Pistons & Rods

Pistons take the force of combustion and the connecting rods pass it to the crankshaft. Forged pistons and stronger rods are the usual choice when an engine is rebuilt for more boost, higher revs or sustained load. This category also carries the small parts that go with them: ring sets, gudgeon pins, pin locks and ring compressor sleeves.

For pistons, start with the bore. The piston has to suit the bore as it will be after machining, listed as standard or oversize in steps such as 0.25, 0.50 and 1.00 mm. Then choose the compression ratio, set by the crown shape (dish, dome or flat), to suit the fuel and boost you plan to use. Listings also give the alloy (mainly 2618, with some 4032), any crown or skirt coating, and whether a ring set is included.

Rods and pins have their own details:

• Rods are listed by length, beam type (I-beam or H-beam), big-end bore, and bolt size and material.
• Gudgeon pins are listed by diameter, length and wall type, with case-hardened or alloy steel options.

Much of the result depends on assembly. Hone the bores to suit the actual pistons, measure piston-to-wall clearance at the point the maker specifies, and file-fit the ring gaps to the maker’s figures for your use. Fit pistons the right way round, as marked on the crown, and tighten rod bolts by stretch where the maker calls for it. A new piston and rod set changes the balance of the rotating assembly, so have it balanced, and follow a proper running-in before using full load.