Valvetrain for Subaru Impreza GD

The valvetrain opens and closes the ports in the cylinder head, letting the charge in and the exhaust out at the moments the camshaft sets. The category covers valves, guides, stem seals, seat inserts, springs and keepers, plus rocker arm studs and valve cover fastener kits.

A valve must seal the cylinder and shed heat. An exhaust valve passes most of its heat into the seat while closed and the rest through its stem. A poor seat or a worn guide therefore runs it hotter, and a loose guide lets the valve rock, wearing the seat and drawing oil down the stem.

The spring must close the valve on time and keep the follower on the lobe at full revs. Too little spring pressure lets the valve float or bounce, while too much wears the cam and followers.

Parts are listed by engine and size; start from the head you are building:

• Valve material: stainless steel for most intake and road use. Exhaust valves, especially under boost, often use a nickel-based superalloy that stays strong where steel softens. Some have sodium-filled stems to cool the valve head; black-nitrided valves have a hard, low-friction stem.
• Head and stem size: heads are marked (Std) for standard or (+1) for oversize, such as 31.50 mm (+1). Stems are given to the hundredth, such as 5.96 mm, which sets the guide clearance.
• Guides and seals: manganese bronze guides, with some parts in nickel aluminium bronze, a material often chosen for hot, heavily loaded heads. Stem seals are FKM or polyacrylic; FKM takes more heat, which suits exhaust guides.
• Springs and keepers: spring kits give the seat pressure at an installed height, such as 80 lb at 35.70 mm. Single- and triple-groove keepers must match the groove on the stem.
• Seat inserts: rings, often steel, sized by outside diameter, inside diameter and height, such as 34.00 × 26.00 × 8.00 mm; the order of the figures varies by maker.
• Studs and cover hardware: rocker arm stud kits for pushrod engines, and valve cover kits with 1/4-20 or M6 threads and 12-point or hex heads.

Valve work belongs in a machine shop. Exhaust stems need slightly more guide clearance than intake stems, as they expand more. Cut each seat to an even, concentric band, shim the springs to installed height and check them on a tester.

A higher-lift cam calls for checks on coil bind, retainer-to-seal and valve-to-piston clearance. Fit new keepers and seals with new valves, and tighten cover fasteners lightly, as the gasket does the sealing.