Oldsmobile 403 V8 Engine Guide

An introduction to the Olds 403

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Oldsmobile released the small-block 403 in 1976 for the 1977 model year. General Motors designed the 403 to replace the big-block Olds 455, which GM decided had outlived its usefulness.

The Olds 403 was designed to be a big block in a small-block package. As a result, it has a bore of 4.351 inches (110.5 mm), giving it one of the widest bores of any factory small-block V8. Oldsmobile discontinued the 403 after the 1979 model year. General Motors used the Olds 403 as a ‘corporate engine’ across its line of brands. Oldsmobile, Pontiac, Buick, and GMC offered the Olds 403 in passenger cars and RVs.

Engine builders know that it’s often easier to generate high RPM power with a short stroke. This is where the Olds 403 shines. In addition to its wide bore, it also has an unusually short 3.385-inch stroke.

These characteristics make the Olds 403 an excellent choice (on paper) for high performance builds, as it has the ‘perfect combination’ of bore and stroke.


From the factory, the engine never lived up to its design potential due to fundamental shortcomings. However, engine builders found creative workarounds and made this 6.6-liter GM V8 a respectable powerplant.

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Olds 403 Specifications

The Olds 403 was found in a variety of General Motors vehicles from model year 1977 to 1979. The famous 1977–1978 GMC Motorhome also came equipped with an Oldsmobile 403. A 1977 Oldsmobile Delta 88 served as the official Indy 500 pace car, marking the fourth consecutive year that Oldsmobile held the title.


Oldsmobile vehicles equipped with the 403 CID:

  • 1977 Cutlass
  • 1977 442
  • 1977 Vista Cruiser
  • 1977–1978 Delta 88
  • 1977–1978 Toronado
  • 1977–1979 98
  • 1977–1979 Custom Cruiser

Buick vehicles equipped with the 403 CID:

  • 1977 Century Estate
  • 1977–1978 Riviera
  • 1977–1979 Electra
  • 1977–1979 Estate Wagon
  • 1977–1979 LeSabre

Pontiac vehicles equipped with the 403 CID:

  • 1977 Bonneville
  • 1977 Grand Prix (California Emissions Package)
  • 1977-1979 Catalina Safari
  • 1977–1979 Trans Am
  • 1977–1979 Firebird Formula 6.6L

Oldsmobile 403 Problems

The primary issue with the Olds 403 is its wide bore. Remember, it’s not a big block, and so its large cylinders are too close together for proper cooling. These engines are known for overheating and, in severe cases, blowing head gaskets.


Excessive engine temperature becomes a bigger issue as the motor ages and rust fills coolant passages. Above is an image of a gunked-up 403 water jacket. Buildup of rust and dirt caused this motor to overheat shortly after starting.

Emissions Components

Additionally, emissions regulations forced GM to choke the big-bore Oldsmobile V8 with restrictive heads. The factory heads have large combustion chambers and inadequately small ports.


These characteristics are typical of early smog vehicles from the 1970s. Here we have the Oldsmobile 403 vacuum diagram illustrating the location of required emissions parts.

The EGR valve is a common failure point, though inexpensive to replace. The smog pump (or ‘air pump’) can also cause problems and is known to seize up. Before replacing emissions components on an Olds 403, replace all vacuum lines first. Over time, old rubber lines fossilize, crack, and cause vacuum leaks.

Thanks to the emissions components, the stock 403 had a low compression ratio, bad cylinder heads, and poor fuel economy. From the factory, the Olds 403 was a typical 1970s smog-era motor.

Windowed Main Webs

Another Olds 403 problem is its windowed main webs. Unfortunately, GM decided to hollow out the main webs which (in the simplest terms) increases the likelihood of failure at high RPMs. Essentially, bottom-end strength was sacrificed. Take a look at the picture below. See the holes? The ‘windowed’ main webs are the hollow spaces to the left and right of the crankshaft bearing.


Some Olds 403 engines may have come with solid main webs. Forum rumors and some Oldsmobile experts believe that General Motors briefly produced the solid main 403 in 1976 or 1979 for a few select vehicles, including the Pontiac Bonneville and the Pontiac Trans Am (with tow package). Chris Witt (an Oldsmobile Forums legend) supposedly offered a $1 million bounty for the elusive engine.

Olds 403 Performance Upgrades

The hot rod community found solutions to all the problems that plague the Olds 403. Some modifications are bolt-on, but real performance engines require some extra custom work. But due to its big bore and short stroke, the 403 can be a formidable power plant.


One of the primary power-killers on the Olds 403 are the heads. But this problem is somewhat easy to overcome. Companies produce high-flowing aluminum heads for the Olds 403.

Replacing the heads, removing smog components, and installing a proper exhaust system are the easiest way to dramatically increase horsepower, torque, and even fuel economy. Upgrading the stock cast iron intake manifold and camshaft also provide noticeable benefits.

Solid Main Webs

Some engine builders rework the bottom end of the Olds 403 and weld in patches for the main webs. This custom modification is expensive and time-consuming, but it increases the durability of the motor at high RPMs.

Cooling System

The Olds 403 has a tendency to run hot due to thin cylinder walls. Adding a performance radiator, water pump, and thermostat can reduce problems with overheating. Though if the block is rusty, it may need to be torn apart and cleaned.

How to Fix an Overheating Olds 403

If your Olds 403 overheats, check the usual suspects (radiator cap, coolant level, thermostat), run a pressure test, and flush the cooling system. If problems persist, check the water pump and make sure the stock radiator isn’t clogged.

Is the Olds 403 a Good Motor?

Overall, the Olds 403 is a good motor. It’s reliable when maintained, and has a surprising amount of torque for a stock smog small block.


Cars like the 1977 Oldsmobile Delta 88 Royale (pictured above) continue to drive on American roads. This particular Oldsmobile sat in a Colorado field for 12 years before becoming a daily driver with few repairs.

Plus, the naturally-aspirated Olds 403 can be modified to produce around 600 horsepower. The Olds 403 is perfect for a stock restoration or a daily driver, and also has performance motor potential.

Where to Find Olds 403 Tips and Assistance

There are a number of resources available to Olds 403 owners who need in-depth or specific assistance with their car. The Classic Oldsmobile Forum is an excellent spot for questions and troubleshooting. The G-Body Forum and other classic car sites also provide a wealth of expertise.

Forums are one of the best ways to find useful information on these old motors. However, detailed shop manuals are available used on eBay and other sources. Performance builders can check out books such as Bill Trovato’s Oldsmobile V-8 Engines: How to Build Max Performance, which was published in 2015.