REAMING VALVE GUIDES
If it becomes necessary to ream a valve guide to install with an oversize stem, a reaming kit is available which contains a oversize reamers and pilot tools.
When replacing a standard size valve with an oversize 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.
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.
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 oversize valve 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° 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.
The valve must be lapped into their seats after resurfacing, to ensure proper sealing. Even if the valve have not been refaced, they should be lapped into the head before reassembly.
Set the cylinder head on the workbench, combustion chamber side up. Rest the head on wooden blocks on either end, so there are 2-3 in. (51-76mm) between the tops of the valve guides and the bench.
- Lightly lube the valve stem with clean engine oil. Coat the valve seat completely with valve grinding compound. Use just enough compound so that the full width and circumference of the seat are covered.
- Install the valve in its proper location in the head. Attach the suction cup end of the valve lapping tool to the valve head. It usually helps to put a small amount of saliva into the suction cup to aid it sticking to the valve.
- Rotate the tool between the palms, changing position and lifting the tool often to prevent grooving. Lap the valve in until a smooth, evenly polished seat and valve face are evident.
- Remove the valve from the head. Wipe away all traces of grinding compound from the valve face and seat. Wipe out the port with a solvent soaked rag, and swab out the valve guide with a piece of solvent soaked rag to make sure there are no traces of compound grit inside the guide. This cleaning is very important, as the engine will ingest any grit remaining when started.
- Proceed through the remaining valves, one at a time. Make sure the valve faces, sets, cylinder ports and valve guides are clean before reassembling the valve train.