E21 M42 Drivetrain Swap

Over the past couple of years, Tim has really noticed the effects of age and use. The car is now 30 years old and has reached nearly a quarter of a million miles. The 1.8 Liter M10 4 cylinder engine has been showing its age and has been significantly down on power. Besides the wear and tear, the age and nature of this engine necessitate regular tune-ups. In 1981, the E21 320i was caught in the middle of BMW's transition to fully electronic engine management systems. The result was a hybrid of new and old technology. Thirty years later, finding a competent mechanic to maintain one of these cars can be difficult and costly. Tim started doing research and looking for ways to breathe new life into his car while improving the power output, reliability, and fuel consumption.

Tim decided that he wanted to transform his car by completely changing engines. There are several options when doing this, but each has its own advantages drawbacks, and challenges. After weighing the options, Tim selected BMW's M42 1.8 Liter 4 cylinder engine. The M42 was introduced to the U.S. market for 1990 in the e30 318i and remained in production until November of 1995 when it was replaced by a very close variant designated M44.

The 1981 U.S. spec M10

The BMW M42

In past generations, BMW had relied on the single overhead cam, rocker on valve, mechanical valve train. Classic motors like the M10, M20, and M30 had powered almost every variant of BMW for many generations. These motors were robust, reliable, and seem to last forever. In their time, they were powerful and efficient when compared with the contemporary offerings from other car companies. Compared to modern motors, their limitations are due to the technology of the valve train and only having two valves per cylinder. The M42 was a remarkable engine when it debuted and it represented a sharp deviation from BMW's tried and true technology of the proceeding thirty years. It pointed in the direction that BMW continues to follow today. The M42 is a twin cam motor and has four valves per cylinder. Its design features hydraulic lifters that are self adjusting. There are no valve adjustments to be done like in the prior generation of engines. The M42 has forged internals and has been described as "overbuilt." It has a 10:1 compression ratio making the most of its 1.8 liters. BMW also designed a tuned exhaust header for the m42. The engine management for the m42 is Bosch Motronic 1.7 which was a sophisticated and efficient system for the time. The end result was an incredibly robust motor putting out 140 horsepower. It is smooth revving, spirited, and efficient. The M42 has approximately 40 more horsepower than the 320i's M10 and will achieve that while using less fuel.

Since the M42 was never offered in the E21, it is more of a complicated swap than some of the other choices. Using this motor means that we will have to modify the front subframe and relocate the mounting points for the motor. The transmission from the 1991 318i will be used along with the driveshaft. Mounts for the driveshaft center bearing will also need to be fabricated. The M42 radiator will be adapted to the E21 and the battery will be relocated to the trunk. In addition, the exhaust will be customized to fit the e21. Along the way, we will be refreshing many bits and pieces, such as the shift linkage. We will also be adapting a 13.6 pound flywheel for this project from a European E21 323i. When the mechanical work is complete we will do all of the necessary wiring to combine the electrical system of the car to the new engine.

The build will be detailed here. Stay tuned for pictures and information on the project.

E21 M42 Swap

The receiving car is a U.S. market 1981 320i 5 speed car. The donor is a 1991 318i 5 speed car.

Parts being used from donor:

Engine (M42B18)

Engine Mounting Arms

Transmission and crossmember (Getrag 240)

Shift Linkage

Driveshaft (complete W/ center bearing and flex disc)

Complete Wiring Harness and ECU

Radiator and associated cooling lines

Air Flow Meter

Air Box

Parts from E21 320i

Wiring Harness (will be modified for use with M42 harness)

Front Subframe (will be modified)

Other Parts

E30 M20 motor mounts

E21 323i flywheel (13.6 pounds)

E21 323i throw out bearing

E30 325i clutch/pressure plate

M20 starter

The car was delivered to us with the engine and transmission already removed. The first step, and the most important, is locating the engine in the engine compartment. We want to make sure that the engine is as far back as possible, centered, and sitting at the proper angle. We started by dropping the motor into the bay, but soon realized that the subframe would need to be modified before the motor could be properly placed in the engine bay. The M10 mounting section on the driver's side needs to be removed. See here:

M10 mounting plate to be removed

We used an angle grinder and a cut off wheel to remove the section boxed in red above.

Once that piece was removed, we were able to put the subframe back in the car and test fit the motor. We wanted to put the motor back toward the firewall to produce the best handling characteristics for the car.

Since the steering rack bolts to the back of the subframe, it creates approximately 10 inches of width from front to back. The oil pan has to straddle the subframe/steering rack, but not touch it. The stock mounting points on the M10 subframe are much to low for the M42. We decided to use M20 E30 325 motor mounts with the M42 mounting arms. Using this arrangement, we measured that we would have to raise the mount significantly. You can see the process of our design in the next few pictures. We selected sheet metal that is the same gauge as the subframe. It is strong, but more difficult to work with due to its thickness.

The angle of these plates was carefully measured to make sure it matched the angle of the motor mounts. To finish off the subframe, we decided to add a large flat plate to the top of the angled bracket. This was done in order to allow the rubber motor mount to sit on a flat stable surface while the offset allows the motor to shift toward the firewall as desired. The end result might be a little overbuilt, but it came out very nice.

Once the fabrication was completed, the subframe was thoroughly cleaned, scuffed, epoxy primed, and painted in a durable satin black.

Since this is a completely custom application, there are many ways to fabricate the subframe. You could start with an e21 m20 subframe, but it would still not be ideal. We had an e21 323i subframe on hand and we looked at what the differences would be. The m20 subframe would still not provide the height needed and it would not be ideal for moving the engine back toward the firewall. It may have required slightly less total fabrication, but in the long run either subframe will work.

The engine bay had many years of built up road grime. We started taking a look and found that the M10's header had been getting hot enough to cause a little heat damage to the inside of the passenger side strut tower. We spent quite a bit of time scrubbing the engine bay in preparation for some repair and paint work. The battery tray was removed as well as the bracket for the air flow matter. There are a few small holes here and there from modifications and mounting of aftermarket accessories. All of these will be welded up, epoxy primed, and painted with some factory color paint. We had some stratos blau (150) mixed up for the repair work.

You can see the heat damage on the passenger side and the evidence of a past battery acid spill all over the driver's side.

Before Cleaning:

Battery Acid spilled on strut tower

After lots of scrubbing:

The motor was also cleaned up. This particular M42 came with a maintenance history. The two weak points of the M42 are the timing components (tensioner, guides, etc.) and the profile gasket. Maintenance records indicate that this motor had already had both of those remedied. The motor was essentially ready to be used in its current condition. There are a few vacuum air lines that will be replaced.

Before putting the motor in the car, Tim decided that he wanted to upgrade to a lightweight flywheel. He selected the e21 323i flywheel. This particular flywheel weighs 13.6 pounds and should be a significant upgrade over the very "massive" M42 flywheel. In order to use the M20 lightweight flywheel, some other things had to be taken into consideration.

M42 Left, M20 right

When using an M20 flywheel, you need to account for the difference in tooth count between the M20 and M42 ring gear. You can change the starter drive gear on the M42 starter, or you can just use an M20 starter. We used a Bosch unit from an e30 325i. There has been some controversy over the use of the m20 starter on an M42. I can assure you that the M20 starter works with no issue. It bolted right up and the wiring is an exact match as well. In order to use the M20 flywheel, you will need to have the engine side of the flywheel machined very lightly in order to clear the 10mm bolts that secure the rear main seal housing to the block. The other option is to use the spacer plate that is on the transmission side of the M42 dual mass flywheel. It is secured by two rivets and they are drilled out in about five minutes. It is placed between the flywheel and the crankshaft.

One complication will be that the spacer from the M42 flywheel is not machined with one large hole to fit over the sleeve in the crank. You will need to increase the diameter of one of the bolt holes in the spacer. This is not difficult, but it is necessary. In order to secure the flywheel to the crank, you will need the corresponding shorter flywheel bolts to use with the M20 flywheel. The M42 flywheel bolts are now significantly too long. As for the clutch, we used a new OEM Sachs E30 325i clutch kit for a single mass flywheel. However, we chose not to use the 325i throw out bearing. The e21 323i throw out bearing is optimal because it will maintain the proper throw of the clutch when using a much thinner flywheel. I also thought I would mention this because I have seen it omitted, but all BMW single mass flywheels use a shim plate between the flywheel bolts and the flywheel.

Shim plate sitting on flywheel

Summary of the m20 flywheel upgrade:

E21 323i flywheel (13.6 pounds)

e30 325i clutch disc and pressure plate

E21 323i throw out bearing

E30 325i starter

E21 323i flywheel bolts (M12 x 1.5 x 28mm)

Flywheel shim plate (part number 1122126282

We took some time to address the transmission before installation. The selector shaft seal was replaced. This is a known leak point for most BMW manual transmissions. A tip that I recently learned is to insert the new seal without removing the old seal. You will have two seals stacked up as a result. There is no harm in doing this and may buy more time before it starts to leak again.

Selector Shaft Seal Replaced on Getrag 240

Since this particular Getrag 240 calls for ATF, we drained the old fluid and filled it with some fresh Red Line synthetic ATF. BMW specified different fluids for many of their transmissions. Some transmissions are clearly marked and some are not. This particular gearbox was stamped "ATF" in black right on the top of the bellhousing. There are also many theories on gearbox oil. Do a little research and use whatever meets the specifications and makes you feel comfortable.

Red Line ATF

We also replaced the shift console bushing and the white shift cup bearing. The rubber bushing was in bad shape. Replacing these parts will tighten up the shift linkage and make for a more positive feel.

New Shift Bushing and Bearing

New Shift Console Bushing Installed

Work on the engine compartment continued. The frame rails were cleaned up, sanded, epoxy primed and painted with factory color stratus blau. The heat damaged area on the passenger side and the area under the battery tray were also repaired. The modified subframe was reinstalled as well as the steering rack. We took this opportunity to install a set of Jesse's (Jester323) urethane steering rack bushings. You can see the red bushings in the ends of the steering rack. These bushings are excellent quality and Jesse has put a lot of time and effort into creating quality products. They are available at http://eurometric.com

Check out the full product line of urethane mounts and bushings for the E21 at eurometric.com

The motor is in the car. The mounting system we came up with worked out very well. We began organizing the wiring harness and deciding where to mount all of the components. The M42 has individual coil packs. We came up with a custom bracket to mount them on the inner fender. This is a bracket fabricated from one piece of sheet metal. It is mounted below the washer bottle and forward a safe distance from the exhaust header.

Custom fabricated coil mount bracket

The next step was laying out the wiring harness. We started by isolating the main fusebox connector from the E21 harness. This is the 15 pin rectangular connector that plugs into the fusebox. BMW wiring diagrams refer to this connector as the "engine connector." We cut open the E21 wiring harness and pulled out all the wires associated with the E21 engine connector.

E21 Rectangular 15 pin "Engine Connector"

Since the M42 wiring harness came from an E30, we chose to source an E30 body harness connector from a '91 E30. This is also known as the C101 connector. You want to find the female side from a donor car and cut it out with as much wire as possible. The connector from a 318i or 325i(s) will work.

C101 Pin out from late E30

The next step is to splice the E21 connector into the E30 C101. This is pretty simple process. However, use care and good products because this is a critical component that the car relies on to function. Each of the splices were soldered and individually insulated with heat shrink. The unused wires were sealed with heat shrink and taped off. Then the whole thing was sealed in large heat shrink. Remember to slide all the heat shrink on the wires before you make your final connections. Here is a chart that I put together to show how we wired the two ends together.

E21 (U.S. 320i) to E30 C101

Once this harness is made, you can plug it into the E21 fusebox and the the C101 will plug right into the M42 Harness.

Some attention was given to the driveshaft. We are using the E30 318i driveshaft. Please note that before we decided on a final position for the motor, we did take into account the length of the driveshaft. As it turns out, the E30 318i driveshaft is ideal. It will bolt up to the E21 differential without any modification. The only complication, and it is considerable, is that the center support bearing will not bolt into the E21 mounting points. New mounting points will have to be fabricated if the E30 318i driveshaft is used. The driveshaft was cleaned up and the center support bearing was replaced with a new Febi unit.

1991 E30 318i Driveshaft

The flex disc from the E30 318i and the E21 320i will both work. They are the same. The one that came on the driveshaft was falling apart. We replaced it while the driveshaft was out of the car.

Cracked and Deteriorated Flex Disc

We started assembling the driveline next. We used the E30 Getrag 240 crossmember to secure the transmission. It bolts right in with one small modification. The mounting holes need to be slotted to line up with the E21 mounting points. After slotting, the subframe will bolt right up. This is a good time to take a measurement and make sure the transmission is centered. Measure between the frame rails because the transmission tunnel is not symmetrical.

The next step was to address the shift linkage. With the transmission mounted in the correct place, it is time to make measurements on the shift linkage. The shift linkage measurements will depend on where you mounted the engine. In our case, the linkage needed to be shortened approximately 1.25 inches. We measured this by finding the amount that needed to be removed in order to place the shift cup on the console in the center of the shifter hole in the transmission tunnel. The E30 aluminum shift console was shortened by removing the specific amount and then TIG welding it back together. If you don't have access or the ability to TIG weld, this would be a good task to have someone else do. The shift rod was shortened the corresponding amount and MIG welded back together.

Shortened Shift Console and Shift Rod

 You can see that the E30 rubber shift boot will fit perfectly in the E21 shifter hole in the tunnel floor.  Also, it turns out that the E30 console mount will attach to the E21 mounting tab that is spot welded to the transmission tunnel. With the way that we mounted all of the components, we found that turning the console mount upside down created the perfect geometry for the shift linkage.

The E30 318i driveshaft will work just fine with regard to overall length. However, as mentioned earlier, the center bearing mounts will have to be moved, or new mounts will need to be fabricated. There are other driveshaft options, but this was our choice. We decided on fabricating new mounts. This measurement will, again, depend on your placement of the engine. In our case, we fabricated new mounts 6.5 inches to the rear of the original mounts. We took careful measurements to ensure that the driveshaft was centered in the tunnel and the height maintained the proper angle at the U-joint. See the picture of the fabricated mounts below. They have been seam sealed and epoxy primed. They have since been painted.

New CSB mount Before Paint

Remember to pre-load the center bearing and tighten the collar nut on the driveshaft. These are tough pictures to capture, but here is a picture of the driveshaft installed.

After the mounts were completed, epoxy primed, and seam sealed, they were painted with black paint and an undercoating application. At every step of this process we are being careful not to create any rust producing modifications to any part of the car.

We began relocating the battery to the trunk and routing the necessary wiring and battery cables to do so.  We ran the battery cable along the passenger side of the car. It was run under the carpet under the backseat and through a pre-existing hole in the rear seat pan. A hole was drilled in the top of the rear seat frame to run the cable straight up to the new battery location. All holes were protected with rubber grommets. Joel welded up a stud for the new body ground coming off of the battery's negative terminal.

Battery Cable and Ground Stud

Since this car had an oxygen sensor wire that ran through the firewall, it created the perfect spot to route the battery cable into the engine compartment and a power distribution box. It was run behind the kick panel up the firewall and out the hole that was previously occupied by the oxygen sensor wire.

Battery Cable into Distribution Box

The new ECU was mounted in the glove compartment similar to the old ECU. The harness hole was drilled straight through into the glove compartment just above the glove box mounting frame.

New ECU Mounted Above Glove Box

The hole in the firewall was drilled so that the E30 harness grommet could be used on the firewall. You will also have one main harness ground on this side of the engine bay. We added one nut to the strut mount stud and grounded it there. There are also three harness wires that need 12v supply. These ere extended and routed into the distribution box. There is also the main body power supply that exits out of the bottom of the fuse box (nearest the front of the car.) This wire will need to be extended and routed to the distribution box, as well.

We chose to mount the C101 and the diagnostic port with the relays on the blower motor cover on the driver's side of the firewall. Joel fabricated a very cool custom mount for the C101 and the diagnostic port. There is also a custom mount for the relays that allows them to slide into place as they would have on the side of the E30 fusebox. Keep overall height in mind when you locate the harness components. When you close the hood, height will be an issue that you need to take into account.

Mounts have also been created for the radiator. Joel fabricated these to fit over the top of the radiator and pull them against the core support. The radiator is mounted with a rubber washer in between the radiator and core support.

In order to get the radiator to sit at the right height, the M10 radiator mounting brackets were cut off and lowered. This needed to be done to maintain the alignment of the radiator hoses to the engine. When doing this, the bottom of the fan shroud also had to be trimmed a very small amount.

Lower Radiator Mounts Lowered to Accommodate M42 Radiator

The existing 320i throttle cable was not quite right with the M42 throttle bracket. So, Joel decided to weld up a new one. It is quite impressive. Pictures don't really do it justice. Measurements were taken so that the 320i throttle cable would hit the bracket right in the middle of its adjustment range.

While Joel built the new throttle bracket, I replaced every air line on the engine, including the throttle body heater assembly. This was relatively inexpensive since most of the hoses were about $6-$8 each. It is good insurance to prevent vacuum leaks. They were all in terrible shape and left me wondering how the engine was able to idle with that much unmetered air.

One of the decisions we had to make was how to run the air intake. The stock air box is quite tall. In the position it wants to naturally sit in, it tends to rest on the alternator. Adjusting it to the perfect spot was putting stress on the intake boot. Since this design has the air intake on the opposite side of the exhaust manifold, it was decided that a cold air intake could be used. A cold air kit was found that is specifically for the E30 318i. This includes the rectangular adapter plate that fastens to the air flow meter. The complication with the cold air kit is the mounting. Joel set out to make a custom mounting for the new cold air kit. He started by removing the E21 air flow meter mounting bracket and welding it back on rotated down 90 degrees.

Next, sheet steel was used to make a custom piece that would mount the air flow meter to the modified bracket.

Custom Mount for AFM

The two pieces will be bolted together with a rubber bushing in between.

Test Fit of New AFM Mount

After test fitting, the modified AFM bracket was epoxy primed and painted body color. The new mount was epoxy primed and painted black. It came out pretty well.

Finished Mounting for AFM

At this point, we have moved on to small tasks. We have been cleaning up the engine and engine bay, tidying up the wiring, and preparing for the exhaust. We also put the hood back on. Here are a few pictures of the engine and bay. There are a few more small tasks, but most of the work is done.

The exhaust is the last task to be completed. The plan is to utilize the factory header, remove the catalytic, and run a center resonator back to a Magnaflow muffler. The header was left intact down to the first flange where it attached to the catalytic converter.

The exhaust is complete. At Tim's request, the catalytic converter was removed from the E30 318i system. The E30's factory resonator was retained (BMW calls this component the "front silencer.") The factory resonator now attaches to the end of the header pipe with the original factory compression clamp. Behind the resonator is a custom two inch pipe that runs up over the subframe and down to the muffler in the factory e21 exhaust exit. The custom pipe was cut and welded in sections and not bent. It is done this way because bends decrease the overall diameter and the bend while cutting and sectioning the turns maintains the overall shape of the pipe.

The above section leads to the factory resonator. The outlet of the resonator was welded to the new pipe section that leads up over the rear subframe and exits in the same spot as the E21 320i exhaust. The muffler selected is a Magnaflow 6 inch round with a single 2.25 in and out. It is the Magnaflow 12615. It gives a nice low throaty sound at idle and comes to life at higher rpm. There is a little crackle when you let off. This set-up is not excessively loud, but it is not quiet either. If you were looking for a subdued exhaust note, this is not the route to go. I think this design is perfect for a sporty sound.

Well, after a few tweaks to the exhaust, we drove the car around for about an hour to do sort of a "shake down." Everything is working well and the car drives great. The M42 is a wonderful compliment to the e21. It is a spirited and peppy motor that is a great balance for the E21. By pushing the motor back and moving the battery to the trunk, the balance of the car has been shifted. It is now how it should be, in my opinion. The additional forty horsepower is apparent and the car is clearly quicker than the M10.

Here are a few pictures snapped before the car went home.

Interior Shot With E30 Shifter

Thanks for following the progress of the project. If you have any questions or comments, feel free to leave a comment here. Or, please feel free to email me at pldlnr@gmail.com.