By: Ron Schmidt

BMW Air Head Engines 1970 onward

4-Cycle Engine Basics

Currently, all BMW motorcycle engines are 4 cycle engines. This is a brief explanation of what happens inside that block of aluminum every time you use it.

The term “4-cycle” (sometimes called “4-stroke”) describes the engine operations that make it function. The cycles are normally called “intake”, “compression”,” power”, and “exhaust”.

Like the chicken and the egg, it is debatable which of these cycles happens first, as at least two will be happening on our multi-cylinder engines when you hit the starter button. I like to start with the intake cycle because until you get something in the cylinder to burn, there will be no power and since power is what we all are really interested in, we’ll begin there.

I am going to over-simplify the engine cycles by telling what would be happening if the engine was designed to be operated at extremely low rpm. Also, I will assume you know that there is a piston that goes back and forth in a cylinder, and there are at least 2 valves for each cylinder, one for intake of fuel & air, and one for exhaust.

The gasses have considerable mass and are reluctant to be moved. The operation of the valves is to allow the movement of those gasses either into or out of the cylinder. So, the actual valve timing for your BMW will be advanced from what we will discuss to allow for the time to accelerate the gasses. Also, the spark timing is advanced to allow time for the mixture to be burned more thoroughly.

Intake Cycle
The piston is at Top Dead Center or TDC (could really be “side dead center” on the airhead engine!). The exhaust valve is closed, the intake valve is opened. As the piston goes down to the bottom of the cylinder it will allow a mixture of air and fuel to fill the cylinder.

Compression Cycle
The intake valve is now closed, exhaust valve is still closed. As the piston moves back up to the top of the cylinder, the air and fuel mix is compressed. This compression raises the temperature of the fuel-air mix, making it more combustible.

Power Cycle
The piston is back at top dead center. All valves are closed. The sparkplug sparks, a big bang happens. The burning fuel air mix is expanded and pushes the piston down. Yahoo!! Power!

Exhaust Cycle
The exhaust valve is opened. The intake valve remains closed. As the piston goes back up to the top, what is left of the burned air fuel mixture is expelled through the exhaust valves and out through the exhaust system. As an interesting note, the exhaust sound that you hear through the muffler is the release of these dead gasses—well after the power has been made—so never confuse a loud exhaust with power!

Valve Adjustment Procedure
BMW air head (2 valve) engine — 1970 and newer

Preface: Those of us who own or appreciate the old BMW air head motorcycle have learned to love what they are and have the practiced ability to overlook what they are not. They are a throw back to the times when mechanics, not technicians, worked on them. While in many ways they are simpler than the newer engine designs, the fact that they were designed in a time when it was just assumed that the person working on them had a functioning brain rather than a flat rate mindset, there are many more ways to mess them up! So, take your time and enjoy the process. The very fact that you are doing this work on an old bike probably means you are not in a big rush anyway!

Tools and other items you will need to perform this service:
• 10mm open end wrench
• 12mm open end wrench
• 12mm socket
• 13mm socket
• 14mm or 15mm socket, depending on model
• 13/16” sparkplug socket
• Handles of choice for sockets
• High quality torque wrench – preferably readable in Newton meters (nm)
• Feeler gauge set 0.05-0.20mm
• Scraper, gasket remover spray, bandages and antiseptic if valve cover gaskets need to be
changed.
• Rags
• Drain pan
• “Special tool” – see below if you need this and how to build it.

The Procedure:

1) The engine needs to be cold. It is best to leave it set over night before performing the adjustment. At the very least, you should not feel any warmth from the engine when you touch it.

2) Park the bike on the center stand and secure it in some manner that will not allow it to tip over. You will be tugging on it some, so make sure it is secure!!

3) STOP NOW AND READ THIS!! IF YOU MISS THIS STEP YOU CAN CAUSE YOURSELF LOTS OF GRIEF!! Look all around the outside of the valve cover and around the spark plug area. There will certainly be dirt there; lots of dirt if it is a properly used GS. This dirt is your worst enemy and must be removed BEFORE you take the sparkplug out or remove the valve cover!! The dirt can generally be blown away with compressed air (be careful to shield your eyes if you remove the dirt this way!). If you do not have an air compressor at home you could even use a garden hose and flush it away. Whatever means you use, make sure the area around the spark plug and all around the outside of the valve cover is squeaky clean.

4) Remove the sparkplugs from both heads using a 13/16” sparkplug socket.

5) Remove the rubber plug from the timing view hole. It is on the left hand side of the engine, just to the rear of the oil dip stick. This plug is about 20mm (3/4”) in diameter.

6) Put a drip catcher pan under the left valve cover. The cover is held on with three fasteners; two 6mm studs that have nuts and wave washers, and one 8mm stud with a wave washer. The 6mm studs are part of the valve cover and protrude through the head. They are secured with nuts and wave washers just behind the second fin on the head. The one on the right side (as you face the engine) is just below the horizontal center line of the head. The one on the left side is just above the horizontal center line of the head. The 8mm stud is part of the cylinder head and the fastener for it is the fancy chrome nut in the center of the valve cover.

Remove both of the nuts on the 6mm studs using a 10mm open end wrench, and the fancy chrome cap 8mm nut from the center of the valve cover using a 13mm socket. Don’t forget to get those wave washers that are behind each of the three fasteners! The valve cover can then be pulled straight off the head. “Pull” can be anywhere from where the valve cover almost fall off by itself, to having to VERY LIGHTLY persuade it with a soft mallet. If the valve covers are stuck and you do not have a soft mallet, you can use a regular hammer—just put your fingers between the cover and the hammer so you do not damage the valve cover or be tempted to hit it too hard. When you remove the cover a bit of oil will come out.

7) Put the transmission in high (4th or 5th) gear. Turn the rear wheel in its normal direction of travel and watch the intake valve. The intake valve is the rear valve. You want to see the intake valve completely open (pushed down all the way) then close (all the way out). When it is completely closed, look into the timing view hole and continue to turn the back wheel in its normal direction of travel until the “OT” marking tab on the flywheel is centered in the hole. The engine is now at Top Dead Center (TDC) with the left side on the compression cycle, which is where it needs to be to adjust the valves for the left side.

Note that once the intake valve has closed, the engine will be near TDC; if you have to turn it very far you probably have missed the timing mark and will be on the wrong cycle when you see the OT mark come up in the window. When the engine is at TDC on the correct cycle, there should be clearance between the end of the valve stem and the rocker arm on both the intake and exhaust valves.

8 ) Now that you have the valve cover off you will see that there are four 10mm studs with flanged nuts on them. The flanged nut looks like a regular hex nut with a built in washer on it. The nuts will be either 14mm (on /5 and very early /6) or, 15mm (on /6 and later bikes) outside hex dimension. These nuts hold the upper and lower rocker arm positioning blocks (RAPB). The built in washer should be touching these blocks. I often see them installed upside down, so make sure they are correctly positioned. Also note that there are two more nuts (of either 14 or 15mm-they will be the same size as the flanged nuts), one just behind the hole for the sparkplug and another between the pushrod tubes on the bottom of the head.

Rocker arm styles: Before we go farther, we need to discuss briefly the four primary types or styles of rocker arms.

Style #1 The very early /5 models had rocker arms that were pivoted on a bushing. This style of rocker arm was narrow vertically and had a spacer on the pivot shaft about the same height as the rocker itself. They were prone to early wear so most of them have been updated to one of the newer styles described below. If you still have a BMW with this style rocker it would be a good idea to remove the rocker arms one at a time and check for wear in the bushing and the pivot shaft. If wear is evident, it would be best to update the rocker arms to a more modern style.

Style #2 The later /5s and /6 models had full length rocker arms that filled the whole space between the RAPB’s. These were pivoted on caged needle bearings, two (or a pair) per rocker arm. On this and the above mentioned Style #1 rocker arm sets, the RAPB’s were not indexed to the head. The RAPB’s simply sat on a tube that the 10mm studs went through and had enough slop in them to adjust the position of the RAPB’s. This is where being a mechanic becomes very important! Not only does the vertical clearance need to be set (as described below) but the entire shaft + RAPB’s + rocker arm + pushrod assembly must be set exactly square on the head and positioned so that the pushrod does not hit the inside of the pushrod tube as the valve opens and closes.

Style #3 The /7 through 1984 models had full length rocker arms that were pivoted on a pair of caged needle bearings per rocker arm. These had RAPB’s that fit loosely over a bushing that was pressed into the head and went around the 10mm studs. On this style rocker, the vertical clearance still needs to be adjusted, but there is no need to be concerned about the pushrod hitting the inside of the pushrod tube.

Style #4 Found on 1985 and through end of production for air heads, this was the most advanced and best of the rocker arm systems. The rocker was pivoted on two caged needle bearings per shaft. They incorporated a plastic noise dampening bushing into the rocker arm. The RAPB’s were precisely indexed into the head and the vertical clearance is set with shims that are available in various thicknesses. Once this style of rocker was run in and properly set, it was very unusual to ever have to reset the vertical clearance; and, there was never a concern for the pushrod ever touching the inside of the pushrod tube.

9) Check the vertical clearance of the rocker arms — the amount of vertical clearance between the rocker arm and the RAPB’s. This clearance needs to be nil, meaning that there can be almost no clearance but no drag on the rocker arm as it is moving. Excessive clearance in this area (more than 0.10mm) will allow the rocker to move vertically which causes noise. But even more importantly it can result in the rocker arm needle bearing cages cracking, dumping the broken needle bearings into the oil sump. This will ruin the rocker arms and shafts and in worst case instances the broken needle bearings could be ingested into other parts of the bottom end of the engine. This is very good for our shop business, but very bad for you!

Special tool — necessary only for rocker arm styles #1 – #3. If you need this tool, it can easily be made to set the rocker arm vertical clearance.

A brief explanation of what you are actually going to do with the tool will make the tool easier to understand. The pivot shaft for the rocker arms simply slides (only a small amount) inside the vertical holes in the RAPB’s, and the RAPB’s can move on the 10mm studs through their horizontal holes. So, to adjust the vertical clearance between the rocker arm and the RAPB’s, we simply need to “squeeze” the RAPB’s together to minimize the space between them and the rocker arm.

To build the special tool you will need an 8” sliding carpenter’s clamp and two end caps for a 5/8” brass pipe. Glue, solder, braze or bolt the end caps to the pads of the clamp so that their open ends are facing each other. The open ends of the caps can be placed over the top and bottom of the rocker arm pivot shafts and will touch the RAPB’s which can then be pushed together to obtain the correct clearance

10) Setting the vertical clearance. For all styles of rocker arms, use a 12mm socket and loosen the locknut for the threaded rocker arm lash adjuster, then use a 12mm open-end wrench and back the lash adjuster

out to the end of it’s travel so that you can pivot the rocker arm and feel if it has any drag or vertical clearance.

● Styles #1 and #2 — Loosen the flanged nuts on the intake rocker arm to finger tight using a 14mm or 15mm socket as is correct for your particular bike. Install your newly made special tool and squeeze the RAPB’s together to make the vertical clearance nil.

NOTE — The amount of force you need to put on the special tool is VERY LITTLE! You should almost be able to do it with just your fingers. If you have to turn the screw on the clamp hard at all, you probably do not have the flange nuts loose enough.

With the clamp still installed, torque the flanged nuts to 20 NM (newton meters). Remove the clamp and check that the clearance is still nil. Repeat this procedure on the exhaust rocker arm. Then, torque the four flanged nuts to 35 NM in the following order: (a) upper intake, (b) lower exhaust, (c) upper exhaust, and (d) lower intake. Next, loosen the other two head nuts (the one aft of the sparkplug hole and the one between pushrod tubes) ½ turn, and then torque to 35 NM. Now double check that the vertical clearance on both rocker arms is still nil. Also check that the pushrod is not going to hit the inside of the pushrod tube by turning the crankshaft through two complete revolutions and watching the pushrods as each valve works though it’s entire movement. If the pushrod touches or gets close to touching the inside of the pushrod tube, you will have to repeat the entire procedure, moving the RAPB’s so that the pushrod does not hit the tube. Quit whining –be a mechanic! With some practice you will be able to make the vertical clearance and the positioning of the rocker assemblies correct on the first try.

● Style #3 — Follow the same procedure as for styles #1 and 2 above, but you do not have to be concerned about the pushrods hitting the inside of the pushrod tubes because the RAPB’s are loosely indexed so that this is not a possibility. Also on these bikes (actually 1975 and later) the pushrod tube has a larger inside diameter further eliminating that possibility unless the engine is grossly overreved.

● Style #4 — The RAPB’s on this style are precisely indexed into the head and are therefore not adjustable. The vertical rocker adjustment is made by varying the thickness of the shim pack between the top of the rocker arm and the bottom of the upper RAPB. Use a feeler gauge to measure the clearance between the rocker arm and the shim packs above the rocker arm. The clearance should be nil to 0.05mm. If the clearance is more than 0.05mm you will need to remove the rocker arm and re-shim it. Generally there are 2 or more shims already in the shim pack. You will have to add or subtract or exchange shims to make the clearance correct. For example, you measure the clearance to be 0.07mm. The shims are available in 0.05 increments; you need to increase the thickness of the shim pack by 0.05mm to make the clearance correct. If one of the existing shims is 0.30mm, you would replace it with a 0.35mm shim. On reassembly, the clearance would then be 0.02mm, which is between nil and 0.05mm. After you have corrected the shim pack (or, more likely, measured it and found it to be correct) proceed with the torquing procedure as outlined above.

11) Adjust the valves. Make sure you are back at the TDC compression stroke as described above if you have turned the engine over to check the pushrod to pushrod tube clearance for Styles #1 and 2!

I suggest that you adjust the intake valve first, but I have no idea why as it really does not matter— it’s just the way I like to do it! Slide a 0.15mm feeler gauge between the top of the intake valve and the rocker arm. There should be a slight drag as you slide the gauge through. If there is no drag, the valve is loose. If there is a lot of drag, the valve is too tight. What is a slight drag compared to a lot of drag, you ask? A good rule of thumb is that the drag should be just enough to hold the feeler gauge in position when you let it go. If in doubt, a little loose is better than a little tight.

I have found that good mechanics will vary about 0.02mm according to what they feel is the correct amount of drag, and argue at length about who’s feel is right. In view of the fact that the clearance is temperature sensitive and will change as the engine warms up, don’t loose sleep over it. If you want to check your work, try a 0.17mm gauge; it should feel very snug, and a 0.13mm should feel very loose. To adjust the valve, loosen the locknut with a 12mm socket; turn the threaded adjuster with a 12mm open end wrench. Turn it clockwise to decrease the gap, anti clockwise to increase the gap. Then torque the locknut to 20 NM and recheck the clearance. Correct if needed. Move to the exhaust valve, using the same procedure, but the using the 0.20mm gauge instead of the 0.15mm gauge.

12) Wipe all the oil off the valve cover, and the gasket, and the head. The valve cover gaskets are usually good for many adjustments, but if yours looks sad or you noted a leak before you removed the cover, renew the gasket. It will probably be between the hardness of granite and diamond, and will be very fond of the head it has been glued to for the last decade(s) so be very careful not to gouge the aluminum head as you scrape it off. Reinstall the 6mm nuts and the fancy 8mm nut with their wave washers; torque the 6mm nuts to 10 NM (like you could actually get a torque wrench in there!) and the 8mm fancy nut to 20 NM.

13) Move to the right side of the motorcycle and repeat steps 6 through 12.

14) Reinstall or renew the sparkplugs using anti-seize on the threads of the plug. NOTE: The correct amount of anti-seize is the least you can put on and still see it! Do not slop it on, and make sure to keep it away from the electrodes as it is conductive and will ruin the sparkplug. Torque to 25 NM.

15) Reinstall spark plug wires. Note that a bit of dielectric grease on the inside of the plug wire clips where they slide over the sparkplugs will make them easier to remove next time!

16) Put the rubber timing-hole plug, that was removed in step #5, back in.

17) Start engine, let it run for 63 seconds, turn it off and check for leaks at the valve covers. Be careful to note that the exhaust headers will be hot enough to burn the poop out of you already! Correct leaks if they exist.

18) Ride the bike until it is fully warmed up; then set the idle speed adjustment and mixture and synchronize the carbs. Carburetor synchronization will be covered in another seminar!