A General Motors 235 engine is often called a "Straight-6" because it has six cylinders configured in a straight line from front to back. The 1955 235 traces it origins back more than a quarter-century, when GM first introduced its workhorse inline six-cylinder engine, and was the last inline six-cylinder engine from GM that had mechanical valve tappets installed at the factory. You need to know how to adjust the 1955 Chevrolet six-cylinder valve tappets to tune this engine correctly.
Drive the car for 30 minutes before you adjust the valve tappets. Put the car into neutral and apply the parking brake. Place a wheel chock behind the rear wheel.
Turn the engine manually until a small, steel-ball alignment mark on the flywheel lines up with a pointer located above the starter through a hole in the transmission bell-housing. When the mark is aligned with the pointer, the number one cylinder will be at top dead centre (TDC).
Remove the number one spark plug and spark plug cable. Remove the distributor cap and note where the rotor is pointing. When the number one cylinder is at TDC, the rotor will point to the empty plug where the number one spark plug cable was connected to the distributor. If the pointer is pointed 180 degrees in the opposite direction, you will need to turn the engine over until the steel ball aligns with the pointer again to be at TDC for cylinder number one.
Remove the valve cover. The first and second rocker arms should be loose, with the number one cylinder at top dead centre. Insert a .020 inch feeler gauge in between the first rocker arm and the top of the valve spring. Loosen the rocker arm adjustment nut and turn the rocker stud with a screwdriver until the .020 inch feeler gauge fits snugly. Tighten the adjustment nut while keeping the stud in position using the screwdriver.
Insert a .020 inch feeler gauge in between the second rocker arm and valve spring. Repeat the process of adjusting the gap for the second rocker arm as for the first, only this time adjust the gap to .020 inches. Repeat the process of adjusting the valve tappets in this order: number three to .020 inches, number five to .0018 inches, number seven to .020 inches, number nine to .020 inches.
Rotate the engine a complete revolution until the steel ball lines up with the pointer again. The number six cylinder is at TDC now and you can finish adjusting the rest of the valve tappets in this order: number 12 to .020 inches, number 11 to .020 inches, number 10 to .020 inches, number eight to .020 inches, number six to .020 inches and number four to .020 inches.
Tighten the number one sparkplug back into the engine. Secure the distributor cap onto the distributor. Connect the number one sparkplug to the distributor with a spark plug wire. Replace the valve cover using a new gasket and tighten.
Run the engine with the valve cover off to check that oil flows up through each rocker arm. Some oil will escape but to be sure each valve is getting enough oil means avoiding internal engine damage in the future.
Debris can easily fall into the engine head during a valve tappet adjustment. Do not permit objects to fall into the engine head or you risk damaging the camshaft, valves or even the crankshaft and pistons.