How BMW geeks use Alfa shims for creative valve adjustments
Last year, I wrote a piece detailing the procedure for valve adjustment on single overhead cam engines with eccentric adjusters. In it, I described how you rotate the engine to put it at top dead center (TDC) for a cylinder, verify that the rockers are on the flat part of the back of the cam lobes, use a feeler gauge between the adjuster and the top of the valve stem to check the gap, adjust it if necessary using a small wrench and an Allen key or equivalent tool to move the eccentric, and then rotate the engine to move to the next cylinder in the firing order.
What happens if you can’t adjust the valve? Well, as you sometimes read on the Internet, there’s this one weird trick.
First, when I say “What happens if you can’t adjust the valve,” what I mean is that sometimes, if you have one loud valve that’s going tick-tick-tick, when you try to adjust it, you find that the adjuster runs out of travel. This sometimes happens with eccentric adjusters, so-named because they employ a circular disc with an off-center hole in it. Rotating the thick part of the disc downward decreases the clearance between it and the top of the valve stem.
Normally, if you rotate the eccentric far enough, it stops at the top of the valve stem (or pinches the feeler gauge you’ve inserted between the two), but you may find that, instead of coming to a stop like it should, the eccentric simply spins around in a circle.
If this happens, it’s almost always due to the engine having a lot of miles on it and the valvetrain components consequently being badly worn. The odds are that if you take the rocker arms and shafts out of the head, you’ll find that the rocker shaft or the bushing inside the rocker arm (or both) for the un-adjustable valve has had a groove or a flat spot worn in it by the repetitive motion of the rocker arm. That wear has caused the tip of the rocker arm to move slightly away from the end of the valve stem. The eccentric, even when rotated into its furthest-extended position, can no longer make up the difference.
In a do-it-once-do-it-right world, the answer to this is to replace the worn-out components. Unfortunately, that usually means pulling the head and disassembling the valvetrain. And, because valvetrain components that are that worn are indicative that the entire engine is nearing the end of a wear cycle, down the slippery slope you go to an expensive engine rebuild.
There is, however, a way to dig in the crampons and the ice axe, not slide down the slippery slope, and instead simply make the valve adjustable again, stop the loud tick-tick-ticking, and eke out another driving season: use a valve shim, also known as a lash cap.
Let me back up a step. As I said in last year’s article, overhead cam engines can use one of three methods of valve adjustment. They can have hydraulic lifters which are self-adjusting (though hydraulic lifters are employed far more often in pushrod engines than in overhead cam engines). They can have a manual adjustment mechanism such as eccentrics or threaded studs and a locknut. Or they can use a bucket-and-shim configuration where shims of selectable thickness sit on top of the valve stems. The “weird trick” if your eccentric runs out of adjustment room is to employ a shim on the top of the valve stem.
What you’re doing when you put a valve shim over the very top of the valve stem is effectively extending the stem enough to allow the eccentric to get close enough to achieve the appropriate clearance. Professional mechanics may warn you that if so much has worn off a shaft or the inside of a rocker arm that the eccentric won’t reach the valve stem, your oil pressure is going to suffer due to the oil’s ability to flow out through this wear area instead of inside the shaft to other rockers. In extreme cases, that’s true of almost any badly-worn oil distribution passage. I don’t believe that quieting the valvetrain down with lash caps will hasten the engine’s demise. In fact, once the valves open the proper amount, there should be less mechanical travel on the worn parts, so it may even buy you some time. As long as you’re not deceiving yourself, or a potential buyer, I don’t see the downside. Since it’s one method of valve adjustment bailing out another, I even think it’s kind of cool.
In the vintage BMW world where I spend so much time, it’s been well-known for 40 years that valve shims from vintage Alfa Romeos fit just about perfectly over the tops of BMW valve stems. Since, on an Alfa, the shims are the primary method of valve adjustment, they come in a range of thicknesses. You’re looking to make up a small gap caused by wear, so you want one of the thinner shims. A little cajoling may be necessary to get the shim over the top of the valve stem. You may need to depress the valve spring slightly to allow enough clearance at the tip of the valve stem to get the shim over it. I usually lean down on the top of the valve spring with the rubber-coated handle of a hammer. Be sure to temporarily plug the oil drain holes in the head with paper towels so, in case you let go of the shim, it doesn’t wind up in the oil pan (ask me how I know).
There is a circumstance other than excessive valvetrain wear that can result in the eccentric’s not having enough extent to adjust the valves. If the stock cam has been replaced with a reground cam, the backs of the camshaft lobes will be lower than on the original cam. This may make it so that the standard eccentrics can’t reach the valve stems. It was this situation that caused me to press the old valve shim trick back into service.
Last summer, I was resurrecting a 1975 BMW 2002 that I’d bought back from a friend after nearly 30 years of disuse. I’d built a hot motor for this car in the 1980s—10:1 pistons, Weber 40 DCOE side-draft carburetors, hot cam. When I bought it back, I did a leakdown test and found that two of the valves weren’t sealing. I pulled the head, had it redone at the machine shop, and when I reassembled it, was perplexed by the fact that two of the valves couldn’t be adjusted. It made sense once I remembered that the cam was a 300-degree regrind, but there weren’t valve shims in it before. I can only surmise that wear patterns and minor differences in the position of the valvetrain components after reassembly were responsible. These days, at least in the BMW world, you can simply buy oversized eccentrics that are made specifically for this application. But locating my cache of 35-year-old shims and installing two of them solved the problem.
So, if you have an un-adjustable valve on a high-mileage engine, the valve shim trick may help you eke another season or two out of it. If the engine has no other obvious signs of wear, you may be able to pull the head and refresh the valvetrain. If compression and performance are also low, and the engine visibly burns oil, then the entire engine is well into a wear cycle and should be refreshed as a whole.
Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel magazine for 30 years. His most recent book, Just Needs a Recharge: The Hack Mechanic™ Guide to Vintage Air Conditioning, is available on Amazon (as are his previous books). You can also order personally inscribed copies here.