How to Convert a 350 Engine to a 358 Engineby John Willis
Engines, especially racing engines, are overbored due either to cylinder wear or for slightly better performance. Not coincidentally, NASCAR has a limitation of 358-cubic-inch displacement engines, limiting the popular 350-cubic-inch V8s. The process is done with a kit that includes new, larger pistons and rings and connecting hardware. The cylinders have to be bored out with a machine mill to match the larger-displacement setup.
Disassemble and remove the valve covers, timing chain and cylinder heads. When disconnecting timing chains, twist a lightweight wire around both ends of the chain so it cannot fall down into the timing cover, making retrieval difficult. Carefully wrap shop towels around the pistons and connecting rods so the edge of the engine block doesn't mar or damage any parts.
Thoroughly clean the cylinder heads.
Machine the heads -- or have them machined by your machinist -- according to the specifications in your overbore kit. It is a fairly light bore, as it is only an additional 8 cubic inches of displacement for 8 cylinders, or 1 inch per cylinder. However, there is not a standard diameter for all engines, as different makes of 350s use different ratios of bore and stroke -- the size of the cylinder, relative to the height of the cylinder. Subsequently, your 358 overbore kit will be specifically matched to the bore and stroke of your make and model 350 engine.
Reassemble the engine using new wrist pins to connect the new larger pistons and rings. Use new gaskets and torque the heads to factory specifications in factory-specified torque patterns so the fastening tension is increased evenly. Refasten the timing chain.
Readjust the valves and double-check your ignition timing before starting. Replace valve covers.
For a street car, break in the new piston and rings gradually, letting the bearing surfaces mate, in the same way you would for a new car.
- These small overbore kits are an opportunity to upgrade when there is piston wear or cylinder damage. They are different from large "big bore" kits that dramatically increase displacement, including "stroker kits" that require the bottom end of the engine to be disassembled and a new crankshaft installed.
Items you will need
- Overbore kit
- Machine mill
- Torque wrench
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