How to adjust the valves on a M20 engine
(Originally posted on the E30 Zone by e30adam)
In this article we will discuss adjusting the valves for the E30 6 cylinder BMW. In this case we will be using my 325i. This method applies to all M20 engine variants including 5-series BMW's. Keep in mind that this article does not apply to the E30 318i or the M3.
Why adjust the valves? You want to keep your valves adjusted as a regular part of your vehicle’s maintenance. Often times, a poorly running engine can purr like new simply by adjusting the valves. When valves do go out of adjustment, they can cause a variety of problems, such as poor fuel economy, loss of power, and even overheating. Many times, failure to adjust valves can result in the valve actually breaking off and causing major damage to the engine.
I would always adjust the valves when the engine is stone cold. If you've run the engine recently, allow at least 6 hours for it to cool before attempting adjustment.
You’ll want to begin by disconnecting the battery. This is just common sense, as you will be working around fuel lines and electrical components, and you can cause serious damage to both you and your car by failing to do this. The battery is located in the rear trunk, on the passenger side under a small access cover on the coupe's or under the bonnet on all other models.
- 1. Begin by undoing the 8 rocker cover bolts and the 4 bolts for the bracket which attaches to the inlet manifold. Also slacken the jubilee clip for the breather hose and separate the hose from the rocker cover as shown in the picture below.
- 2. Next remove the plug leads and plugs. If your leads are not already labelled then label them up, cylinders 1 to 6. The *cylinder nearest the front of the car is cylinder 1.
- 3. Ensure that you keep the plugs in the order in which they were removed, clean or replace the plugs if necessary.
- 4. Once you've removed the valve\rocker cover, you should be faced with a similar view to the below picture. This is the valve train and contains the camshaft, valves and rockers.
- 5. The valve clearances are adjusted by an eccentric attached to the end of the rocker shaft as highlighted in the below picture. The small bolt on the side loosens it and inserting a small implement such as a 3mm allen key into the hole, you can adjust the clearance.
- 6. Before you start to adjust the valve, you need to ensure that the camshaft is in the correct position for each valve in turn. The camshaft has 12 lobes on it. You need to turn the crankshaft until the cam lobe for the rocker that you are adjusting points straight down. as shown in the following diagram
Having the camshaft in this position ensures that both intake and exhaust valves are closed and there is no tension on the valves. If you get beneath the front of the car, you can put a 22mm socket and ratchet on the crankshaft to turn it until the cam is in the desired position.
- 7. The gap that is being adjusted is shown in the following picture
- 8. Begin by releasing this 10mm bolt on the side of the rocker
- 9. You will need a set of feeler gauges to set the gap correctly to 0.25mm. Feeler gauges can be purchased from any motor factors
- 10. Once you've released the 10mm bolt, put a 3mm allen key into the small adjuster hole and then slide the feeler gauge underneath. You now need to tighten the eccentric by moving it towards you for the exhaust valves or away from you for the inlet valves until you feel a slight resistance when you try and slide the feeler gauge in and out.
- 11. Once you've achieved the desired adjustment, retighten the 10mm bolt and then recheck by sliding the feeler gauge again. This may take several attempts to get right for each valve.
- 12. Repeat this procedure for each valve in turn ensuring that you turn the crankshaft so that the cam is in the correct position before adjusting each valve.
- 13. When complete, reinstall the spark plugs and rocker cover and take for a test drive. If your valves were badly out of adjustment, the car should feel a lot better to drive.
- 14. Daft as it sounds - make sure you remember to remove the wrench / spanner from the crank!!