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My church is looking to buy a used high milage van from a dedicated lease company. Good reputation. I have education but little hands on and the church uses my input for van preinspection. I'm not looking for advice on whether to purchase, I just want information on common practice. Upon inspecting this van I found some minor deficiencies but there was also a small oil leak from the head leaking externally. They are removing the head and having it machined before sale. My question is: Isn't it common practice to do both heads because I would suspect the most common cause for a warped head is overheating problems in the past.

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TL DR: Not an engine killer, but not the way I'd do it.

Not necessarily. There is an old adage when it comes to working on vehicles: If it ain't broke, don't fix it. The meaning here is, if you try to fix something which doesn't have a problem, you can introduce new problems, which once fixed can give you other issues.

That said, there are some considerations. The main one being, when you machine a head, you remove material from the head surface, which shrinks the combustion chamber size, which in turn raises the compression ratio of the cylinders involved. Raising the compression ratio raises the power level. As a rule of thumb, a single point of compression is worth about 3% power increase on a typical V8 engine (I'm assuming this is what is in the van, since it has two heads). This, in and of itself, would get me to do the same to both heads to ensure the engine stays equal on both cylinder banks. While this wouldn't be an engine killer, it will make it so things are not balanced, which might cause issues down the road.

I'm not sure why they went directly to machining the head. Most of the time a head gasket change would most likely fix the issue. They are the ones there who checked the head, though, so maybe they have a better handle on things looking directly at the head. Like I said though, if it was me, I'd do both sides at the same time to ensure power balance in the engine stays relative.

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obviously I didn't see the vehicle but an external oil leak i would assume wouldn't be a head gasket at all but a valve cover... also i would almost certainly change the other side gasket if you got it apart already rather than rip it apart again in 20,000 miles why not?

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  • Both the valve cover gasket and the head gasket are possible sources for an oil leak. I would inspect thoroughly before assuming the source.
    – Solar Mike
    Sep 24 '20 at 10:20
  • I had a good view of the source. Even without dye it was obvious that it is coming out between head and block. Confirmed by sellers mechanic. I called and said that if they remove, inspect, both heads and repair as necessary, and send wet\dry compression test results I would recommend purchase. They already have one head off, sent it for inspection for trueness. They agreed to do both heads and knock off $400 because they were embarrassed that they missed things during their own pre sale inspection.
    – Jupiter
    Sep 24 '20 at 13:17
  • An update: I am now speaking directly to the mechanic doing the repairs. When they sent the right side head to the machine shop, it was not warped. But they did find that the valves needed attention. So the engine is out of the van and they will machine both heads and do a valve job and still give us the price they requested minus $400 for our inconvenience. Looking like a winning situation.
    – Jupiter
    Sep 29 '20 at 20:10

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