Many years ago, I rebuilt the Muncie 4-speed transmission in my 1968 Chevelle SS396. The installation seemed to go well, but I did have to adjust the linkage of the new Hurst shifter somewhat to align the shifter rods properly into the neutral position. Upon completing the installation, I took the car out for a test drive. When shifting from 1st to 2nd, it felt like a very hard downshift, the rear tires squealed and the engine RPM increased sharply! I then quickly shifted it into 3rd, and it almost stalled. I don’t believe that I tried the top gear, and I managed to limp the car safely home.

I crawled underneath and examined my work while looking over the new shifter installation instructions. I soon realized that I had installed the shifter lever arms upside down on the transmission! This effectively changed the shift pattern from the standard 1-2 : 3-4 to 2-1 : 4-3. So on the test drive I had shifted normally, but due to the upside down levers, I started out in 2nd gear then shifted to 1st and then to 4th. After repositioning and adjustment of the levers the shifter worked perfectly.

If I had read the instructions and looked at the diagrams before installing the new shifter, the additional work could have been avoided.

Rob in Tennessee

The procedure for replacing a leaky cylinder head gasket traditionally includes having a machine shop resurface the cylinder head so it is smooth and flat. Unfortunately, many engine designs did not include a lot of extra cylinder head thickness. Grinding what seems like a small amount of metal off a cylinder head's mounting surface might take it out of specification and potentially cause problems.

With a thinner cylinder head, an overhead camshaft moves closer to the crankshaft. This may loosen the timing chain and retard camshaft/valve timing. Camshafts that are directly driven by gears rather than by a chain/belt might experience binding of the gears due to out-of-spec gear backlash.

A thinner cylinder head may also reduce the size of the combustion chamber. This could increase the compression ratio enough to cause knocking. Engine damage might happen immediately if exhaust/intake valves are now close enough to hit piston tops.

The computer(s) that control modern engines will likely generate a flood of trouble codes as sensors send data showing that multiple systems (camshaft/crankshaft timing, emissions, compression...) are out-of-spec. Intake manifolds, brackets or other parts that bolt onto the cylinder head may no longer fit because mounting holes no longer line up.

Of course one solution is to not grind off too much metal so cylinder head thickness stays within the engine manufacturer's original specifications. Another solution is to replace the excess metal removed with a cylinder head spacer shim or a thicker head gasket. What works best varies for different engine designs. The huge RockAuto.com catalog is especially useful here because for many engines it offers a choice of proven solutions from multiple parts manufacturers. When the old cylinder head is just unusable, RockAuto often has remanufactured and/or new cylinder heads as well.

Cylinder head spacer shims are typically made of steel or copper and add 0.020 inches (0.51 mm) of space. The shim is usually sandwiched between the head gasket and the engine block, but that varies from engine to engine. It is important to always follow the installation instructions that came with the cylinder head spacer shim.

Steel & Copper Shims (top left), Oversize Head Gasket (top right) & Cylinder Head (bottom)

Thicker head gaskets typically build the 0.020 inches of extra space right into the head gasket. Do not be tempted to install a thicker head gasket whenever one is available. If cylinder head thickness is still within the engine manufacturer's specification, then installing a thicker head gasket may add too much space and cause problems on the opposite end of the spectrum (timing chain that is too tight, combustion chamber that is too big, etc.). The extra thickness of a thicker head gasket is typically there to act as a shim and not intended to somehow seal better than a normal head gasket.

See all the cylinder head related parts available for your specific vehicle under "Engine" in the RockAuto.com catalog.

Tom Taylor,
RockAuto.com

To read more of Tom's articles, click this link and choose from story titles on the Newsletter Archives page.