Engine Swap - M42

The four-cylinder engines get a lot of stick from E30 owners. "Feeble", "weak" and "two pistons short" are the usual complaints made against the rattly M10 and its M40 sibling. And while it's true that the four-pots can feel a bit asthmatic after driving a big six-cylinder M20, it doesn't mean all four-pot engines are rubbish; after all, the M3 was powered by the four-cylinder S14 lump.

If you want more poke from your 318i without the drinking problems inherent of the bigger engines, then look no further than the M42 engine. This fizzy little four-banger features the same 1.8 litre displacement as the other E30 engines, but with double the valves (16 in all) to crank the power up to 140bhp. Combined with a simple drop-in swap procedure and easy-to-find spares from the E36, the M42 is the best engine swap to liven up the smaller-engined E30 models.

Identification

The M42 engine is found in two cars; the 318is versions of the E30 and E36. However, the E36 318is was later "upgraded" to the M44 engine, which is not as desirable for a simple swap due to its increased complexity. To make sure you're getting the right engine from a donor, the most obvious way to identify an M44 engine is by the crank sensor being at the rear of the block, not the front timing case:

M44 crank sensor location:

M42 crank sensor location:

If you plan on fitting an E36 M42 you need to change the inlet manifold to the E30 one (E30 one is better than the E36 one any way), you will also need an E30 M42/M40 sump (upper and lower) and engine mounts.

Parts List

You need:

1. M42 engine
2. E30 M42 engine loom and ECU (will work with E36 engines also, saves having to adapt E36 looms and causing issues!)
3. E30 M40 or M42 engine mount arms
4. E30 M42 engine mounts
5. Coil packs and HT leads or COP conversion
6. E30 M42 exhaust manifold
7. E30 318iS exhaust system
8. E30 M42 coolant hoses
9. E30 M42 AFM to TB boot
10. Getrag 240 gearbox
11. 318iS propshaft
12. E30-specific oil sump from an M40 or M42 engine. An M10 sump will not fit.

If your current car sports an M10 engine, you'll also need:

• Radiator upper and lower mounts are different and need to be fabricated
• Engine subframe/mounts are about 10mm too wide for the M42 engine legs
• Coil pack needs to have a mount made and attached to hold in place
• M42 engine requires a fuel pump in the tank, or in-line AND a fuel return pipe to be added. On many cars the wiring loom does cater for an in-tank fuel pump
• On carburretted M10 engines the fuel tank will need replacing to provide an in-tank swirl pot.
• Air filter needs a bracket / mount made
• Wiring loom requires a swap (2 wires, oil and temp need swapping) and there is a LIVE off the engine loom the goes directly into and EARTH on the multi plug!
• M40 or M42 instrument cluster. This is down to an issue with the temperature gauge.

Gearbox

The gearbox you're looking for is the Getrag 240 gear box. However, there are two types: one was fitted to the M40 and the other to the M42. Despite sharing a model number, they do have different internal ratios, with the M42 box being more desirable. The distinguishing feature is the output flange; you need the one with the flange further away in order to use all standard 318iS parts.

Here is the back of the M42 box:

Here is the back of the M40 box:

Whichever you choose you will need the corresponding prop shaft (M40 box -> M40 prop or M42 box -> M42 prop)

If you're using a gearbox from an E36, you'll notice that it sits at an angle compared to E30 boxes. This will give you an issue regarding both the gearbox mounts and the gear shifter. These challenges can be fixed by some simple creativity, which involves beefing up one side of the gearbox mount with welded patches, and bending the gear shifter rod to accommodate the angle. More importantly, the bolt pattern between the gearbox bell housing and the engine block is not identical. The 4/5 BIG bolts will go in but the smaller bolts that hold the plate steady are different and cannot be used. This is still safe as the small bolts are not load bearing, and are only used as locators.

This gearbox was also fitted to the E34 318i.

Clutch

The clutch you use is specific to the gearbox. However, M40 and M42 clutches are the same, and E30 and E36 clutches are the same, so no matter what you choose it'll fit fine.

Differential

The preferred differential is 4.10. This gives a good combination of speed and economy, although the economy can be improved further with a 3.91 diff, at the expense of your 0-60 time. On the flip side, a 4.27 or even 4.45 diff will turn your car into a rocket, but your motorway economy and comfort will suffer for it.

As you can see on the table in the Differential section of the Wiki, the 4.10 diff was a popular unit fitted to a lot of cars in the E30 range, so there's a large chance your car will have one already.

Coil packs

If you opt for standard coil packs, you will need the coil packs and HT leads. You will also need the brackets to mount them to the wing, which can be had from BMW. The part numbers are: 41 14 8 106 938 and 41 14 8 106 938. They mount on the drivers side of the engine bay:

Another option that costs about the same is COP (Coil On Plug). This works by fitting a set of M52 (P/N: 17 480 17) coil packs directly over the spark plugs mounted on a special plate. Here is how it looks:

Learn more about installing coil-on-plug.

Flywheel

Word on the street is that M42 came with a dual mass (DM) flywheel. This is a big part that people stumble on. I have yet to see an M42 in the UK that has been fitted with a DM flywheel they nearly all came with single mass units. The single mass are exactly that and are one lump of steel. These are still heavy but these can be lightened unlike the DM ones.

Single mass:

Dual mass (note the extra circles around the main bolt holes):

Ancillaries

If you pull all ancillaries from your M40 out of the engine bay prior to lift it will make it all easier.

Bear in mind the 318iS (which came with the M42) came with rear disc brakes and 51mm front struts, which you won't have on the M40 car. It's heavily recommended to You might want to upgrade the suspension. The radiators for the M40 are also slightly smaller than the M42 ones but doesn't make much odds.

Upgrades

There are a number of things you can do to your M42 engine prior to fitting it. Learn more about common M42 upgrades.