive car in the eyes of many buyers. Declare the conversion honestly at sale and keep receipts and photographs of the work.
  • Torque figures, clutch alignment procedures and overdrive wiring must be taken from the official Triumph workshop manual for the specific gearbox fitted, not from a summary.

Steps

  1. 01. Understand what you are taking on before you buy parts

    A factory-optioned manual-overdrive Stag is generally the more valuable car, so converting an automatic can protect long-term value — but only if the job is done properly and reversibly. Keep every automatic part you remove, labelled and boxed, so a future owner can put the car back to its original specification if they wish.

    Warning:This is not a bolt-in swap. It affects the pedal box, transmission tunnel trim, wiring, propshaft and speedometer drive. Cost the parts and the trim work in full before starting.

  2. 02. Source a matched gearbox, bellhousing and overdrive

    Buy the gearbox, bellhousing, clutch fork and slave cylinder as a matched set from a known Stag or TR6 donor where possible. Confirm the overdrive engages and disengages cleanly on the bench, and inspect the tail-housing for wear before committing. A rebuilt overdrive unit from a marque specialist is money well spent.

  3. Owner-uploaded photo slot
    Donor gearbox and overdrive on the bench. The 4-speed gearbox with its Laycock overdrive, cleaned and inspected before fitting.

  4. 03. Remove the automatic gearbox, torque converter and cooler lines

    Drain the automatic, disconnect the cooler lines at the radiator and the shift linkage at the selector, then support and lower the gearbox on a proper gearbox jack. Cap the cooler lines to keep debris out and set the automatic aside intact — do not scrap it.

  5. 04. Fit the manual flywheel, clutch and bellhousing

    Fit the correct flywheel with new bolts to the torque specified in the workshop manual, followed by the new clutch cover and driven plate using an alignment tool. Bolt the manual bellhousing to the block and offer the gearbox up on the jack, engaging the input shaft squarely — never pull it in with the bellhousing bolts.

  6. Owner-uploaded photo slot
    Flywheel and clutch fitted, gearbox offered up. The manual flywheel and clutch in place, with the gearbox on the jack ready to mate to the bellhousing.

  7. 05. Fit the manual pedal box, clutch hydraulics and gear lever

    Swap the automatic pedal box for a manual assembly with a clutch pedal, then fit the clutch master cylinder, pipework and slave cylinder. Cut and trim the transmission tunnel and centre console as needed for the manual gear lever, and fit the correct gaiter and knob.

  8. Owner-uploaded photo slot
    Manual pedal box and clutch master fitted. The manual pedal box installed, showing the clutch pedal, master cylinder and pipework.

  9. 06. Wire the overdrive and inhibitor switches

    Run a fused feed to a column-mounted or gear-knob overdrive switch via a relay, taking the switched earth through the gearbox inhibitor switches so overdrive can only operate in top (and third, where fitted). Test the solenoid clicks in the correct gears before road use.

  10. 07. Fit the correct propshaft and speedometer drive

    The manual gearbox is a different length to the automatic, so a propshaft of the correct length must be fitted (or an existing one shortened by a driveline specialist). Fit the speedometer drive of the correct ratio for the manual box and confirm the reading against a GPS on a level road.

  11. Owner-uploaded photo slot
    Correct-length propshaft in place. The propshaft of the correct length fitted between the gearbox output flange and the diff.

  12. 08. Bleed the clutch, road-test and re-torque

    Bleed the clutch hydraulics thoroughly, check gear selection with the engine off and then running, and take the car on a light road test. Re-check gearbox mount bolts, propshaft flange bolts and bellhousing bolts after the first fifty miles.

Related faults

Further reading

Figures and procedures should be confirmed against the official Triumph workshop manual; for safety-critical or specialist work, consult a specialist.

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