Back in 2010, I built what was rare for me in my later career, an American-made engine. The Lincoln KB V-12, circa 1932, was a big cast iron monster, very different from the mostly French and Italian motors I’d been working on in that era at Phil Reilly & Company.

This story isn’t about the Lincoln, per se, but about a lesson I learned from that project.

It was always hammered into me by the senior staff at Reilly’s that shims are bad, m’kay. Especially any shim thinner than .060″, or a millimeter and a half. Thin shims get beat out, torn up, they aren’t tough enough.

My main experience making shims came from the Maserati Birdcage gearbox arena, where each set of new parts… gears, shift forks, dog rings, etc… required re-shimming the gear stacks to get the gear engagement correct. I had learned that thinner shims wear quickly and loosen the tension on the gear stack that is pinched together by a nut at one end.

Eventually, we only made new shims from Nitralloy steel, machined them to a rough thickness, had them hardened, then finish ground them to size. That helped the gear stack live longer without being beaten loose.

So, onto this Lincoln V-12. The project went well and the motor performed great on the dyno… Endless buckets of torque. But after the motor was in the car and with the owner for a while, a problem arose.

There was a loud tapping noise that occurred only in the few moments after starting the engine from cold. The car came back to assess the situation. Opinions varied on what the issue was. Leadership was certain that it was a valve guide issue… a tight guide making noise and opening up as the temperature rose.

I took the heads off and removed the valves to measure everything. The dimensions were where they were supposed to be, but we decided to give all 24 valves a very slight increase in the stem to guide clearance. It all went back together, I started it up, and “tap tap tap tap tap…”, no change.

It was very difficult to locate the source of the noise. I took the sump off of the motor so that I could look at the rotating parts to figure out what was happening. It still was not immediately obvious.

It turns out, and this still blows me away, that the camshaft was the source of the noise. There was too much end float on the cam, because there was a shim in the bronze casting that located it. The shim broke and fell out, so there was an extra .035″ or so of end float.

When the engine first started, the cam was being pushed back by the force of the chain drive, and one cam lobe was clipping the edge of the valve lifter that was adjacent to it. When the oil pressure came up, it pushed the cam forward and off of the offending lifter and the noise went away.

Wow. That was one of the hardest issues I ever had to track down. Shims suck!

The original casting that located the cam had a weld repair in a previous life and someone used a shim to make up for material that had been machined off. I should have lobbied to make a new part to delete the shim, but I thought it would be OK. I was wrong.

In the end, a new bronze cam bushing was made, and the cam was properly located, with no possibility of losing a shim. Hooray!

Unfortunately, shims are a necessary fact of life in many aspects of automotive engineering. Just about every mechanical device has some kind of spacer or shim inside of it. The key is to know when it is OK to use a thin and fragile shim and when it isn’t. AKA, when is “good enough” actually good enough.

Have you experienced a mechanical disaster as the result of a bad shim? Share it in the comments form below!