old_man
NAXJA Forum User
- Location
- Loveland, Colorado
I have been asked this question several times, so I thought I would just put together a short tutorial on valve guides and make it available to be found with a search. If your engine has what I call Burger King smoke, in other words if you have the vehicle warm and have been running down the road, then pull into the drive through at BK and end up waiting for your order for a few minutes, and when you get to the pickup window, you see a cloud of smoke behind you, you most likely have worn valve guides and or shot valve seals.
As the valves open and close, the stem of the valve moves up and down within a long cylindrical hole drilled in the head called the valve guide. The tolerances are quite tight. On the intake valve, the vacuum tries to suck oil from the top of the head where the springs and rockers reside. Most modern engines use what is called a valve seal to reduce the oil being sucked into the intake and ultimately taken into the cylinder where it is burned and turned into the blue smoke we sometimes see on cars.
There are two basic types of valve seals. The most common looks almost like an umbrella that the valve stem sticks up through. It actually goes up and down with the valve. It keeps oil splash from directly hitting the area where the valve stem exits the head. The second is what is called a positive seal. A positive seal is attached to the head and the valve stem slides up and down through the seal. Positive seals require a particular profile to be machined into the head that the seal snaps on.
As an engine wears, the clearance between the valve stem and the head gets larger. This allows more oil to be sucked into the engine. Positive seals tend to help minimize this but are not 100% efficient.
So how do you get rid of this oil burning. In the old days you could actually buy valves that had a larger diameter valve stem. You would then simply run a precision reamer though the valve guide to make it match the new valve stem diameter. The problem is that these valves are getting hard to find, not to mention more expensive.
The cheap method used today is called knurling. A tool that looks similar to a tap is threaded down though the valve guide. The thing that seperates this tool from a tap is that it does not have a cutting edge. As it is screwed in, it displaces some of the metal, pushing it up. It is then followed by a reamer to cut the tops of the displaced material down to the correct clearance for the valve stem. If you think about it, the valve will not wiggle in the guide any more, but you end up with what looks like a set of screw threads down though the guide. They count on the fact that the length of the thread is long enough that the suction can't pull much oil. That is all fine and good, but think....the amount of metal that is supporting the valve stem is less due to the spiral groove. That means that the smaller amount of metal will wear faster. This is why buying an off the shelf "rebuilt" head many times works fine for 15-20k miles, then the smoking starts.
Other than going to a larger valve stem, there is really only one way to make a head as good as new...or even better. It is called sleeving the guide. The last time I had it done it cost about $5 per valve. The machinist actually drills out the valve guide significantly larger. A bronze tubular insert is then driven into the guide. The insert is then reamed back to the correct size. As it turns out, the bronze is actually a better material than the original iron of the head. It isn't done to new heads for one reason, cost.
I have seen several people take a worn head and simply go with new valve seals. This tends to help, but only for the short term.
Good luck getting rid of the smoke.
As the valves open and close, the stem of the valve moves up and down within a long cylindrical hole drilled in the head called the valve guide. The tolerances are quite tight. On the intake valve, the vacuum tries to suck oil from the top of the head where the springs and rockers reside. Most modern engines use what is called a valve seal to reduce the oil being sucked into the intake and ultimately taken into the cylinder where it is burned and turned into the blue smoke we sometimes see on cars.
There are two basic types of valve seals. The most common looks almost like an umbrella that the valve stem sticks up through. It actually goes up and down with the valve. It keeps oil splash from directly hitting the area where the valve stem exits the head. The second is what is called a positive seal. A positive seal is attached to the head and the valve stem slides up and down through the seal. Positive seals require a particular profile to be machined into the head that the seal snaps on.
As an engine wears, the clearance between the valve stem and the head gets larger. This allows more oil to be sucked into the engine. Positive seals tend to help minimize this but are not 100% efficient.
So how do you get rid of this oil burning. In the old days you could actually buy valves that had a larger diameter valve stem. You would then simply run a precision reamer though the valve guide to make it match the new valve stem diameter. The problem is that these valves are getting hard to find, not to mention more expensive.
The cheap method used today is called knurling. A tool that looks similar to a tap is threaded down though the valve guide. The thing that seperates this tool from a tap is that it does not have a cutting edge. As it is screwed in, it displaces some of the metal, pushing it up. It is then followed by a reamer to cut the tops of the displaced material down to the correct clearance for the valve stem. If you think about it, the valve will not wiggle in the guide any more, but you end up with what looks like a set of screw threads down though the guide. They count on the fact that the length of the thread is long enough that the suction can't pull much oil. That is all fine and good, but think....the amount of metal that is supporting the valve stem is less due to the spiral groove. That means that the smaller amount of metal will wear faster. This is why buying an off the shelf "rebuilt" head many times works fine for 15-20k miles, then the smoking starts.
Other than going to a larger valve stem, there is really only one way to make a head as good as new...or even better. It is called sleeving the guide. The last time I had it done it cost about $5 per valve. The machinist actually drills out the valve guide significantly larger. A bronze tubular insert is then driven into the guide. The insert is then reamed back to the correct size. As it turns out, the bronze is actually a better material than the original iron of the head. It isn't done to new heads for one reason, cost.
I have seen several people take a worn head and simply go with new valve seals. This tends to help, but only for the short term.
Good luck getting rid of the smoke.
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