Spark Plug FAQ

General E28 FAQs.
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Spark Plug FAQ

Post by wkohler »

Problem: You need to be replacing your sparking stoppers, but you don't know which ones to buy. You searched for the answer. Honest.

Cause: You used poor search terms and came up with threads about sunroof maintenance

Solution: Look at the following information and then post a thread about how it is confusing.

We're not covering glow plugs.

Spark plugs vary from model to model, so just because you see that someone uses a certain plug in their car, it doesn't necessarily mean that you would use that same plug in your own car. Hopefully, the following information will help you choose the right flicker corks for your E28.

***A very important note: If you take the information you find here to one of your local discount auto parts suppliers, you will more than likely be offered a set of Platinum plugs. The sales person will probably share some story about how the spark plug rescued his or her dog/father/mother/aunt/baby daddy/hair stylist from an untimely death in a swimming pool underneath a fiery dragon's lair. None of that will have ever actually happened. Platinum plugs will cause running issues with M10, M20 and M30 motors, so they are to be avoided.

**A not-so important note: The motors sent to the budding North American market in the '80s were designed to run Silver electrode spark plugs. If you cannot find spark plugs with silver electrodes, copper plugs are a suitable substitute.

Here's how a spark plug works (and you thought I'd just spoon-feed you which ones to buy for your Eta :rofl: ):

Spark plugs are simply insulated plugs that are threaded into the cylinder head to ignite the air/fuel mixture in the combustion chamber of an internal combustion engine. Simple, yes, but...

They also have another use. They transfer heat out of the combustion chamber. The amount transferred out is dependent upon the length of the insulator, and the materials of which they're made from. You might have heard the terms 'hot' or 'cold' referring to plugs. Those generally refer to the heat ranges, which are determined by the size of the insulator. Colder plugs have smaller insulators, which allows more heat to leave the combustion chamber. Hotter plugs are the opposite. Colder plugs are generally used in high power motors since heat dissipation is key to preventing pre-ignition (also known as pinging or knocking). Hotter plugs can keep the combustion chambers cleaner (burning off carbon deposits, etc) and more efficient by keeping temperatures more even. Lower compression motors are not as susceptible to pre-ignition, so they will more than likely run a hotter plug.

Hopefully that was enough information to give you a basic understanding, but not enough info to keep you from being confused.

How do you know if you need plugs?

Great question. Spark plugs should be changed every Inspection II service, or roughly 50,000 miles. I'll change mine a bit earlier than that, but when you do a valve adjustment, you can inspect your spark plugs.

Thank you to the Haynes manual folks for giving us this great color chart to "read" spark plugs:


I am an E28 owner and need plugs. What kind of plugs should I buy?

That's a fantastic question. There were several different motors offered in the E28 across the globe, and they don't all use the same plug! These are the plugs originally specified for these applications. Some of the plugs might not be in production any longer, so be careful when purchasing. Using the links below on how to decipher the part numbers, you can find a suitable plug for your motor.

Let's start with the US stuff:

528e M20B27 up to 12/1986: Bosch WR9LS
528e M20B27 12/1986 on: Bosch W8LCR
533i M30B32 up to 1/84: Bosch WR9DS
533i M30B32 1/84 on: Bosch WR9LS
535i M30B34 all: Bosch WR9LS
M5 S38B35 all: Bosch X5DC

And the Euro stuff:

518 M10B18 all: Bosch W8DC
518i M10B18 all: Bosch W8DC
520i M20B20 all: Bosch W8DC
525e M20B27 all no catalyst: Bosch W8DC
525e M20B27 up to 12/86 with catalyst: Bosch W9LC
525e M20B27 from 12/86 with catalyst: Bosch W8LCR
525i M30B25 all: Bosch W8DC
528i M30B28 all: Bosch W8DC
535i M30B34 no catalyst: Bosch W8DC
535i M30B34 with catalyst: Bosch W9DC
M5 M88B35 all: Bosch X5DC

I only decided to cover the motors originally included in E28s, but for the E28 community here in the United States, there's a rather common motor swap. The M30B35 from '88-89 E24 635s, E32 735s and E34 535s. These are not the same as the M30B34 used in the E28 535i, and therefore use a different plug. The Bosch W8LCR.

You are not restricted to Bosch plugs. Many people run NGK plugs with good results. Remember, when cross-referencing that NGK heat range codes are the inverse of Bosch. A Bosch 9 is an NGK 4.

Here is some fun info:

Bosch Part Numbers Explained

NGK Part Numbers Explained

Bosch-NGK Cross Reference

I hope you found this useful. Thanks.
Last edited by wkohler on Jun 01, 2012 2:44 PM, edited 4 times in total.