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The BMW M20 is an inline-six, single overhead camshaft engine that was produced from 1977 to 1993. The M20 powered multiple BMWs, but it went down in history as the unit that powered the iconic E30 3-series. The German manufacturer intended for the M20 to replace its larger displacement inline-four engines, as it believed that it had greater potential.
Over its decade-and-a-half-long production run, the M20 engine was available in four iterations, each with a different displacement. Only one of these made it to the North American market, but they were all notable in their own regard.
A Smaller M30 Engine
The M20 was developed to sit between the M10 inline-4 and the larger M30 inline-6. The M20 debuted as a 2.0L, and the first cars to feature this engine were the E12 5-Series and the E21 3-Series.
It was produced alongside its larger sibling, the M30, which at the time was in a 2.5L form. Unsurprisingly, the engineers also drew inspiration from this larger unit while designing and developing the M20. Like the M30, the M20 featured an iron block, an aluminum head, and a SOHC valve train with 2 valves per cylinder. It also had a traditional rocker arm design, missing out on hydraulic tappets. That’s where the core similarities ended.
The M20 had a timing belt instead of a timing chain, and its bore spacing was slightly less than that of the M30’s (91 mm (3.6 in) vs. 100 mm (3.9 in). Lastly, the slant angle of the engine was 20 degrees, while the M30’s was 30 degrees.
Available Versions
The BMW M20 engine was available in four iterations — the M20B20, M20B23, M20B25, and the M20B27. Only the last of these made its way to America, but since the rest were also rather impressive units and a significant part of the manufacturer’s history, we’ll take a quick look at what they had to offer.
M20B20
The E12 520/6 and the E21 320/6 were the first cars to feature the M20 engine. This version of the engine was dubbed the M20B20, and as its name suggests, it was a 2.0L engine. Well, almost. The 1,991cc engine was initially carbureted and produced 121hp at 6,000rpm and 120 lb-ft of torque at 4,000 pm.
In 1981, the M20 gained a fuel-injection system, and peak output figures saw a bump to 123hp and 125lb-ft of torque. The FI system was updated the following year, and a larger port cylinder head and lighter block were also added.
This engine was the longest-running of the M20 and was on sale for the entirety of the platform’s lifespan, up until 1992. It featured on the following models:
M20B23
In early 1978, about six months after the M20 was first launched, BMW introduced the M20B23 — a fuel-injected, larger engine version. It used the same head as the M20B20, but a longer stroke meant that it displaced slightly more, with a capacity of 2,316cc (2.3 L). Peak power and torque figures were 141hp at 5,300rpm and 140lb-ft at 4,500rpm.
In 1982, BMW swapped out its K-Jetronic FI system for the L-Jetronic and tweaked the compression ratio to achieve more torque; it now produced 137hp at 5,300rpm and 151lb-ft at 4,000 pm.
The last of this engine was produced from 1983-1987, and it featured a new exhaust and camshaft, taking output figures to 148hp and 151lb-ft. Over its decade-long life span, this engine featured on the 1977–1982 E21 323i, 1982–1984 E30 323i, and the 1984–1987 E30 323i.
M20B25
In 1985, the M20B25 was introduced as the eventual replacement to the M20B23. It displaced 2,494cc and initially produced 169hp at 5,800rpm and 167lb-ft at 4,000rpm. The exhaust system on this engine did not have a catalytic converter. Two years later, a catalyzed model was introduced along with a reduced compression ratio. However, it also featured a more advanced engine management system, and so, peak output figures remained nearly identical — apart from a 1lb-ft drop in peak torque.
This engine made its way to the 1985–1993 E30 325i, 1989–1990 E34 525i, and the 1988–1991 Z1.
M20B27
The M20B27 was the last of the M20 engines and the only one to make it to the North American market. This engine was designed for efficiency and produced some of the lowest power figures of the M20 engine despite having the largest capacity.
The M20B25 featured a slightly longer stroke (75 mm to 81 mm), taking displacement to 2,693cc. BMW equipped the car with a new head that featured smaller ports, a different camshaft, and softer valve springs to improve efficiency.
As a result, the M20B27 wasn’t as sporty as its predecessors and had a relatively low redline of 4,800rpm — compared to the 6,000rpm and over redlines on the previous M20 engines. Initially, the M20B27 engine produced 123hp at 4,250rpm and 177lb-ft of torque at 3,250rpm.
In the United States, the first car to use the M20B27 was the US-market 528e in 1982.
In 1985, the engine was returned, taking power and torque output to 127hp, despite a lower compression ratio.
The M20B27 engine powered the 1982–1987 E30 325e, 1982–1988 E28 525e (528e in North America), and the 1989–1992 E30 325iS.
BMW M20 Common Problems
The M20 generation of engines belongs to the era when BMW made bulletproof motors. However, no engine is perfect and rarely does one old age without experiencing some sort of an issue. The same goes for the M20 series. Here’s a short list of everything you can expect to deal with if you plan to get one of these motors.
Cylinder Heads Cracking
One of the more common issues you’ll hear about whenever an M20 is mentioned is a cracked cylinder heads. This issue ties into the cooling system issue we’ll discuss further down, and is almost always a direct result of engine overheating. Cylinder heads generally don’t like to run ho. However, the issue with stock M20 cylinder heads is that they don’t need much to crack. Even a slight overheating issue will cause them to fail.
There are several reasons why this happens, and most of them have to do with low-quality casting process that BMW used at the time. Long story short, they’ve created a number of thin stress points inside the cylinder head. When the engine overheats, these stress points crack, which requires cylinder head replacement.
Timing Belt Failure
Unlike the larger M30, the M20 uses a timing belt instead of a timing chain. BMW recommends replacing the timing belt every 60,000 miles. However, that recommendation has proven to be too optimistic on BMW’s part. Pushing this engine to 60,000 miles increases the risk of timing belt failure by a decent margin. Instead, it’s much safer to replace the timing belt every 40,000 to 50,000 miles. That way you’re almost guaranteed to avoid dealing with a broken timing belt and the catastrophic damage it can cause in all interference engines, including the M20.
One thing to note, when replacing the timing belt, you should also replace the water pump. The belt tensioner is pinned against the water pump, making this the perfect opportunity to swap the pump as well.
Valve Cover Gasket Leak
Valve cover gasket leaks are synonymous with BMW engines. Just about all of them spring a leak around from the valve cover when the valve cover gasket inevitably fails. The same applies to the M20 family of engines. Depending on how you maintain your M20, there are plenty of opportunities to change the valve cover gasket. You should get a new one every time you adjust the valves or the eccentrics. However, sometimes they don’t last that long.
When replacing the valve cover gasket, make sure to replace the rocker shaft plugs as well. Often overlooked, these little plugs often go brittle, resulting in a whole different type of oil leak. Changing them when you replace the valve cover gasket seal is a great way to prevent any issues in the future.
Oil Pan Gasket Leak
An oil pan gasket leak is usually not a big deal on most cars. However, the M20 is a bit different. The pan is long, extending deep towards the rear of the car. Depending on the car you own, the pan goes above the subframe which is why you often have to drop the said subframe in order to reach the bolts in the rear.
Considering how difficult it is to drop the pan, and how old these engines are, an oil pan gasket leak is almost a given.
Camshaft Oil Seal Failure
Another potential oil leak location is the camshaft oil seal flange. There are two O-ring seals associated with the flange — one that is pressed into the flange (main camshaft oil seal), and the camshaft flange O-ring that mates the flange to the head. The best way to solve this issue is to replace both O-rings at the same time since you have to remove the flange to reach both. If you notice an oil leak behind the cam pulley and it’s not the valve cover leaking, check the cam oil seals.
Crankshaft Rear Main Seal
Speaking of oil leaks that are hard to spot, the M20 is known to spring a leak on either the front main seal or the rear main seal. Replacing the rear main seal is complicated since you need to drop the transmission to swap it out. Because of that, you should always check this seal when you do your clutch.
Oil Pressure Sensor Failure/Leak
Oil pressure switches don’t fail too often, but when they do, they usually cause an oil leak. There are two ways this can happen. One type of oil leak is caused by a worn brass ring that sits between the block and the oil pressure switch. The other type of leak, a less common one, happens when the oil pressure switch core fails, and you get oil coming through the center of the switch.
Idle Control Valve
Rough idle is often caused by ignition issues of some type. However, on the M20, it is commonly caused by a failed or dirty idle control valve, or IAC. In addition to a rough idle, you will also get a check engine light on the dash.
In some instances, taking the valve apart and cleaning it will solve all your issues. However, don’t be surprised if your IAC is toast and in need of replacement.
Cooling System Issues
The cooling system on the M20 can be finicky. The radiator, coolant hoses, thermostat, and water pump — all need to be in top shape for this engine to perform. Most people run into issues when they try to do a coolant flush. As it turns out, it’s fairly difficult to get all the air out of this engine’s cooling system when you bleed it, which often causes overheating issues when you push the car. Paired with a badly cast cylinder head, you’re looking at a potentially dangerous combination.
It’s highly recommended that you give the cooling system a proper shakedown during the pre-purchase inspection.
Crankshaft Position Sensor
There are few things more annoying than having your car crank, but not start. On M20 engines, especially on E30 325i, one of the likely culprits is the crankshaft position sensor. This sensor tells the engine how fast the crankshaft is spinning and which way it’s oriented. When it fails, it can result in a no-start condition. Some M20 engines have two of these, so make sure to check both.
BMW M20 Engine — A Tuner’s Playground
Although it wasn’t the S14 from the E30 M3, BMW’s M20 engine has grown a massive following over the years. The last of the M20-powered BMWs were sold 30 years ago, but even today, it’s still a car that you can find aftermarket support for. The M20 is a superb, robust example of German engineering at its finest, and that’s the reason it also got so much love and attention from the tuner community.
Owners have added turbos and swapped the sportier head from the older M20 engines to replace the one on the M20B27 — the list of experiments done with this engine is rather elaborate. You’ll find examples with race camshafts, remapped ECUs, high-performance air filters, sport exhaust, high-flow injectors, and even modded engines featuring bigger valves.
It’s one of the few BMW engines to use a timing belt instead of a chain, but reliability was never one of its issues. Most M20s out there have a fair bit of mileage on them, considering how good of a daily driver they were and still are.
It’s All About the Parts
The key to keeping any BMW engine reliable is using quality parts. The same applies to the entire range of M20 engines, both the heavily tuned ones, and the rare stock ones. We carry everything you could need to keep your M20 alive and purring.
From OEM parts to aftermarket performance parts, you can use our catalog to find just about anything that’s for your car. Simply select your BMW’s year, make, and model using our navigation tool, and you’ll be presented with all the results.
