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Simon Briggs' 1971 Triumph Stag V8 conversion

Simon Briggs · 1971 Triumph Stag

Triumph Stag

britishv8.orgas published in BritishV8 Magazine, Volume XVI Issue 2, October 2008

Read the full write-up at britishv8.org ↗

Photo: britishv8.org

A 1971 Triumph Stag rebuilt with a small block Chevy V8

Simon Briggs rescued an abandoned Stag project car from a Virginia field and replaced a failed odd-fire Buick V6 swap with a small block Chevy V8 sourced from a Nova donor car. Over time he moved from a stock 350 and TH350 to a rebuilt, bored and blueprinted engine paired with an automatic overdrive transmission, tackling rear end gearing, halfshafts, and cooling along the way. It is a useful read for anyone weighing up how much a Stag's stock radiator and driveline can really take under a V8.

Specs

As the owner states it.

Engine
Small block Chevy V8, 4 bolt mains, bored to approximately 6 liters, balanced and blueprinted, Edelbrock Performer intake, 600cfm carburetor, and cam; Jacobs computerized ignition; remote oil filter with thermostatically controlled valve and oil cooler
Exhaust
Iron manifolds, rams head style center-dump on passenger side, later-model manifold on driver side for steering clearance; custom exhaust pipes fabricated by owner
Cooling
Alloy radiator and electric fan
Transmission
TH400R7 automatic; custom driveshaft fabricated by owner
Rear End
Stock Triumph Stag rear end with 3.45:1 final drive (in lieu of original 3.7:1); custom fabricated halfshafts using larger Triumph U-joints similar to TR4/TR250/TR6
Suspension (front)
Stock, with urethane bushes
Suspension (rear)
Stock
Brakes
Stock, with cross-drilled rotors
Wheels/Tires
8-spoke alloy wheels, "+1" size
Interior
Sport seats, re-veneered dashboard, stock electrical system, custom audio system with Blaupunkt radio/CD player in a custom console

Parts used

What the owner names in the write-up, grouped the way the Parts page groups them. Where one of the shops we index lists the same thing for this car, it is shown underneath with a link to the shop.

Engine 2

  • Small block Chevy V8 (bored to ~6 liters, 4 bolt mains)

    Rebuilt by owner to a higher spec, balanced and blueprinted, replacing the donor engine that did not hold up

  • Remote oil filter with thermostatic valve and oil cooler

    Added for oil temperature control on the V8 swap

    In the shops
    Oil Cooler Kit with Spin-On Filter ↗Moss Motors – Triumph TR6/TR250· $234.50 USD

Fuel & Ignition 4

  • Edelbrock Performer intake manifoldEdelbrock

    Fitted to the rebuilt small block Chevy

    In the shops
    Intake Manifold Gasket, Rear ↗Moss Motors – Triumph TR6/TR250· $0.00 USD
    Intake Manifold Gasket, Front ↗Moss Motors – Triumph TR6/TR250· $0.00 USD
    Intake/Exhaust Manifold Gasket ↗bpnorthwest.com· $8.99 USD
  • Edelbrock Performer carburetor (600cfm)Edelbrock

    Paired with the Edelbrock intake on the rebuilt engine

  • Edelbrock Performer camshaftEdelbrock

    Part of the higher-spec rebuild of the small block Chevy

  • Jacobs computerized ignition systemJacobs

    Ignition system fitted to the V8 conversion

Cooling 2

Gearbox & Drivetrain 4

Suspension & Steering 1

  • Urethane front suspension bushes

    Fitted to the stock front suspension

Brakes 1

Wheels & Tyres 1

Interior 2

  • Sport seats

    Fitted as part of the interior refresh

  • Re-veneered dashboard

    Refinished as part of the interior work

    In the shops
    Dashboard-Walnut Veneered ↗MGbits4U· £318.36 GBP

Electrical 1

  • Blaupunkt radio/CD playerBlaupunkt

    Mounted in a custom console as part of a custom audio system

    In the shops
    Radio/CD Player (NC) ↗MX5 City· $67.65 USD

Photos

All 4 pictures in the write-up, in the order they were posted, shown from britishv8.org. A picture opens larger here, with the way through to the page it is on.

Simon Briggs' 1971 Triumph Stag with Small Block Chevy V8 EnginePhoto: britishv8.org
small block chevyPhoto: britishv8.org
this cowl induction hood scoop is pretty cute, eh?Photo: britishv8.org
Donate Now!Photo: britishv8.org

How it was done

Starting from an abandoned project

The Stag began life as a parts car found deserted in a Virginia field, where a previous owner had started fitting an odd-fire Buick V6 and a 2-speed automatic. That combination never ran, so Briggs started again.

First V8 transplant

Briggs bought a 1972 Chevy Nova as a donor car for a supposedly rebuilt small block Chevy 350. He fitted this engine with a TH350 transmission and kept the original Stag radiator. It ran, but revs were high because of the stock rear end and small wheels.

Cooling under real conditions

The stock Stag radiator, paired with an electric fan, coped with the Chevy 350 on the highway and even in traffic, only struggling once summer temperatures reached the high 90s and the car stopped moving. Briggs took this as evidence the factory radiator was not the real cause of the Stag's reputed cooling problems.

Engine rebuilt to a higher spec

The donor engine did not hold up well, so Briggs rebuilt a small block Chevy himself to a higher specification, with four bolt mains, overboring to around 6 liters, balancing and blueprinting, and an Edelbrock intake, carburetor and cam. He also swapped in a different transmission at this stage.

Driveline and running gear changes

To suit the V8, Briggs changed the final drive ratio from the standard 3.7:1 to 3.45:1 and replaced the original peened-over U-joint halfshafts with custom fabricated ones using the larger U-joints from the TR4, TR250 and TR6. He also fitted urethane front suspension bushes and cross-drilled brake rotors.

Exhaust and finishing touches

Briggs fabricated his own exhaust pipes and fitted iron manifolds, choosing a rams-head style on the passenger side and a later-model manifold on the driver side to clear the unmodified Stag steering. Inside, he fitted sport seats, a re-veneered dashboard and a custom audio console with a Blaupunkt radio and CD player.

A summary in our own words. The full account, with all its detail, is the owner's: read it at britishv8.org ↗

Worth knowing

  • A stock Stag radiator with an electric fan can cope with a V8 in normal driving, only struggling in extreme heat while stationary
  • Buying a donor car with a reputedly rebuilt engine is not a guarantee of quality, plan to rebuild it yourself
  • Stock small wheels and rear end gearing can leave revs uncomfortably high with a V8, worth upgrading both together
  • The Stag's original peened-over halfshaft U-joints are a weak point and benefit from being replaced with larger Triumph U-joints
  • Manifold choice may need to differ side to side to clear the unmodified steering system

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