Unlike other DIY projects for the E30, if you do a quick search on the internet for “BMW M20 replace rocker without removing head” you won’t find much in the way of quality advice. I found one lengthy writeup which may be the single most poorly written and profane set of DIY instructions I’ve ever seen. At least it provided some inspiration and I was able to figure it out myself. So the answer is yes, Virginia, you can replace a broken rocker on the BMW M20 engine without removing the head. Here’s how. (Disclaimer: Use at your own risk, your experience may vary.)
I broke the intake rocker on the #6 cylinder. You will get a range of advice on replacing just one rocker. Some say just to be safe, replace them all. If you have the improved rocker from the later years of the series, you can probably get away with just replacing the broken one. That was my case.
Here’s what you need to consider before you begin: you will need to remove the camshaft gear to get to the rocker arm shaft. To get to the cam gear, you need to remove the radiator and the timing belt. If you replace the timing belt, you should replace the tensioner, and the two seals behind the cam gear. If you don’t remember the last time you replaced the water pump, you should replace it as well. I replaced my water pump recently so I just went for the seals, tensioner, pulley, and new hoses. You will also want to replace the valve cover seal as well. Since you’re going to pull the spark plugs to make turning the engine by hand easier, you may want to replace them as well. Since you’re going to drain the coolant, you need to buy a new supply of coolant. I also changed the oil while everything was apart just in case any bits of rocker fell into the oil. (Turns out it was a clean break.) Replace your AC, PS, and Alternator belts too if you can’t remember the last time you replaced them. And of course you’re going to need a new rocker, eccentric, washer, bolt, nut and clamp.
Since these parts are relatively cheap, pick up a couple of spares in case you get into the project and realize any more rockers are cracked. Of course, having your trusty Bentley Manual is a must. Now you’re ready to start. Give yourself at least twelve hours of work time to do this if you’ve never done it before. You will also need an assistant once you get to step 29.
1. Place your car on jack stands and remove the front wheels, plastic under-tray, and hood. Take the transmission out of gear.
2. Drain the coolant out of your radiator using the drain screw at the bottom of your radiator and the 19mm screw plug on the block. You will make a mess so be prepared. The M20 engine holds about 3 gallons of coolant.
3. Remove the hoses to the radiator and remove the radiator by removing the bracket that holds it at the top and lift it out of the car. There will be some coolant left in the radiator so be sure to drain that too. If you are replacing all of the hoses, remove the rest of the hoses that go to the water pump as well. At a minimum you will need to remove the metal pipe that crosses in front of the waterpump and attaches to the timing belt cover.
4. Remove the mechanical fan using a long thin 32 mm wrench and fan pulley holder.
5. Remove the distributor and the rotor using a 3mm allen key.
6. Cut you belts or loosen the brackets for your AC (if you have one, I don’t), power steering pump, and alternator and remove your belts.
7. Remove the pulleys from your water pump and vibration damper.
8. Remove the cover that protects the position transmitter wire that crosses the timing belt cover.
9. Remove the timing belt cover, both halves.
10. Remove your spark plugs. (Optional, I found it easier to rotate the engine manually with them removed.)
11. Remove your valve cover and gasket.
12. Remove the oil pipe that runs the length of the head so you don’t damage it.
13. Note which rocker is broken. (I’m describing what worked for me with intake #6 rocker.)
14. Using a 22 mm socket, rotate the engine clockwise until it reaches Top Dead Center (TDC). This will be marked on both the cam gear and the crank. On the cam gear there will be a line on the head and a mark on the cam gear indicating TDC. On the crank, there is notch on the crank gear wheel. These two marks should both line up, otherwise your timing belt is installed wrong (not uncommon to be off by one tooth.) Freshen the markings so you can be sure you can find TDC again.
15. Rotate the engine manually in the clockwise direction until the pressure is relieved on the remaining 5 intake rockers. This will occur when the cam lob for the broke rocker is at its highest. Rotate until you are just ahead or just past the peak, but the other 5 rockers remain without tension. This will make replacement easier as you’ll see later.
16. Using a different color paint, mark this point on your cam gear and your crank gear wheel at the same line on the engine as TDC. Once we remove the timing belt, you want to be sure neither the crank nor the camshaft has turned while there is no tension between them so you don’t bend a valve.
17. Relieve pressure on the timing belt tensioner and remove the spring.
18. Remove the timing belt.
19. If you’re replacing your water pump, follow the instructions in the Bentley manual and replace the water pump now.
20. Remove the cam gear using a Torx socket. The Torx bolt is a T50. The corresponding socket is an E12. (Older cars may have an allen head bolt.)
21. Remove the bracket that locks down the two rocker arm shafts at the font of the cylinder head.
22. Remove the blind plug closest to the front (cam gear end) of the cylinder head. Use a screw driver to carefully pry it out and discard it. You’ll want to replace all four plugs while you have the valve cover off.
23. Remove the clamps that go over the rockers on each of the intake rockers.
24. All six rockers (including the broken one) should be loose on the rocker arm. Slide them off the valve perch toward the back of the engine. They should slide freely on the rocker arm shaft. You may need to lube to get them moving. Loosen the eccentrics if you don’t have enough play.
25. Lube the rocker arm shaft and see if it spins freely. Try to move it forward without prying. You are trying to slide it out the front of the engine far enough to reach the broken rocker. If you can’t get a good grip on it, wrap a piece of brass around it (or split a small piece of copper pipe) and use vice grips to get it to move. You want to be sure not to damage the surface of the rocker arm shaft.
26. Replace the blind plug at back end of the cylinder head.
27. Place your new eccentric into your new rocker along with the bolt, washer and nut. Be sure to put it in correctly so the adjusting hole is up and away from the rocker arm. Spin the eccentric so it creates the largest gap to the valve perch when installed. It will be adjusted later.
28. Slide the new rocker on the end of the rocker arm shaft.
29. Have a helper climb up in the engine bay with a large screw driver and press down on the valve spring of the rocker you are replacing. Remember that since the lob is up, the piston is down in the cylinder so you won’t damage the valve. You need to apply quite a bit of down ward pressure to free up the rocker on the rocker arm so you can move it back into position. Once the rocker is fully on the shaft and while keeping the pressure on the spring, use a rubber mallet to move the rocker shaft back into position. Be sure to align the rocker in the correct position so you can reinstall the clamp.
30. Spin the rocker arm shaft so the indentations are parallel for the bracket to be replaced that holds the arms in position. Reinstall the bracket.
31. Move the 5 loose rockers into the correct position above their valves and reinstall the clamps on each rocker.
32. Put a new blind plug at the front of the rocker arm shaft. (Replace the two plugs on the exhaust rocker arm while you’re at it.)
33. Before you reinstall the cam gear, you should replace the two seals behind it. Pull the guide cover off. Discard the o-ring and replace it with a new one. Pound out your old shaft seal, clean up the guide cover, and use the old seal to press in the new one. Lube it up along with the new o-ring and replace. You’ll be thankful you did this when your new timing belt puts additional tension on your old cam gear seal and you’re taking all of the timing belt parts off again next weekend because your cylinder head is leaking oil after you put on your new timing belt. (Don’t ask me how I know this….)
34. Reinstall the cam gear. Tighten it but don’t torque to spec until you have the timing belt back on. Check that the timing lines (the one you painted, not TDC) still line up.
35. Follow the instructions in the Bentley Manual to install your new timing belt tensioner and timing belt. Torque the cam gear nut. Verify your marks line up for TDC. Rotate the engine clockwise 4 cycles to make sure they still line up.
36. Follow the instructions in the Bentley Manual to adjust your valves.
37. Put the oil pipe back on the top of the cylinder head.
38. Put a little RTV Black on the Rocker Arm Shaft bracket as well as along the tops of the four blind plugs. Put on a new valve cover seal, and reinstall your valve cover.
39. For the rest, as they say in the Bentley manual, installation is the reverse of removal…