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GM G-body 3.8L V6 to Olds small block V8 swap; Olds small block to big block swap

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.


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.


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.

The second way to make this work is to simply cut out a flat metal extender tab that will go from the top outermost hole in the bracket. This is the hole in the bracket that the A/C compressor pivots on when you adjust the belt. In the above picture, it is the upper left most hole. The bracket you cut will connect the upper left hole to the long bolt that runs through the compressor.

    The second problem is getting the power steering and alternator mounted. If you use the power steering pump from the donor big block, the biggest problem is that the flared fittings on the end of the power steering hoses will not mate to the metric O-ringed connections in the steering gear box. I have heard that there is an adapter fitting to solve that, but I have no information on where to get it. The best thing to do is simply use all of the 307 brackets and alternator and power steering pump, EXCEPT for the one I am pointing to in the picture. Get that one off of a early 70's to 1976 Olds vehicle with a 455. A 1975 or 1976 Delta 88 would be a great donor car. The key is to look for a vehicle with the same type of power steering pump the G-body has. The earlier (1960's) 455's had a very different looking power steering pump that kind of reminds me of a Genie's bottle. You want the bracket that looks just like the 307 triangular bracket, except it is a little SMALLER (not larger).

Click on the picture for a larger view

Now you will need a longer A/C belt, and possibly a longer alternator belt. If you do, just read the stock belt lengths and get something an inch or two longer. The belt part numbers tell their length. For example, a 15545 belt is a 15 series, 54.5 inches long. A 15440 is 44.0 inches long. Simple!

The Olds high performance W and Z manifolds will fit a 400-425-455 in a G-body without problems. These manifolds can be identified by a large letter "W" on the driver's side manifold, and a large letter "Z" on the passenger side manifold. The manifold runners are smoothly swept, almost like a shorty header, and the center 2 ports are NOT siamesed - there are 4 individual exhaust runners. The only trick will be getting new down pipes made. Torque Tech performance exhaust may be able to help, although they do NOT currently make an exhaust kit for the G-body using a big block Olds. See http://www.torquetechexh.com/gmmanifold.htm

As far as header fitment goes, the only direct fit header currently available is the Hooker Super Competition #3203 (1-7/8" primary tube), but as of January 2004 Kooks Custom Headers is developing a set of INSIDE the chassis 1-7/8" and 2" primary tube G-body headers! Click here to go to the Kooks Custom Headers home page. The Hooker Super competition headers are somewhere around $500, and and have 1 removable tube on each side that wraps around the frame rails. This necessitates narrow front tires and rims to be able to turn the tires all the way left and right. Kooks headers are some of the best headers around - they have very thick laser cut flanges, and their workmanship and design is so nice they look more like art than an ordinary header. I have seen many people use small block 1-3/4" headers, and they usually come too close to the oil filter, and also run right into the power steering shaft on the driver's side. The usual fix is to almost flatten the tube(s) to clear - ugh! If you are a good fabricator and can cut and weld header tubes, Hedman Street Hedders #58070 (1-3/4" small block, $150 from Summit Racing) or Hooker Super Competition #3108 (1-3/4" small block, $315) may be a good starting point.

I recently modified a set of Dynomax # 85096 (1-3/4" small block G-body, $121) headers to fit the 455 in a G-body. The modification of the passenger side header is straightforeward - I simply lengthened the 4 tubes where they all point straight down about 1-1/4". The driver's side header took some reworking though, as the big block's higher deck height raises the first 3 tubes into the steering shaft. I had to reroute 2 of the tubes (swap them), and also modify the 3rd tube. 2 dents were also needed. The driver's side header was a bit of work to complete, with 15 welded joints. See the pics of the Dynomax 85096 header modification below.

Dynomax 85096 driver's side as purchased

Dynomax 85096 driver's side, the first 2 tubes cut

Dynomax 85096 driver's after all modifications (notice welds)

Dynomax 85096 passenger's side as purchased (right) laying next to the coated passenger side header on my '86 Cutlass

Dynomax 85096 passenger's side after lengthening 4 tubes

A-body headers do not fit because their chassis is much wider than the G-body's, making the header tubes or collector run into the lower A-arm perch. The rear tube on the driver's header also hits the fire wall because the G-body's fire wall is very close to the back of the cylinder head. CLICK HERE to go to the '86 Cutlass page to see more on the inside the chassis headers I fabricated for my car from Dynomax A-body headers.

What follows is directly copied from Steve O., screen name "olds72468", a member of www.RealOldsPower.com who used Hedman #58070, 1-3/4" small block G-body headers on a big block (click HERE to go directly to his header modification page):

"The Hedman headers fit, what a freaking nightmare!!!

I got the big stock starter in there, the passenger side is in but I will have to either cut off the filter and run the remote setup I allready have. Or swap the filter adapter to something from a later engine? I have an old Toro filter adapter, it would work if it didn't stick out so far towards the frame. I think im just gonna run the filter thats allready on there until I can get to the junkyard and look for a filter adapter off of a 307. I need one that sits up about 1 inch higher towards the top of the engine.

The drivers side header is the problem. I had to put a small dent in the #3 cyllinder tube to make it clear the steering shaft. I also had to cut a little bit off of the drivers side control arm to clear the same tube (there's still plenty of meat on there, just trimmed some off of the back side by the rear bushing with the trusty sawzall).

I had to take out 1/8" of shims from the back of the control arm to get more clearance for that damn #3 tube too. I took 1/8 out from the passenger side also to keep things even. I reduced my positive caster slightly, and increased positive camber, The wheels were leaning in on top anyway (negative camber) Im gonna have to give it my complete driveway alignment anyway. Also had to hook the wiring to the starter and install the header and starter at the same time.

Other than all of that BS they do fit the car!!! They have good ground clearance. The starter is all wraped up in those tubes worse than I've ever seen though, could cause problems."

Click HERE to go to Steve O.'s web site. Again, I myself have not tried the #58070 small block headers on a big block, so I cannot personally verify the above report of the job. Click HERE to go to a post on Oldspower.com to see Steve O.'s header fitment job pictures. NOTE: I would NOT recommend removing alignment shims to get headers to fit. I would assume the above headers will require some welding to use. In a later post, Steve eliminated the steering column shaft interference by cutting out a short section of the #3 and 5 tubes, and welding in new pieces (see Oldspower.com thread for pics). Unless you like to cut and weld header tubes, the W & Z iron manifolds start to look better and better for a street car. I had a friend with W & Z iron manifolds on a 8-1/2:1 compression 455 that ran the 1/4 mile in 13.4 seconds with 2-1/4" full dual exhaust, so a street big block can perform well with the manifolds.

A 455 with an Edlelbrock Performer intake, a Quadrajet, and a 2-1/2" tall open element chrome drop base air cleaner WILL fit under the stock flat hood. It will be so close it will rub the hood insulation, and you will need a very low profile wing nut, but it will fit. You will also need to hammer a dent into the floor of the drop base to clear the Q-jet's accelerator pump arm.

Make sure you have a good cooling system with a good thermal clutch fan with your new big block. Do NOT use the "heavy duty" clutch fans, as they are for towing and are too stiff, and will result in dramatic power loss from the fan turning too quickly. A new standard 307 thermal clutch fan is best. Use a water pump for a G-body with a 307. Heavy duty aluminum water pumps are available from GMB as well as General Motors. I would recommend an upgraded radiator too. The best ones are from Be Cool or Howe - 2 row aluminum radiators. They have 2 wide rows instead of 4 narrow ones, so they cool better than copper 4 core radiators. They are rather pricey though. Griffin makes a more generic one that may fit, but I believe it does not come with an internal transmission cooler. Some people have adapted larger 4 core radiators from earlier cars. Some have used the GM plastic/aluminum radiators. Some have purchased custom 4 core copper radiators. I would measure the stock radiator width and height, and start searching!

To be continued...


* Updated 01/18/04 - More technical info coming soon!