Sep 25 2017

Tech Tips: Valve Adjust #brent’s #motorcycle #works, #bmw, #airhead, #/2, #slash2


Tech Tips: Valve Adjust

How to adjust your valves, and Sign and Symptoms of valve recession and why it occurs

Valve adjustment should only be done when the engine is cold. The first thing to do is to remove the valve cover which is held on by three fasteners. I always do the left side first then the right, because the timing marks on the left side. Remove the timing plug to observe the marks on the fly wheel (see side note below about lightened flywheels). After taking the valve cover off and removing the timing plug cover, bring the piston up on the compression stroke. Do this by watching the intake valve move from open to closed position in the correct rotation of the engine. I like to use the kick starter lever for this. On later airheads without kick starters, place the transmission in third or fourth gear work and turn the wheel in the forward direction. Then observe the timing marks as you rotate the engine. The first mark that you will observe is the F mark – that is the full advance mark. The next mark that you see is the S mark – this is where the timing is set from static. The next mark is OT (see Photo 1) which is top dead center. This is where the valves are to be adjusted from. In the valve timing aspect this is the low spot on the cam where the valve is allowed to rest in the valve seat and held by the spring in place with no other forces on it.

The first thing I look for: Is there any clearance noted to start with, and how much? The next thing is the rocker play – the amount of movement that can be felt by moving the rocker up and down. There should be no play but the rocker should still move freely. This needs to be corrected before any adjustment is done to the valve clearance. This little bit of play can make up 2 thousandths difference in the clearance measured in the valve stem to rocker measurement. Ever notice that some engines are much quieter than others? This may explain part of this. Most of the time this needs to be addressed before setting the clearance to the valves. This is also the time to re-torque the head bolts. Always do this in a cross pattern. I always like to do the R69S starting with the short bolts behind the spark plug and on the other side of the head, then a cross pattern.

Setting the valve clearance is not hard. Place the correct sized feeler gauge between the valve stem and the rocker. The correct clearance for the intake valve is 0.15mm <.006ths> and for the exhaust is 0.2mm <.008ths>. This is a feel thing – there should be a slight drag when pulling the feeler gauge past the rocker and the valve stem. A double check is to try to pass the next sized feeler gauge: for instance a .007ths gauge in the intake side. If it can be passed then the clearance is a little too loose. As I stated it is a feel thing, and practice is the key to getting this correct every time. See Photo 2.

Loosen the lock nut using a wrench. On the R50s and R60s, it is a 12mm nut. The adjustment stud for the R50s and R60s is 11mm. The R69S takes 12mm on the locking nut and the adjustment stud. Using the appropriate wrench, hold the adjustment stud, loosen the locking nut, then adjust the clearance by turning the adjustment stud to gain the proper clearance. Lock in place, and re-check the clearance. See Photos 3 and 4.

Valve Recession

Once you’ve mastered valve adjustments, it is helpful for assessing valve recession. Recession is when the valve or the valve seat is wearing away faster than it should, and becomes evident when you need to adjust the valves more frequently then every five thousand miles or so. The other thing to look at is how much adjustment is left on the adjustment screw. As the valve recesses into the head, the valve stem will recede farther into the head thus loosing the ability to adjust the valves.

Why Valve Recession occurs: Back in the 1980s lead was removed from the gasoline that we used to buy. This was good for the environment but it was a bad thing for the older valves, and it was a mini disaster for older cars and bikes. Many of the valve seats were made from cast material which was a little softer. Because of the lead, which gave the valves lubricity and a cushion, they did not need to be all that hard. In fact back in the thirtys some engines had bronze seats that did very well. The lead also went past the rings and filled small scores in the cylinders and went though the oil and filled some of the old babit bearings giving a little help to some of the older engines. It also helped fill the slingers on old BMWs – the old gray pasty stuff you dig out of the slingers.

What we noticed: When only unleaded fuel became available, after a valve job bikes would lose valve clearance much faster than before. The harder the bike was ridden the faster the clearance would close up.

This started the search for an unleaded fuel conversion for older bikes.

What we learned from this: The seats need to be harder, and the valves also need to be harder material than the original design. The valve stem clearance needs to be much closer than before. The guide material also needs to be better in order to hold better tighter clearances. My contention is that after 1982-3 the valve guide clearance needed to be much tighter than the factory was setting them.

The problem is twofold: The way the rocker works is that it pulls the valve stem toward the center of the head as it depresses the valve. This adds to the wear factor because this is back and forth movement and up and down movement as the valve is moved back and forth. This allowed the seat area to also wear at a faster rate thus compounding the problem. Also a weak valve spring would allow the valve to bounce on the valve seat. Thus the demise of the pushrod motor and better designed overhead cams and on to the next best thing – no cams but solenoids to open and close the valves via computer controlled valve timing.

Other issues to be noted back in the late 1950s and up thru mid 1960 the R50-R50/2 and R60-R60/2 were known to drop a valve and destroy the motor. The intake and exhaust valves rely on physical contact with the valve seat and guide for cooling. About 75% of the combustion heat that is conducted away from the valve passes through the seat, so good seat contact is essential to prevent burning. The remaining 25% of the heat is dissipated up through the valve stem and out through the guides. If the valve does not receive adequate cooling, It can overheat, burn and fail. This was caused by the valve being made of two different materials the stem of one and the valve being made of another type (the best of both worlds so they thought). This was welded together and did well for the most part. In the last year I have seen five failures. The root causes may be old age, excessive heat (the valves got too hot), they lost temper. and the weld joint failed. I have heard and seen many of these go this way. The story goes I was riding the bike hard, parked it, got on it, in about one mile or less the motor made a bad noise and that was end of it. See photo 5.

Noted here are some pics of a suspect valve. Note the erosions on the interior on the head of the valve. This prevents adequate heat transfer to cool the valve, thus leading to eventual parting of the weld and the separation of the stem and valve face. See Photos 6, 7, and 8.

In this picture note the valve is deep in the valve seat with worn valve guild. This can also lead to valve failure. The valve head may get pushed to one side, get caught on the seat, the valve starts to bend and then can’t close. The piston comes up and hits the valve and that starts the destruction of the motor. Also note the spark plug whole insert is starting to pull out. See photos 9 and 10.

Side note about lightened flywheels

On flywheels that have been lightened and timing marks obliterated, it would be good and prudent to check that the marks for the OT (top dead center), S (static), and F (advance) are correct. Not all mechanics are diligent about replacing the timing marks correctly after lightening flywheels. This has given me headaches when setting the timing and valves on some customer’s bikes. Characteristics of incorrect timing marks (and therefore incorrect timing) show up as poor performance, difficulty starting, and higher than normal engine temperatures. Some timing marks I’ve measured have been as much as 12 degrees off.


Learn to adjust your valves correctly, be able to note when you lose clearance, understand the reasons for it, and what you can do about it. Finally, if you are really going to ride the bike and put miles on it, it is prudent to do an unleaded fuel conversion. I believe this applies to all airhead bikes.

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