The valve guide seals are simple and relatively cheap and always the problem. If the guides are wore the seals won't last as long. Even poor guides are usually sealed for 10-20k with new seals.

There is no reason to rebuild the whole head untill the seals don't last but 10k or so

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician