1964 and later small block Oldsmobile V8 engines are the 260, 307, 330, 350, and the 403. All of these engines share the same external dimensions and mounting hole locations.

Big block Oldsmobile V8 engines are comprised of the 400 (early and late), 425, and the 455. All of the big block engines share the same external dimensions and mounting hole locations, but are larger than the small blocks. The main difference is that the big blocks have a 1.3" taller block deck height. This means the cylinder heads are 1.3" farther away from the crankshaft. The engine is about 2" wider and 1" taller. The big block intake manifold is noticably wider.

3.8L BUICK V6 to OLDS SMALL BLOCK V8 SWAP:

First, let's cover converting a G-body Cutlass originally equipped with a Buick derived 3.8L V6 to a small block Olds V8. There are other ways to do this than the way I list, but my way will result in a car that appears exactly like a Cutlass originally equipped with a factory 260 or 307.

The way I prefer to do the swap is to have a "donor" V8 Cutlass. The main parts you will need to convert a 3.8L V6 Cutlass to an Olds V8 Cutlasses are:
   - The steel motor mounts on frame
   - The 307 Cutlass rubber motor mounts (they are different than the ones on the A-body Cutlasses)
   - Engine bay wiring harnesses - the main wiring harness for the gages and alternator unbolts easily once you get the front clip back a few inches and undo 1 bolt from the fire wall connector
   - Computer wiring harness and ECM
   - Battery and charcoal canister are on opposite sides (batt on driver's side on V8 Olds)
   - Fuel lines routed differently - need the main fuel feed and return lines on the car's passenger side for Olds V8's; fuel feed line is on the driver's side w/ V6
   - 3 row V8 radiator
   - A/C system
   - Radiator shroud
   - Battery + and - cables
   - V8 engine front accessories - get the complete front accessories from a 260/307 V8 Cutlass including the power steering pump and brackets

It depends how nice a swap you want to do. I bought a 1985 Cutlass Supreme with a 350 Olds put on top of the 3.8 mounts! The motor looked like it was launching out of the engine bay, and the motor mounts were severely bent from tightening the mount bolts down over the narrower 3.8L mounts. The previous owners said the engine "bolted right up" to the 3.8L mounts. They also put a tiny plastic flex fan on so the fan would clear the shroud... they ran rubber lines across the engine bay to connect the fuel lines... had the battery on the wrong side, and grounded the main negative terminal to the tiny 1/4" water pump studs... a disaster. I pulled the motor and re-installed the engine, and swapped the entire V8 A/C system from a donor car without opening the lines (this takes 2 people).

After removing the V6 engine, remove the cover from the heater box. This is in preparation for swapping the entire A/C system without breaking the lines open. Remove the clips that secure the A/C condensor (the thing in front of the radiator). Now start removing the front clip. This sounds like a lot of work, but it really is quite easy and should only take an hour. Be sure to remove the front tires and the front air dam under the radiator core support before pulling off the front clip. Mark the location of any body shims for reinstallation later. Now move the A/C condensor and secure it away from the front clip. Now remove the front clip. Then CAREFULLY remove the A/C system all in one piece. DO NOT over stress the lines and connections! Wear gloves and safety goggles in case you rupture a line. Have one person supporting the condensor and the compressor, and another supporting the evaporator core and lines. It all should lift out cleanly. After the front clip has been removed, remove the V6 motor mounts from the chassis. Install the 307 V8 mounts in the appropriate holes. The larger mounting tab on the 307 mounts should go up as shown in the picture to the left. While the front clip is off, you may want to take some time to scrub down and paint the frame rails and engine bay.

There are 2 separate wiring harnesses under the hood of the G-body cars. One is for the oil and water gages or dummy lights, alternator, blower motor control, etc. The other is for the ECM (Electronic Control Module = the computer) and the computer sensors. They are completely independent of each other. First remove the single long bolt from the center of the plastic fire wall plug on the driver's side of the car. This is the plug for the harness that has the wires for the gages or dummy lights and alternator, etc. Remove the V6 harness and plug in the correct V8 harness. Reinstall the long bolt.

If you are going to use the ECM and CCC carburetor and distributor:

Unplug the computer wirning harness from all of the sensors in the engine bay. Now remove the 2 small screws that hold on that metal strap that goes over the ECM harness plug on the passenger side fire wall. Go inside the car and pull off the kick panel that contains the ECM - it is to the right of the passenger's feet. Remove any other sensors or connections as needed. Now you should be able to pull the ECM harness out into the passenger compartment through the hole in the fire wall. Reverse the procedure to install the V8 ECM harness.

If you are going to delete the ECM and CCC carburetor and distributor:

Follow the above removal instructions. Make a thin sheet metal block off plate to cover the hole that will be created by the missing ECM harness. Use the metal strap as a template. Now simply install the blockoff plate with the metal strap and the original 2 screws. Use silicone between the plate and the firewall. Now to keep the check engine light from coming on, remove the check engine light driver. This is a small box under the dash board, very near where the ECM was installed. It should be behind the glove box. It will be a pale green or yellow in color, and is a plastic rectangle a few inches long. It will have several wires going into it, and will have a "door" on it. Open the plastic door, and you should see a small removable circuit board. Remove the circuit board, and place it in a plastic bag and mark it. Save it for later.

Now reinstall the front clip! Carefully install the V8 A/C system all in one piece by the reverse of the removal instructions above. Re-run the fuel lines as needed. You should be able to avoid running a new 5/16 vapor line by rebending the stock line or replacing the last few feet so it runs across the engine crossmember. Move the battery & hold down and the vapor canisters such that the battery is on the driver's side of the car.

Before installing the new engine, compare the intake and exhaust manifolds. If you are installing an engine that came from a vehicle without a computer and EGR, and are going to use the ECM and EGR, you will need the intake and exhaust manifolds from any GM car with a 307 with 5A code heads ('81-'84). Do NOT use the intake manifold from an '85 and later 307 with the 7A code heads - they have smaller ports which will not line up with the heads. The intake you want should have 3 NPT ports at the front of the intake for the coolant temperature light, the computer's coolant temperature sesor, and another coolant activated emissions sensor. The rear of the intake should have 2 NPT's - 1 for the heater control valve and 1 for the EFE valve (on the driver's side). The passenger side exhaust manifold needs a provision for the oxygen sensor. If installing an exhaust manifold, replace all of the manifold to head bolts and crossover pipe to manifold bolts with stainless steel bolts, and lube them with anti-seize. Lube the down pipe to manifold spring bolts too. You'll thank me a couple years down the road.

There are 2 plugs on the front of the CCC carb. They plug over the idle mixture screws. With the carb off the car, either take a drill bit and drill into them, then use the hole to pull the plug out, OR from the bottom use a chisel and chisel them out. This will allow you to adjust the idle mixture later if you need to. Basically you will drive the car and use the computer to tune it. If it is too lean at idle & highway cruising, and the computer cannot compensate, you will get a check engine code 44 - lean exhaust. First make sure you do not have a vacuum leak. That is the most common problem creating a code 44, which can be mistaken for having a larger engine. If you do need to richen the mixture a little, turn the idle mixtyure screws counter clockwise (out) 1/2 turn, and clear the computer by unplugging the battery, then drive the car again. The opposite could happen too and you could get a code 45 rich exh.

Many people wonder if the CCC system will work without the Air Injection Reaction system... it will. Click here to read more about the AIR system. Although technically not "legal", removing the AIR pump, metal lines into the heads, and the line to the catalytic converter will clean up the engine bay considerably, and make the engine MUCH easier to work on. The majority of the clutter with CCC engines is from the AIR pump and lines. The emissions output of the car when hot will not increase much, because the AIR system functions primarily at cold startup, and on overly rich conditions such as full throttle and deceleration. If you have an engine without the holes and taps in the heads for the AIR lines, they can be added by drilling and tapping the heads. The threads are an odd sized 3/8" straight (NOT tapered) pipe thread.

Heads with the AIR taps in the heads can be plugged off. If you remove the AIR pump, and you remove the AIR pump bracket, you may need to install shims to re-align the A/C compressor and belt. Some engines have a separate bracket for the AIR pump, some engines have the mounting position as part of the AC bracket. Pay attention when you take it apart.

Another side note... the CCC system will funtion perfectly with a 160 degree thermostat. If you drive the car in northern U.S. winters, you may want to use a 180 degree thermostat. I believe the 160 or 180 degree thermostat extends engine life, and the Olds engines defenitely make more power with a cooler thermostat.

Now begin installation of the V8 engine. As you can see, starting with a V8 car to begin with is easier, but you are much more likely to find a cheap Cutlass with a nice body if it has a blown up V6 in it.

OLDS SMALL BLOCK V8 to BIG BLOCK V8 SWAP:

The main thing to remember here, is that as far as external parts are concerned, the only difference between the small blocks and big blocks is that the heads moved up and out 1.3". The camshaft centerline, the water pump location, and the motor mount locations do not move.

If your donor engine is a Toronado engine, you may need to swap the oil filter mounting pad with one from a 350 or 307. Use the steel frame mounts and the rubber motor mounts for a G-body with a 307! Do not use earlier A-body or other mounts! This is the most common mistake people make! If you do not use the G-body steel and rubber motor mounts, the engine placement will not be correct, and you will create numerous other problems.

Install the engine and transmission and begin reassembly. I have used an upper radiator hose for a '76 Olds Cutlass with a 455. The lower radiator hose would be the same as stock (G-body with a 307). Begin to install the front accessories. I always like to use all of the 307 front accessores, although you do not have to. If you use all the 307 accessores, the installation will be a little neater and easier.

The first problem is that Olds quit making the big blocks in 1976. The R4 style of radial A/C compressor was not used until 1978, so there are no correct A/C brackets to directly bolt the G-body's A/C compressor to the 400/425/455 engines. To solve this, the G-body will work with minor modifications. The rear bracket bolts to the front of the cylinder head, and has a brace that runs to the second cylinder head bolt. There is also a smaller brace that runs to the intake manifold. This smaller brace will not fit. You may be able to get away without it, but if you have access to a torch and a large hammer, you can cut the brace shorter, and pound it flat again at the new required angle and length, and drill a new hole. As far as the front bracket goes, there are 2 ways to make it work. The best way is to get some 3/16" thick hot rolled steel bar. Home Depot sells this for a few bucks. You will need to cut out 2 pieces so that the front A/C bracket mounting locations on the water pump studs can be lengthened 1.3 inches as in the following picture. The tabs you make should be Z-bent such that the new mounting tab will be in line with the original one. If you have access to a welder, weld or have someone else weld the tabs on after trial fitment and bolting on.