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Go Back   ClassicBroncos.com Forums > 66-77 Ford Bronco > Bronco FAQ > Engine FAQ's

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6 Cylinder to V8 Conversion 6 Cylinder to V8 Conversion
For Those Thirsty For More Power
This thread is intended to provide a checklist of parts needed and point you to a write-up on how to convert an Early Bronco equipped with the stock inline 6 to a small block V8.

First, A Word Of Advice
Basically, this project involves eliminating everything forward of the transfer case and replacing it with V8 specific parts. Do your homework in advance and don't begin the conversion process until you have ALL the parts required. Otherwise you run the risk of stalling out on your project due to confusion, lack of funds, time scrounging for parts, or just life in general getting in the way. With the all necessary parts acquired and a good gameplan in place, this process can go quickly and minimize the amount of downtime for your Bronco. If only I'd done this, I could have at least still been enjoying a slow 6 cylinder Bronco instead of no Bronco at all for a long time. I now adhere to this principle when undertaking any moderately sized or above project.

Checklist of Needed Parts
Small Block Ford V8- Keep in mind there are no engine parts that are interchangeable from 6 cylinder to V8. You are going to need a V8 complete with intake, carb or EFI, air cleaner, exhaust, fuel pump, fuel lines, ignition, plug wires, starter, water pump and timing cover, accessory brackets (ok, you can re-use the alternator, and A/C or power steering if you have), pulleys, belts and if applicable, bell housing, clutch disc, pressure plate and motor plate. I'm sure I've left something off. The easiest swap is an engine directly from an Early Bronco as it should already be set-up with the appropriate water pump, timing cover, and pulleys. 5.0s and 5.8s can be installed, but may require further ingenuity to make clearance between the front dress of the engine and the radiator. In my opinion, the Explorer 5.0s make the best candidates for V8 conversion because of their short style water pump and timing cover.

Transmission- don't even think about using that stock 6 cylinder tranny
Tramission Adapter (aka Intermediate Housing)
Transmission Output Shaft (aka Spud Shaft)
There are numerous options here depending on your desires and budget. I was able to get the above transmisson pieces as one unit free of charge from a great member on this board. People are always upgrading so check the For Sale or Free Parts section in the Classifieds and you might get lucky like I did.

Shifter Linkages
V8 Clutch Fork
Clutch Linkage- 3 pieces here, if you're swapping in a 5.0 you're also going to need a EFI clutch equalizer bracket (Wild Horses) so that the equalizer bracket has a place to mount to the engine
Motor Towers (Frame Brackets)
Motor Mounts- standard or extreme duty available
Radiator- the smaller 6 cylinder rad will not adequately cool the V8
Radiator Hangers- because the V8 radiator mounts completely different than the I6 radiator
Upper Radiator Hose
Lower Radiator Hose
Throttle Linkage - instead of going with the stock Bronco linkage, you can instead choose a simpler, better looking throttle cable available from BC Broncos or Summit Racing

The Actual Conversion Process
This link will provide an awesome write-up on the how-to process and is very comprehensive. http://www.lonestar-ebc.com/Tech/TechItem.asp?TechID=19 I don't want to be repetitive of his great write-up and I don't think there's anything I'd add to it aside from the above mentioned throttle cable option versus the stock throttle linkage.

If you have anything to add to this post, feel free to add below.

Additional Sources: Tom's Bronco Parts catalog, Wild Horses catalog  

Last edited by bfoldy; 07/26/06 at 11:54 PM..
alien74 on 09/27/07, 11:17 PM
using a 6cyl tranny

22years ago I bought a 6cyl Bronco and after a couple of years conv. to a v-8 351W without much knowledge of the diff. between the two other than the intermediate housing was shorter.

Well needless to say butchered my tunnel and changed the local of crossmember. Well for a couple of years I struggled with breaking carriers and 1st gear in those trannies, I just thought that with 4:11 gears and 38' swampers doing holeshots on hard pavement @ 5 or 6 grand was the culprit and gave up and put an automatic

Well that was 16 years and I have not had any problems with it at all considering almost 400hp but not as much of a led foot.

I wish I knew then what We know now!!!!!!!!!!!!!!!
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Bargain Hunter on 05/18/09, 04:32 PM
I have a 68 with a 170 inline 6. I'm in the process of converting to a 302. I know the output shaft has to be changed to use the factory transmission, but I couldn't help but notice you saying DONT EVEN THINK ABOUT USING STOCK TRANNY! Maybe a dumb question, but why not? This is my first Bronco, just bought it, so expect a few dumb questions.
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wynn68 on 06/26/11, 07:29 PM
Did you ever determine if you had to change the transmission for the inline 6 to v8 swap? I am working on my first bronco project as well. I would like to keep the PTO winch if possible.
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bfoldy on 08/21/11, 07:56 PM
You don't absolutely have to switch transmissions, but you would need to change the transmission to transfer case adapter because the i-6 and v8 transmissions are different lengths. The main reason it's not recommended to use the original 6 cyl trans is because it was not built to handle nearly as much power as the v8 RAT or RAT trannys. In my opinion, it's just as easy to find a v8 3 speed tranny and swap it in than it is to fool with swapping transfer case adapters.

- Sent from my iPhone using Tapatalk
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wyowheeler on 05/12/14, 10:23 PM
The lonestar link above above is no longer a valid address. Anybody know of any similar addresses? I amthinking about swapping my 66 half cab's 6 cylinder to something better. Probably a 302 but have also wondered about a V-6 from a Ford Ranger. Those have decent power and I like the idea of taking the motor, tranny, and tcase all from the same donor, but V-8s Are tough to beat. Can anyone shed some light on this? Thanks
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biofused1 on 01/29/15, 04:56 PM
Doing a 351 motor swap in 66 bronco donor vehicle is 93 ford f150 motor and harness making the change on harness to be mass air have not bought ecu or transmission yet Question 1 the speed density motor and harness that I have can I use the 94-95 mustang ecu. plus is the 94-95 ecu pin locations the same as 88-93
Question 2 If I can use 94-95 ecu does it require anything else on the motor other than the mass air that is needed for the 88-93
Question 3 If I can use 94-95 ecu my harness has a plug for the transmission that is for a e4od is it possible to change plug connections to use 4r70w transmission
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cshepherd70 on 09/23/15, 01:27 PM
Does anyone know how to locate "steps to convert a 6 cylinder to V8 on an early bronco" like the lonestar (from 2006) link states (but not available)? Thank you.
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mynous on 10/12/15, 02:10 PM
for anyone looking for the lonestar-ebc writeup. the wayback machine managed to save a copy (not all pictures are available).

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chrlsful on 06/29/18, 08:10 AM
the 6 cyl 3.03 has 500 HP capacity (local circle trackers use it in tube chassy race cars as it's light'n strong). The RAN v RAT may B about ease of mate-up/length/etc, don't know.
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chrlsful on 06/29/18, 08:11 AM
How bout a bent8 to i6 swap like some of us do (250/4.1 or 300/4.9)?
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broozedss on 10/16/19, 03:28 PM
Here is the Lonestar-ebc write up. The link no longer works
Written, Compiled, and/or Created by: Andrew Lochbaum

Tired of hearing all the people complaining about their old 289/302s and how they are updating them to 90s fuel injected late model 5.0 HO motors, yet you are still running an old straight six with a whopping 100 horsepower? With some minor modifications and financial investment you can be running with the V8 power most Bronco owners had 30 years ago.

I took advantage of all the people updating their stock setups to higher performance engines and transmissions. Those people need to finance their upgrades and on average are selling stock parts at a very good price. Because I could find these parts easily and cheaply, I decided to convert over to a nearly all stock V8 with a manual transmission. The only upgrades over stock were the Duraspark II electronic ignition and the Edelbrock carb and intake. I can then start complaining with the rest of the stock V8 owners about how I need more power and can look into updating later. That may not make total financial sense if I want to start upgrading immediately, but I think I will be happy with the stock V8 setup for years to come.

Parts Needed
In order to do the swap you will need to change the engine, transmission, transmission adapter and radiator.
Notice the six cylinder is shorter due to the difference in the lengths of the engines. The six cylinder transmission is shorter than the RAT V8 transmission and supposedly not strong enough to handle the V8 power. The overall length of the drive train will remain the same, so drive shaft changes are not necessary. Also many of the 6 cylinder linkages will not work with the V8. These include clutch, shifter and accelerator linkages. Most of the existing wiring will work, but some will need to be lengthened. The suspension seems to be sitting at roughly the same height, so you will probably not need new springs. My springs are over 30 years old, so I plan on replacing those soon anyway.

Here is a list of the parts I needed for the conversion:

Complete 302 block (obviously) with heads, oil pan, exhaust manifold, intake manifold, carburetor, air cleaner, flywheel, engine to bell housing shield, bell housing, starter, alternator, water pump and distributor
Motor mount brackets and rubber mounts
Radiator, radiator mounts, radiator L brackets and radiator hoses
Clutch, clutch rod, equalizer bar, equalizer engine bracket, clutch rod and clutch fork push rod
V8 manual transmission
V8 transmission shifter linkages
Transmission adapter for between transmission and transfer case
Accelerator pedal to bell crank rod, bell crank and bell crank to carb rod
In doing the conversion I tried to amass all the parts I needed ahead of time and tried to preassemble them. By doing this I was able to determine if I had all the correct parts, they fit together and I had the necessary fasteners. By doing this I was able to get the Bronco up and running under V8 power within 3 weekends.

Pulling Out The Old Engine
The first step is to pull the old boat anchor out of the Bronco:
Start with the driveline and remove front and rear drive shafts, shifter linkages, clutch linkages and disconnect the speedometer cable. Place a jack under the engine and put a transmission jack under the transmission and transfer case assembly. Unbolt the transmission from the bell housing and remove the transmission and transfer assembly as a unit. Wheel it off to the side for later unbolting.
Next start the process of removing the engine block. Remove the radiator and radiator hoses. Disconnect all the wiring to the engine and label each wire because they will be reused with the V8. Take off the air cleaner, disconnect the accelerator cable from the carburetor and from the accelerator pedal and remove the whole cable. Disconnect the fuel line and plug it with a golf tee. (I also disconnected the fuel line back at the gas tank since I was going to be grinding and welding in the engine bay and didnt want gas fumes floating around.) Disconnect the exhaust pipe from the exhaust manifold and remove the exhaust system from the Bronco. Finally remove the fan, fan pulley and fan belt.

You are now ready to pull the engine. It was not necessary to remove the carburetor, manifolds, alternator or distributor from the engine. Take one last look and make sure there is nothing left connecting the engine to the Bronco. Connect a hoist to the engine, unbolt the motor mounts and pull it. (I connected the hoist to the last exhaust manifold bolt and the top motor mount bolt on the drivers side.)
Now that the engine is out it is time to clean up the engine compartment a little and get ready to weld in the new motor and radiator mounts.

Engine Mounts

I cut the 6 cylinder engine mounts out with a 4 angle grinder. I first cut off as much of the mount as possible with a cut off wheel, and then ground off the rest of the mount with a grinder.
Now get ready to weld on the V8 motor mounts. Probably the best way to determine where to weld in the mounts is to put the new motor on the hoist, put it in position in the engine bay, bolt it to the driveline and then bolt the adapter to the cross member. You can now bolt on the engine mounts and tack weld the mounts in place. (I decided that method was a little too much work and instead took some measurements from another bronco and read the directions from a parts supplier. The measurements were the same between the two: 9 inches back from the front frame cross member. I measured back 9 inches from the cross member and welded the mounts on.) The longer mount is placed on the drivers side. Here is a figure which shows where to weld the mount and a picture of my welded in mounts:
When I assembled everything it turned out that the cross member at the transfer case adapter was " too far rearward. I just grinded out the holes bigger on the top of the crossmember mount so that I did not have to drill new holes in the frame. The only reason I can think of why the cross member bolts did not line up is that there may be a difference in the lengths of the two V8 transmissions. There is only about an inch of clearance between the radiator and the grill, so I would not want it any more forward.

Radiator Mounts

The two engines use totally different mechanisms to mount the radiator. The six radiator is bolted on to sheet metal, while the V8 is clamped between mounts on the top and bottom. To change to the V8 radiator the sheet metal will need to be trimmed out and the new mounts need to be welded in. Here is a six cylinder radiator mount:
The openings thru the sheet metal have roughly the same dimensions, so only the sheet metal from the 6 cylinder mount needs to be trimmed off. I measured the opening for the core to be about 21 x 12 .

After taking measurements from the radiator and from another Bronco, I welded the top mounts in first, then strapped the radiator into position and finally welded in the bottom mounts. The top mounts were mounted such that the bottom of the mount was 3 from the top of the core support. The distance from the center bolt of the hood latch to the drivers side mount hole was 9 1/2 while the distance from the hood latch to the passenger side was 10. This gives a total of 19 1/2 between the holes in the radiator mounts. For the bottom mount I bent the sheet metal so that I had a greater area to weld on. The center portion of sheet metal between the lower mounts was bent down to allow more air flow through the radiator.

Accelerator Linkage

Mount the new accelerator bell crank before the new engine is put in. What worked for me was mounting it pushed up against the top of the firewall and the left edge on a seam (see right picture below). Attach the rod from the accelerator to the bell crank. The rod will attach to the pedal using the same mechanism as the six cylinder's cable.
Here is the factory parts breakdown and how I mounted it:
Drive Train Pre-Bolt Up
Make sure your drive train all bolts up before putting it in the truck. First separate the transfer case from the old adapter. Be careful when removing the adapter as there are needle bearings in the transfer case which may fall out into the case if you are not careful.
Also be careful with the new and old transmissions. The output shaft is not held in place and can slide out causing needle bearings to fall into the bottom of the transmission. In order to put them back into place the transmission has to be almost completely torn apart. You can make a special tool similar to the special ford part to hold it together. I made the tool out of all thread and here is a picture with it:
I bolted the engine, transmission, adapter and transfer case all together and made sure everything fit. I then split it apart between the transmission and adapter in preparation for installing the engine with the transmission.

Installing The New Engine

Now for the moment of truth, installing the new engine and drive train. I put the engine and transmission in as a unit to make it easier stabbing the transmission into the clutch plate. It makes it a little more difficult to drop the engine in, but it will go in. I also bolted the rubber motor mounts to the engine before dropping it in. Now is the time to hoist the engine up and into the Bronco. I attached the hoist chain to an empty bolt hole at the drivers side front head and the other one to the rear of the opposite head. When putting the engine in you will need to tilt the transmission down a few degrees to get everything to slip in. Once everything is lined up bolt the mounts to the frame mounts. I couldnt get the mounts to line up with the rubber mounts bolted on to the engine so I had to unbolt one mount, attach it to the frame mount first and then bolt it to the block. Once everything is bolted in, support the rear of the engine or transmission with a jack. Now stand back and admire that monster under your hood!
Transfer Case Installation
Using a transmission jack lift the transfer case and adapter up and into position. You will probably need to mess with the height of the jack under the engine to get everything to line up. Once the transfer case is bolted up to the transmission install the cross member support. Here is where you will find out if you welded the motor mounts at the right place. As I said earlier it turned out that the holes didnt line up by about for me so I just extended the holes by that amount.

Accessory and Linkage Installation

Install all the accessories which were not already bolted on. Put the radiator in and connect all the hoses up to it, as well as run the heater hoses from the firewall to the engine. If you purchased a V8 stock alternator then this wiring will be the same, hook up the wires to the same posts listed on your labeled wires. The wiring I needed to lengthen/change was the temperature and oil pressure sender units, the starter wire as well as the distributor and coil wiring. Also since I installed the Duraspark II setup, it required a little more fiddling with the wiring. There are a few web pages describing how to install the Duraspark II, so I will not go over it.
The only vacuum lines on my application were from the ported connection on the carburetor to the distributor and from the manifold vacuum to the PCV valve. The fuel line routing will need to be changed based on the carburetor. Attach the rod from the throttle bell crank to the carburetor. Attach the transmission linkages from the shifter to the transmission and mount the clutch linkages. One bracket is bolted on the engine between the block and bell housing. The other bracket on the frame is the same as the six cylinders and can be reused. Install the V8 equalizer bar and run the rods from the pedal to the bar and from the bar to the clutch release arm. Install the drive shafts and exhaust. (I waited until after I fired it up to make sure everything was working before bolting them in.) Take one final look at all the electrical and mechanical connections and when everything looks good, fire that bad boy up!

Last edited by broozedss; 10/16/19 at 03:39 PM..
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