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These guides are the highest precision quality guides available on the market today. |
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Tolerance for OD, ID and concentricity is less than + or - 0.0005" (1/2 a thou!) |
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Designed with today's hostile engine environment in mind. |
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Superior heat conductivity results in reduced exhaust valve temperature. |
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Valve seat recession (a big problem with today's gas) is reduced because the valve is kept cooler. |
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Elimination of oil contamination of the fuel and combustion chamber. |
Important info for fitting/installing these components:
Some engines use the factory supplied stem seal, while others use a special teflon 'Positive' type seal supplied by APT
Stem seals are normally only fitted to the
intake on most engines. There is a risk with the exhaust that a seal that
does not allow any oil whatsoever to enter the guide can end up having a
problem from total lack of lubrication.
If rebuilding a worn engine but are NOT changing the guides then it is
usually OK to fit a not too aggressive seal to the exhaust.
Stem to guide clearance tolerance is
critical for trouble free running.
Because the guide is squeezed in the middle from being pressed into the
head, and because valve stems vary so much in their tolerance, it is
really a good idea to have the guide "sized" by a good machine
shop.
This is really a whole subject in itself, and we plan one day to
put up a complete article dealing with all the ins and outs of this very
important subject.