See Figure 1
Worn valve guides can, in most cases, be reamed to accept a valve with an oversized stem. Valve guides that are not excessively worn or distorted may, in some cases, be knurled rather than reamed. However, if the valve stem is worn, reaming for an oversized valve stem is the answer since a new valve would be required anyway.
Knurling is a process in which metal is displaced and raised, thereby reducing clearance. Knurling also produces excellent oil control. The possibility of knurling instead of reaming the valve guides should be discussed with a machinist.
REAMING VALVE GUIDES
See Figure 2
If it becomes necessary to ream a valve guide to install a valve with an oversized stem, a reaming kit is available which contains an oversize reamers and pilot tools.
When replacing a standard size valve with an oversized valve always use the reamer in sequence (smallest oversize first, then next smallest, etc.) so as not to overload the reamers. Always reface the valve seat after the valve guide has been reamed, and use a suitable scraper to brake the sharp corner at the top of the valve guide.
See Figure 3
Valve guides which are not excessively worn or distorted may, in some cases, be knurled rather than reamed. Knurling is a process in which metal inside the valve guide bore is displaced and raised (forming a very fine cross-hatch pattern), thereby reducing clearance. Knurling also provides for excellent oil control. The possibility of knurling rather than reaming the guides should be discussed with a machinist.
See Figure 4
Valve stem-to-guide clearance should be checked upon assembling the cylinder head, and is especially necessary if the valve guides have been reamed or knurled, or if oversized valves have been installed. Excessive oil consumption often is a result of too much clearance between the valve guide and valve stem.
- Clean the valve stem with lacquer thinner or a similar solvent to remove all gum and varnish. Clean the valve guides using solvent and an expanding wire-type valve guide cleaner (a rifle cleaning brush works well here).
- Mount a dial indicator so that the stem is 90 degrees to the valve stem and as close to the valve guide as possible.
- Move the valve off its seat, and measure the valve guide-to-stem clearance by rocking the stem back and forth to actuate the dial indicator. Measure the valve stems using a micrometer and compare to specifications, to determine whether stem or guide wear is responsible for excessive clearance.