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What is the better practice: replace a burnt exhaust valve and 850 1996

posted by  someone claiming to be Hugh on  Wed Feb 2 08:48 CST 2022 [RELATED]

Hello All,

I recently posted a query involving my 1996 850 GLT wagon, non-turbo with a 5-speed manual which when it hit 430,000 miles (I purchased the car new) burnt an exhaust valve in cylinder # 1. I am having the engine rebuilt at a private garage. The mechanic at the private garage has recommended replacing the burnt valve with a genuine Volvo exhaust valve and having the remaining valves "cut" if I am using the correct term at the machine shop so they sit properly in the head which will also be machined. Another local mechanic, who I have known for many years and whose advice I respect thinks the better practice would be to replace all the exhaust valves. By "cutting" the edges of the old exhaust valves, you are making them thinner and more prone to fail in the future. Hence the better course of action is to just replace them with new exhaust valves. This advice seems to make sense, but I would welcome the opinions and advice of others. Thank you in advance to all who respond. I appreciate your help. This is "new territory" for me.

Hugh








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    posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

    I would 'wait and see' as the valves are pulled and inspected. Obviously, both exhaust valves in Cyl #1 need to be replaced. It wouldn't be hard to inspect the other valves and seats once the head is removed.

    BTW, all 24 valve stem seals must be replaced anyway. :)


    --
    Keeping it running is better than buying new



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      posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

      Klaus,

      The garage that is performing this rebuild is following the procedure you outlined in terms of inspecting the internal condition of the valves, cylinder walls, head, piston and piston rings. The oil seals will be replaced. I hope after spending all this money, I can get another 200,000 miles out of this Volvo. This particular 850 has 430,000 miles at the moment. Thank you for your reply.

      Hugh


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    posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

    At $40 each I'd have to see what the other valves looked like before i replaced all of them. Can the other valves be lapped into the seats rather than ground ?

    Greg



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      posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

      Greg,

      My understanding is the valves can be lapped. Although the mileage is high, I have changed the oil and oil filter every 5,000 miles. Other than the failed exhaust valve in cylinder # 1 I have been advised by the mechanic the interior of the engine appears to be in very good shape.

      Hugh



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        posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

        An interesting question for the more knowledgeable metallurgists amongst us might be: Does grinding the valves compromise any sort of hardening that they may have been treated to during manufacturing? Just curious……



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          posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

          I am neither a materials scientist nor knowledgeable, but I do have some experience in such things.

          Assuming the valve material and its properties are homogeneous, the same all the way through and not just surface hardened, then the grinding process wouldn't affect the strength of hardness properties of the material. That assumes of course that the valve is not subjected to excessive heat (think cherry red) during the grinding process.

          --
          XC60 / Odyssey



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            posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

            Makes sense to me. I just didn’t know the right word for “hardened all the way through”. I guess only the valve manufacturer could answer that one.

            Hmmm, could it be that a good valve grinding setup might incorporate some sort of water or oil cooling in order to keep the temp under control?



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              posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

              Chris,
              What you describe is what I have seen in old black and white films of the manufacturing process of new cars where certain components are being grinded. You will see water or some liquid being poured over the contact point of the drill. I never thought about the fact the liquid was being used to "cool" the drilling process.

              Hugh



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