Ford Explorer, Maintaince And Repair Manual, Page: 476 / 1119

Knurling is a process in which metal is displaced and raised, thereby reducing clearance, giving a true center,
and providing oil control. It is the least expensive way of repairing the valve guides. However, it is not
necessarily the best, and in some cases, a knurled valve guide will not stand up for more than a short time. It
requires a special knurlizer and precision reaming tools to obtain proper clearances. It would not be cost
effective to purchase these tools, unless you plan on rebuilding several of the same cylinder head.
Installing a guide insert involves machining the guide to accept a bronze insert. One style is the coil−type
which is installed into a threaded guide. Another is the thin−walled insert where the 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 the standard size for proper valve
clearance.
Reaming for oversize valves restores normal clearances and provides a true valve seat. Most cast−in type
guides can be reamed to accept an valve with an oversize stem. The cost factor for this can become quite high
as you will need to purchase the reamer and new, oversize stem valves for all guides which were reamed.
Oversizes are generally 0.003−0.030 in. (0.076−0.762mm), with 0.015 in. (0.381mm) being the most
common.
To replace cast−in type valve guides, they must be drilled out, 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 may occur making it impossible to properly machine the seat.
Replaceable−type guides 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 also seals a water
jacket in the head, and if it does, use the recommended sealing agent. If there is no water jacket, grease the
valve guide and its bore. Use the stepped drift, and hammer the new guide into the cylinder head from the
spring side of the cylinder head. A stack of washers the same thickness as the measured protrusion may help
the installation process.
VALVE SEATS
Before any valve seat machining can be performed, the guides must be within factory recommended
specifications. If any machining occurred or if 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. See
the valves section for instructions on lapping.
If the valve seats are worn, cracked or damaged, they must be serviced by a machine shop. The valve seat
must be perfectly centered to the valve guide, which requires very accurate machining.
CYLINDER HEAD SURFACE
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. In either case, contact a professional machine shop for service.
Any OHC cylinder head that shows excessive warpage should have the camshaft bearing journals align
bored after the cylinder head has been resurfaced.
WARNING
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 breakage.
HOW TO USE THIS BOOK
470 REFINISHING & REPAIRING