389 Pontiac Engine Build

Article by Mark Trotta

When the modern Pontiac V8 was introduced in 1955, it displaced 287 cubic-inches. Over the years, displacement increased several times, but external dimensions remained about the same.

1965 GTO convertible restoration

The Pontiac 389 engine was produced from 1959 to 1966, and has a 4.06" bore and 3.75" stroke. It is probably best known for it's Tri-power option, which was optional on GTO's from 1964 through 1966.

Besides a variety of displacements, the Pontiac V-8 block saw a number of other changes over the years, including starter locations, main-journal diameters, engine-mounting points, and transmission bell-housing bolt patterns. Parts interchange is still pretty good, but do your homework before swapping parts.

*********************************

389 Engine Build

With the motor out of the car, the GTO motor rebuild began with cleaning and disassembly, followed by careful measuring of internal components.

Cylinder Bores

The cylinder bores measured .040" over and had no ridges on the top of the cylinder walls. Since all eight bores were in allowable tolerance, honing and re-ringing the pistons was all that was needed.

389 Bottom End

After inspecting the pistons and connecting rods for wear, the crankshaft was measured and showed .010" under stock on both the main and rod journals. While the crank and rods were at the machine shop being balanced, the cylinder bores were honed.

Rear-Main Rope Seal

Having previously rebuilt a 400ci Pontiac motor from a 1969 Firebird, I was aware of the tricky two-piece rear-main rope seal. Installing it so it doesn't leak requires one side to be tucked slightly in and the other side sticking slightly out of its cap. Same thing for the other half of rope seal in the cap.

After the main bearings were checked for proper clearance with Plastigage, the bearings were lightly lubed and set into the block and main caps. The crankshaft gets torqued down to 100 lb/ft for the front four journals, and 120 lb/ft for the rear journal.

For the oil pump, a TRW #50049 high-pressure unit was chosen.

Tri-power Camshaft Choices

Two different camshafts were used for 1965 GTO engines. A 4-barrel carburetor motor used cam #9779067 (stamped "P"). Tri-power engines used #9779068 (stamped "S").

The 1966 Tri-power camshaft is #9785744 and is stamped "H".

The #9779068 cam was selected. It was also fitted to 421 HO engines and several Ram-Air III applications. Camshaft lift (at 1.50:1) is .407" intake and .447" exhaust. Duration at .050 lift is 288 degrees. Stock lifters will work fine up to 6,000 rpm. Recommended adjustment is .010" tighter than zero lash.

After lubing and carefully installing the cam, a new double-roller timing chain was fitted. The cam gear was set to "0" degrees.

389 Pistons and Rings

Piston rings should be checked in their prospective bores before installing on pistons. Top rings were gapped at .016" and the second rings at .013". A piston ring installer is essential for this.

After the pistons and rods were in, the rod bolts were torqued to 45 ft/lbs.

389 Cylinder Heads (1965-1966)

Correct cylinder heads for the 1965 389 GTO are cast #77 and have 1.96" intake/1.66" exhaust valves. For 1966, most GTO models had a cast #093 head. California-bound GTO's in 1966 had different cylinder heads with an internal air galley.

Pontiac 400 Heads

In 1967, the 389 Pontiac engine was enlarged to 400 cubic-inches. The cylinder heads were redesigned and featured larger valves (2.11" intake and 1.77" exhaust). They were better breathing than earlier heads, and made more power.

Early vs Late Heads

Pontiac V8 heads 1967 and up have a different valve angle from earlier heads, and require longer pushrods and different rocker arms. 1967 and up valve covers were redesigned to clear the new valvetrain geometry (earlier valve covers won't fit).

More Power Please!

In stock trim, the 1965 GTO 389ci Tri-power motor was rated at 360 horsepower. In a properly prepared car, that was good for 14-second quarter-mile times. But I needed to make a little more power.

400ci Heads On 389ci Block

The 389 heads that came with the 1965 GTO were not correct, nor were they in good shape. After doing a little research, I found a pair of 1969 Ram-Air 3 heads (cast #16), which have the large valves and better breathing. They fit on the 389 block with no problems and will work fine, providing you use push rods, rocker arms, and valve covers for the 1967-up Pontiac 400 motor.

Pontiac GTO cylinder heads

By using Pontiac Ram-Air III heads on the 389 block, the factory Tri-power engine output of 360 horsepower is significantly higher.

The motor was painted Pontiac "Robins Egg Blue" from a spray can.

1965 Pontiac engine paint

Torque Specs

Pontiac V8's have just ten head bolts, so correct torquing is critical. They are to be tightened to 90 lb/ft in 20 pound intervals.

On hydraulic cam motors, rocker arms are adjusted by tightening them to 20 lb/ft.

Intake manifold bolts get tightened to 40 lb/ft in 10 pound intervals.

389 Pontiac engine build

Completion

After the Tri-power carbs were rebuilt and reinstalled, the GTO engine looked, sounded and ran great. The #068 cam gives a nice lumpy idle and all the torque you'd ever need for the street.

389 Engine Build

*********************************

Related Articles:

Tri-power Carbs and Manifold

Best Oil for Classic Cars

Pontiac GTO History (1964-1965)

1965 GTO Restoration

Dual Quad 1965 GTO

Read: Dual Quad GTO