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The venerable Chevy 350 engine has been a mainstay of the industry, and is a favorite of hot-rodders and backyard mechanics. The torque specifications are required to ensure that the engine components are properly tightened. If the proper torque is not applied the motor may leak fluids or become unreliable. An over-tightened bolt can damage the threads, or break the bolt--causing time-consuming and sometimes costly repair. If the bolt is not tight enough, it may eventually back out of the threads, resulting in severe engine damage.
Lower Engine Components
The piston caps have two bolts per piston that require specific torque. You will also need to torque the main-bearing caps that keep the crankshaft in place. If you over-tighten the bolts you eliminate the small space between the bearing and journal that allows proper oiling. If you under-tighten the piston or main-cap bolts, there will be too much space, and the volume of the oil will not create the lubricated cushion around the journal.
Low-End Torque Specs
Main-cap torque specification on the typical Chevrolet 350 two-bolt-main engine is 70 ft.-lbs. Specifications for the four-bolt-main are 70 ft.-lbs for the inner bolts, 65 ft.-lbs for the 7/16-inch outer bolts, and 40 ft. lbs. for the 3/8-inch outer bolts. The connecting-rod torque specifications are 40 to 45 ft.-lbs. for 3/8-inch bolts, and 35 to 45 ft. lbs. for 11/32-inch bolts.
Top Engine Components
The intake manifold and the heads are the main top-end components that rely on torque specifications. The intake specs vary due to the differences between metals, such as aluminum intake to iron heads, or iron intake to aluminum heads. If you under-torque the intake bolts, this can cause coolant and vacuum leaks. If you over-tighten, the bolt threads can be severely damaged and you risk breaking a bolt off inside the head. The heads require the correct torque and tightening sequence to prevent damage to the head gasket. Again, if you over-tighten them, the bolts may be broken off inside the block.
Top-End Torque Specs
You should always check the year of your engine before torquing any bolts. The specifications given are for the typical 350 engine. The Chevrolet 350 cylinder-head torque specification is 65 ft.-lbs. The intake installed onto cast-iron heads requires 30 ft.-lbs. If your engine has aluminum cylinder heads, the threads are more fragile than the cast iron, so the intake specification is 18 ft.-lbs.
Accessories
The accessories have certain specifications as well. These include the water pump, flex plate or flywheel, spark plugs, timing cover, intake manifold and oil pan, just to name a few. A plastigage is very useful for making sure that all clearances are correct. You can purchase a plastigage at any auto parts store and it comes with instructions for use. You will also need a torque wrench and engine oil. Wipe the oil on the bolt threads to assist the bolts during tightening, and be sure not to cross-thread them. All specifications above are listed for a stock engine. If you purchased aftermarket products you need to refer to the manufacturers specifications.
