Saturday, November 29, 2008
Valve Job - Replacing Guides
The cast aluminum heads on a Volkswagen engine are fitted with four valve guides made of phosphor-bronze. The stems of your valves are installed in the guides. As the valves open and close, the small amount of clearance between the valve stem and the valve guide provides a direct path to the atmosphere. This isn't an especially good idea, so the valve guide is usually fitted with a seal. In the HVX mods I discuss the seal and show how to install them.
As the valves open and close they cause the valve guides to wear. The more they wear, the bigger the gap to the atmosphere and the more the valve will fail to run true. Due to the high temperatures present around the exhaust valve, the problem of a worn valve guide is more evident with your exhaust valves.
We periodically check the valve guides for wear. When the wear approaches the allowable limit, we replace the valve guide(s). The guides for the exhaust valves typically wears about three times faster than the guides for the intake valves. That means we will replace the exhaust valves about three times before we have to replace the guides for the intake valves. The exception to this rule is when you use the shorter valve guides from a water-cooled VW engine for your intake valves. (This modification is done to improve the in-flow at high rpm and does not apply to flying Volkswagen engines.)
To replace the valve guides we use a core drill, a suitable punch or drift, and simply drive them out. To install the new guides we heat the heads to 450 degrees on the Fahrenheit scale, chill the guides in dry ice or propane, and simply drive the new guide into the head. The first picture shows the step-drill and piloted driver used to replace (and install) VW valve guides. The picture on the right shows the stepped core-drill used to drill-out the valve guide before it is driven out of the head. When the valve guide is opened up in this manner it loses most of its grip on the head and can be easily driven out with a suitable punch or drift. The core drill is home-made. It starts out as a 7/16" drill bit. A half-inch (0.500") of the tip is ground down to a diameter of 0.3125"
The picture on the right shows the stepped drift used to drive the valve guide out of the head after it has been opened up by the core drill. The tip of the drift is turned to 0.308" for a distance of 1.125". The shank of the drift is 0.450" for a distance of 3.0". The drift started out as a standard 6" pneumatic drift but any bar of 1/2" steel can be made to serve.
-Robert S. Hoover
-30 Nov 2008