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Discussion Starter #1 (Edited)
The valve components that may have to be replaced will depend on the age and condition of the engine. New exhaust valves are often needed because they run much hotter than intake valves and often "burn" or fail because of erosion and heat cracking. Exhaust valves also stretch with age, which increases the danger of valve breakage. For this reason, you might want to replace all the exhaust valves with new ones regardless of their condition.

Intake valves can generally be reused unless bent or worn. Replacement is required if stem wear exceeds specifications.

If guides are worn, which they usually are, the engine can suck a lot of oil. Evidence of this is usually heavy black carbon deposits on the backs of the intake valves and heavy carbon deposits on the pistons and in the combustion chambers. Minor guide wear can be reduced somewhat by knurling. If integral guides are worn, they may be drilled out to accept thin wall bronze or cast iron guide liners, or reamed to oversize (which requires new valves with oversize stems). Worn guides in aluminum heads can also be lined, or reamed to oversize or pressed out and replaced with new guides.

Unless valve seats are cracked or badly worn, they can usually be reconditioned by cutting or grinding. Damaged or badly worn seats in aluminum heads will have to be replaced. Bad seats in cast iron heads can sometimes be repaired by machining out the old seat to accept an insert.

Engine Rebuild Tutorial Links:

 
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