Valve guides help to control the valve by supporting the loads placed on it during operation. Guides are normally made from nodular cast-iron and machined on the inside and outside surfaces. The inside diameter (I.D.) is sized to give the proper valve-to-guide clearance. The outside diameter (O.D.) is sized to be a few ten-thousandths of an inch larger than the I.D. of the valve guide hole in the head. This is called an interference fit and helps ensure that the valve guide does not slip during operation. Once you have removed the cylinder head, you can replace the guides.
- Skill level:
- Moderately Easy
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Things you need
- Leather gloves
- Propane torch
- Valve guide tool
- Small hole gauge
- Valve guide seals
Don the leather gloves, then invert the cylinder head and heat the combustion chamber with a torch. Heat a broad area around the valve guide, but do not apply heat directly to the guide. Flip the head over and heat the top where the valve guides protrude. Quickly flip the head back over and insert the valve guide tool into a guide, and drive the tool and guide through the head with a hammer. Repeat for each guide.
Measure the I.D. of the valve guide hole in the head with a small-hole gauge and a micrometer. Refer to factory specifications for the right size O.D. for the new guide. Check the O.D. of the valve stem with a micrometer, and check the I.D. of the new guide. Refer to factory specifications and verify that these are within specifications.
Place the new valve guide in a freezer or a cup of ice water to thoroughly chill it and force it to shrink up as much as possible. Heat the cylinder head as in Step 1. Insert the valve guide tool into the chilled valve. Quickly drive the guide into its hole in the cylinder head from the top down until the shoulder on the guide bottoms out on the head. Repeat for each valve.
Install the new valve guide seals onto the tops of the guides by hand.
Tips and warnings
- Always wear gloves when handling the hot heads.
- Do not overheat the head or concentrate the heat at any one point during heating. Localised expansion caused by uneven warming can lead to stress cracks that will ruin the head.
- The guide must be properly sized to its hole. Since it is an interference fit, there is a tremendous load placed on the guide and the surrounding aluminium of the head. If not fitted precisely, these stresses can easily overcome the tensile strength of the aluminium and crack the head, ruining it.