top of page

The 1966 Chevrolet

427/425HP "L72" Engine

The 427/425 HP L72 engine was the pinnacle of performance for Chevrolet in 1966. Even though it was the top engine in the Chevrolet line-up, only 1856 L72 full-size cars were produced out of a total of 1,499,878 units built for the 1966 Model Year (0.0012%). Exact model specific production totals are not available, but most of the cars assembled with this fabulous engine were Impala and Impala SS 2 Door Hardtop models (about 1250). The next highest volume was the much celebrated Biscayne / Bel-air 2 Door Sedan followed by the Impala and Impala SS convertible. Other body styles that received the L72 were Belair and Impala Station Wagons, Caprice 2 Door Hardtops and even the Caprice 4 Door Hardtop (if you ever see one of these, please let me know !) 

So how can you tell if you have one of the 1856 Full-Size Chevrolets built with the L72 engine ? Unfortunately, most of these cars had their engines "donated" to other performance car projects over the years so it is very hard to find a "numbers matching, original engine" L72 Chevy. Below I've tried to outline what makes a L72 car unique, and what things to look for when hunting for a 1966 427/425HP engine.

All 1966 L72 cars came equipped with manual transmissions. No 1966 L72's were ever built with automatic transmissions (The M40 Turbo 400 became available with the L72 in 1969). Also, Air Conditioning was not available with the L72 engine. The standard transmission was the M13 Heavy Duty 3 speed manual, but no records exist to show that any cars were ever built with that transmission. The most popular transmission choice for the L72 was the M21 Close Ratio Muncie at 1595 units. The reason for that was, if the buyer wanted an axle ratio higher than the standard 3.31, Chevrolet forced you into the M21 transmission (For example, if you wanted say a 4.10 rear axle, you had to upgrade to the M21). The only axle ratio available for the M20 Wide Ratio transmission in 1966 was the 3.31. The legendary M22 "Rock Crusher" was also available with the L72, but only 2 cars were ever built with that transmission (Again, if you ever see one of these, please, please let me know !) For the Chevrolet managed assembly plants the Trim Tag was stamped with a "2Q" to signify that the car was equipped with a M21 or M22 transmission. The BOP (Buick, Olds, Pontiac) Assembly plants did not stamp the Trim Tag with "2Q" so there is no way to tell the transmission type on a BOP assembled Chevrolet. 

The 427/425HP engine had a casting number of 3869942 and was stamped as broadcast code "ID" from the Tonawanda Engine Plant. This casting was also shared with the 427/390HP L36 engine, but the L72 had 4 bolt main bearing caps and a larger oil pressure fitting port than the L36 (On the driver's side of the block above the oil filter). The port was 1" in diameter for the L72 verses 11/16" for the L36. 

Other unique attributes of the 427/425HP engine include larger rectangular port cylinder heads (casting number 3873858), an aluminum intake manifold (casting number 3885069), and larger, more header like exhaust manifolds (casting number 3880827 for the left hand side and casting number 3880828 for the right hand side).

Other items that distinguished the 427/425HP engine from other '66 Big Blocks were the 780 CFM Holley Carburetor (Model 4150, List 3246), the 3 groove lower crank pulley (casting 3869978), 2 groove water pump pulley (stamped 3864480), and the infamous "idler" pulley (part number 3765393). The idler pulley was there to help maintain more than 90 degrees of fan belt coverage of the water pump pulley (To make sure that the fan belt was turning the water pump and not slipping during hard acceleration). All of the pulleys for the L72 were of the deep groove variety to make sure the fan belts stayed in place.

To keep the L72 engine cool, a large 4 core "CW" Harrison radiator (part number 3007719) was utilized (The L72 was the only '66 Chevrolet to use the 4 core. Even the HD L36 Radiator was only a large 3 core).

To complete the L72 engine package, below are the other unique parts for this engine (Some parts like the 3872444 Bell Housing are not shown because they were shared with the L35 396 and the L36 427/390HP engines):

Distributor (non-transistorized) - 1111100

Carburetor Intake Gasket - 3881847

Air Cleaner, Dual Snorkel - 6421970

Air Cleaner Decal, 427 Flag - 3874904

Air Cleaner Decal, Horsepower Banner, TurboJet 425HP - 3880870

Air Cleaner Stud - 3869954

Air Cleaner Vent Elbow - 3868842

Air Cleaner Vent Hose - 3846122

Flame Arrestor - 3849856

Camshaft - 3863143

Pistons (11.0:1) - Standard - 3888342 - 0.030 over - 3888344

Crankshaft - 3869952, Casting # 6223

Oil Pump - 3876868 ( Replaced by 3904826)

Oil Pump Drive Shaft - 3865886

Vacuum Advance - 1115360

Vacuum Advance Tube - 3877640

Intake Manifold Gasket Kit - 3955527 (for Rectangular Port Heads)

8" Harmonic Balancer - 3860010

Timing Chain Cover - 3975941

Fuel Pump - 6416459 (AC Number 40358)

Fuel Lines - Pump to Tee: 3886151, Tee Block: 3886153, From Tee to Carb: 3886152 

Valve Covers (Drippers) - 3877631 (LH), 3877632 (RH)

PCV Valve - 6423438 (CV691C)

PCV Hose - 3871298

PCV Hose Clip (at Base of Carb) - 3846117

Choke Coil - 3887148

Choke Rod - 3885070

Idler Pulley Bracket - 3878226

Water Pump Pulley - 3864480

Crank Pulley - 3869978 (casting number)

Alternator Pulley - 3844100

Alternator Fan - 3843346

Exhaust Pipe (LH) - 3866523

Exhaust Pipe (RH) - 3866524

Muffler (LH) - 3928215

Muffler (RH) - 3884800

Tailpipe (LH) - 3883697

Tailpipe (RH) - 3883698

Tailpipe Hanger (LH) - 3883693

Tailpipe Hanger (RH) - 3883694

Tailpipe Clamps - 1368195

7000 Tachometer (6000 Red Line)- 6412549

If your L72 had Transistorized Ignition, here are some additional parts that are required

7000 Tachometer (6000 Red Line) - 6412572

Distributor - 1111074

T.I. Module - 1115005

T.I. Coil - 1115210

42 Amp Alternator - 1100696

T.I. Harnesses, Main: 2988218, Extension: 6287841, Ignition Lead: 2986913

If your L72 has Power Steering (RPO N40) then you also need a deep groove Power Steering Pump Pulley, part number 3873847. The pump, brackets, and hoses are the same as all 1966 Full-size Chevrolet L35/L36 Big Blocks. 

bottom of page