EarlyCJ5.com Technical Library

A technical library for classic Jeeps

CHEVY 4.3 & SM 420 SWAP

by Jason "Neptco19"

One of my goals for my jeep was to have an engine that would require very little maintenance. Something that I could just walk out and start up in any weather and not worry about it. Fuel injection does a very good job of adjusting itself for altitude changes and temperature changes due to the fact that there are numerous sensors checking vital engine components to make sure they are operating like they should be. With a carburetor you can tune them to work well but they do not adjust to weather changes well. Now that I decided on going with fuel injection I began looking for what would be a good candidate for the swap. I wanted to stay with a V6 to save some weight and something that I could drive after I got done with the swap and not worry about being to strong for the stock axles. After much research and asking many questions I decided on going with the Chevy 4.3 V6 due to its power and torque ratings in stock form. Another good part about the 4.3 is that most all of the parts are interchangeable with the Chevy small block V8 since they are both designed around the 90° block.

Now the search for a 4.3 was on. I began watching the ads for a used one and eventually found one that was out of an 88 Astro Van. This would be a great candidate for the swap since it was a TBI system. The TBI’s are very easy to wire up for the novice mechanic especially with a painless wiring harness (part # 60101). The TBI’s are built sort of like a carburetor except they are a fuel injection unit. The engine I located came with everything with it including the factory CPU and wiring harness along with all the sensors. The only other things I needed to purchase for the engine would be a manual flywheel, clutch, and a bell housing. I was able to locate the flywheel (Borg & Beck part # 4130012), clutch (Borg # 5551) clutch fork (Borg # 801) and throw out bearing (Borg # 1705) at a local transmission shop. For the bell housing I got one from a junkyard (any bell housing off a manual Chevy smallblock V8 will work) I had to decide between an aluminum one and a steel one. I decided to go with the aluminum due o its lighter weight.

Once I had the engine it was time to locate a transmission for the swap. I decided to go with the SM 420 since it has the lowest first gear of all manual transmissions. The rumor that parts are getting rare and hard to find is just that, a rumor. There are plenty of suppliers that have the parts to rebuild and repair one if needed. I decided to go ahead and purchase a rebuilt 420 and Dana 18 from Herm TheOverdrive Guy. Now I needed an adapter to adapt the two together. There are 2 companies that I know of that produce this adapter, Novak and Advance Adapters. Herm uses the Advance Adapter in his kits so that’s what he provided for the swap.

So now that I had my engine and rebuilt transmission and transfer case I needed to start figuring out how it would mount up compared the dauntless v6, T-90 and Dana 18 drive train that it would be replacing. I needed new motor mounts so I called Novak and they had mounts to mount the Chevy V6 and V8’s into an early CJ5 (part #MM29). When they arrived I read though the instructions which talked about the placement of the mounts and that they needed to be mounted so that there is at least 4* of front to rear slope of the engine. After reading the instructions I began test fitting the drive train to see how it all fit up. I ended up with the engine a little farther forward than some others that have done this swap due to the fact I didn’t want to notch the firewall so that it would clear the distributor. I also knew that I would be using an electric fan so fan clearance wouldn’t be a problem. The only thing I needed to watch was the clearance between the crank pulley and the front cross member. I also needed to place the engine closer to the driver side than the passenger side so there would be enough clearance for the front drive shaft to clear the starter. So I placed it so that the exhaust manifold collector was about ½” from the frame rail on the driver side. The engine ended up being about 1” off center. So once I had it placed exactly where I wanted I traced the motor mounts on the frame so I would know exactly where to weld them up. The arms that bolted to the engine and extend to the frame mounts had to be drilled so they could be bolted up. They come un-drilled due to the fact they are used for multiple applications and to allow the most flexibility in the swap so you can place the engine where it will work the best for you.

Now that I knew where the motor mounts needed to be welded I began making clearance for the taller shift tower on the 420. It didn’t take too much modification of the floor pan to open up enough space for the shift tower. For the cross member I just bolted it to the transfer case like it was originally and lined it up with the frame rails. Then I drilled new holes to mount it up. Now that all this was done it all had to be pulled back out so that the motor mounts could be welded in. In all I “test” fit the drive train about a dozen times to keep checking for clearance for everything. The biggest problem area I had was with the clutch fork. I had to modify it a lot so that it would clear the master cylinder. I would very highly suggest switching to swinging pedals when doing this conversion. Once I had the clutch fork problem worked out I welded in the motor mounts and it was time to bolt it all in.

First thing I did was hook up the clutch linkage to make sure it was going to clear everything all right. I went with the chain clutch linkage from Advance Adapters. (Part # 716640) It was very simple to hook up and got rid of all the cables and cross bars that the original linkage used.


So now that it’s all bolted in its time to start wiring it up. I went with a painless wiring kit (part # 60101). I highly recommend using this harness when doing this swap, unless you have the time to sort through the factory harness and decide what you need and what you don’t. The directions that came with it were very clear and easy to read and understand. All you have to do is pick a place in the firewall to route the wires through and run them to their destination and plug them in. The plugs in the harness can only go in their correct place so it’s pretty much impossible to put one in the incorrect place. The kit is designed to be used with the 700r4 transmission and since I wasn’t, I needed to hook some of the wires to each other to “fool” the CPU into thinking it was being used with the auto transmission. (The directions in the painless book explain which ones to hook up with each other.) I decided to mount the CPU behind the dash under the cowl so that it would be up out of the way and incase the inside is ever filled with water, the CPU will be out of the way. There is a nice little ridge where the cowl and the bottom part of it meet at, that was just big enough to mount the ALDL to. I mounted it there so it would be easy enough to get to, to check if necessary and out of the way so that it wouldn’t be in the way or be very noticeable. I mounted the check engine light on the dash so it would be easily noticed when and if it is set off.


For the fuel system I needed a fuel pump that maintained around 15psi of pressure to operate the TBI. I wanted to go with the Carter P-5001 but it was on a 3-week back order so I went with a Holley (Part # 12927). I mounted the pump on the frame directly behind where my AA chain clutch mount was. The pump ended up being roughly 8” from the tank by the time I added the filter and a little bit of hose. This should be close enough so that it will be gravity fed by the tank and shouldn’t have to struggle for the fuel to pump. For fuel filters I used two filters, one between the tank and pump and one between the pump and TBI. With this many filters it should keep the system clean of any foreign particles that might ruin the pump or the TBI injectors. The fuel line for the inlet and return line requires a GM flare that has a built in o-ring. Luckily NAPA carries pre-made lines with this flare on them so all you have to do is bend them to shape. (#730-4929 for the 3/8” return line and #730-4928 for the 5/16” inlet line) For extra assurance I added a piece of tube onto the ends of their tubes that have a flare to help hold the 3/8” hoses on, so they don’t come off under pressure. One more thing I had to get was the accelerator cable to go from the gas pedal to the TBI unit. I ended up getting one that was roughly 20” and already had the correct size eye hole on it to fit the throttle body (Pioneer part #CA-8469). The cable also had mounts so that it could be secured in the stock location on the intake and had a fitting that fit into the firewall to give it a real clean look. For the gas pedal I had to bend it into a more of a “C” shape so it would have enough space to pull the cable far enough to open the throttle all the way up.




Elsewhere on the web

Mike Boyink's '64 CJ6 also has a 4.3L Chevy swapped in. Some engine pictures are here.