NOTE: Before lapping the valves to the seats ensure that all related parts are in acceptable condition.
NOTE: Before any valve seat machining and/or lapping can be performed, the guides must be within factory recommended specifications.
NOTE: Match the valves to their numbered cylinders. They must be matched with the same valve seat with which they were lapped.
There is no repair or refinishing possible with the springs, retainers and valve locks. Worn or defective springs, retainers and valve locks must be replaced with new (or known good) parts.
NOTE: If any machining or replacements are made to the valve guides, the valve seats must be machined.
Unless the valve guides need machining or replacing, the only service to perform is to thoroughly clean them.
There are only two types of valve guides used on automobile engines: the replaceable-type (all aluminum heads) and the cast-in integral-type (most cast iron heads). There are four recommended methods for repairing worn guides:
Knurling is a process in which metal is displaced and raised, thereby reducing clearance, providing a true center, and oil control. In some cases, a knurled valve guide will not stand up for more than a short time.
Installing a guide insert involves machining the worn guide to accept a bronze insert. One style is the coil-type which is installed into a guide that has been threaded. Another is the thin-walled insert where the worn guide is reamed oversize to accept a split-sleeve insert. After the insert is installed, a special tool is then run through the guide to expand the insert, locking it to the guide. The insert is then reamed to provide the proper valve stem clearance.
Reaming for oversize valves restores normal clearances and provides a true valve seat. Most cast-in type guides can be reamed to accept a valve with an oversize stem. Oversized valves are generally 0.003 to 0.030 in. (0.076 to 0.762mm), with 0.015 in. (0.381mm) being the most common.
To replace cast-in type valve guides, they must be drilled out, and then reamed to accept replacement guides. This must be done on a fixture which will allow centering and leveling off of the original valve seat or guide, otherwise a serious guide-to-seat misalignment can occur making it impossible to properly machine the seat.
Replaceable-type guides, commonly found in aluminum cylinder heads are pressed into the cylinder head. A hammer and a stepped drift or punch may be used to install and remove the guides. Before removing the guides, measure the protrusion on the spring side of the head and record it for installation. Use the stepped drift to hammer out the old guide from the combustion chamber side of the head. When installing, determine whether or not the guide seals a water jacket in the head. If it does, use the recommended sealing agent. If the valve guides do not seal any water jackets grease the valve guide and its bore. Use the stepped drift and a hammer to drive the new guide into the cylinder head from the spring side (top) of the cylinder head. A stack of washers the same thickness as the measured protrusion may help the installation process. Measure the guide heights to verify correct installation.
NOTE: Before any valve seat machining can be performed, the guides must be within factory recommended specifications.
NOTE: If any machining or replacements were made to the valve guides, the seats must be machined.
If the seats are in good condition, the valves can be lapped to the seats, and the cylinder head assembled.
If the valve seats are worn, cracked or damaged, they must be replaced. The valve seat must be perfectly centered to the valve guide, which requires very accurate machining.
If the cylinder head is warped, it must be machined flat. If the warpage is extremely severe, the head may need to be replaced. In some instances, it may be possible to straighten a warped head enough to allow machining.
NOTE: Any overhead valve cylinder head that shows excessive warpage should have the camshaft bearing journals align bored after the cylinder head has been resurfaced.
CAUTION
Failure to align bore the camshaft bearing journals could result in severe engine damage including but not limited to: valve and piston damage, connecting rod damage, camshaft and/or crankshaft damage.
Certain cracks can be repaired in both cast iron and aluminum heads. For cast iron, a tapered threaded insert is installed along the length of the crack. Aluminum cylinder heads can also be repaired with the tapered inserts; however welding is the preferred method. Some physical damage can be repaired through brazing or welding.
NOTE: Cracks in cast-iron heads require furnace braising to create an adequate seal.