For many it is common practice to leave the rebuilding of heads to a professional machine shop, and in a lot of cases this is sound advice. In order to maintain the correct seal, grinding three angles on valves takes special equipment. Also in many cases the valve guides and the valve seats need to be reworked which also requires machining. With all that said some times a set of heads is in good enough used shape that they could be installed as is, but by using the technique of lapping the valves you can freshen them up to nearly as good as new.
Valve Lapping 101
The basic concept of lapping the valves is really straight forward. The valve is removed, lapping compound is applied to the mating surface of the valve, the valve is inserted into the head, and then the valve is worked back and forth so that the grit in the lapping compound smoothes the valve and mates the valve perfectly to its corresponding seat. It sounds simple enough but there is a little more to it than that if you want the heads to turn out right. Simple tricks and techniques will save time and insure the best possible results.
Tools
Unlike some automotive projects not a lot is needed to lap valves: a lapping tool, a valve spring compressor, and lapping compound. These items can be purchased at any automotive parts house and will be required to do the job. For the sake of this article I am assuming you know how to correctly use a valve spring compressor. If not there are sever very informative articles on the subject in the tech exchange.
Disassembly
There are two viable techniques to deal with disassembly and lapping. The first is to disassemble the heads one valve at a time, clean the parts, lap that one valve and then put it all back together before moving on to the next valve. The other option is to completely disassemble the head and then reassemble it one valve at a time as they are lapped. Both options have advantages and disadvantages.
The biggest benefit to dealing with disassembly, lapping and reassembly one valve at a time is that you cannot accidentally install a valve in the wrong combustion chamber. In other words, each valve is lapped and installed in the same exact section of the head where it was originally installed. This is extremely important because no machining is being done to the heads to true things back up. Basically moving parts get where patterns over time and each valve and valve guide will where together as a matched set. Installing a valve from one chamber in to another chamber will result in unmatched where patterns and typically shorten the over all life of valve train and this is why it is so important to reinstall the valve in its original location.
The biggest benefit to disassembling the entire head first is that it makes cleaning the head much easier. With this technique it is still critically important that each valve be reinstalled in the chamber it original came in. To insure this, the pieces need to be individually labeled. In the case of the heads I was working on for this project I chose this method so that I could have the heads hot tanked. The first thing I did prior to disassembly is use a punch set to mark each head. The heads were labeled A and B and then each bolt hole was numbered 1-4.
I then took a box and divided it in to eight sections and labeled them A and B, 1-4. This gave me a place to put both the intake and exhaust valves from each chamber as I disassembled the heads. I also kept the springs and keepers attached to the valves so that those parts would also be installed in their original location.
When the heads came back from being hot tanked I used a wire wheel in my drill to finish cleaning up and loose rust and gasket material left over resulting in a nice clean bare head.
Lapping by Hand
To begin lapping I checked the marks on the head and grabbed the intake valve for the corresponding chamber I would be working on. The valve and spring assembly was quite dirty so I placed the spring and keepers in a jar of solvent and took the valve over to my wire wheel.
When cleaning the valve I only used the wire wheel on the head of the valve and made sure it never came in to contact with the shaft. The reason for this is even though the wire wheel is only mildly abrasive I did not want to risk changing the ware pattern on the valve. After the valve was cleaned I inspected the mating surface and was pleased to see no pitting of any kind.
That meant there was really little to no reason to even lap this valve, but for the sake of being through I use the fine compound on it anyway. Compound was applied to the mating surface of the valve and it was inserted in to the head.
The lapping tool which is nothing more than a stick with a suction cup on each end was then adhered to the face of the valve and then spun back and forth by placing a hand on each side of the tool and moving it back and forth rolling the tool between my palms.
As the valve spins you will be able to here the grit in the compound rubbing against the valve and seat, but after just a few rotations the sound will change as the compound brakes down. When the sound changes lift up on the tool pulling the head of the valve away from the seat and then push it back into the set. This will pull some of the unused compound back on to the mating surface allowing you to continue with out having to reapply more compound. Once again the sound will change as the “fresh” compound brakes down. As you progress you will need to continue to rely on your ears to know when to raise and lower the valve to refresh the compound.
Since the intake valves were in such excellent shape I only spent about a minute lapping each one. But when I was done I had a nice flat gray stripe on the mating surface.
After lapping I used a clean rag to wipe down both the valve and valve seat to remove most of the compound. It is important to have two separate rags for wiping off excess compound, one for the course grit and one for the fine grit. After the area was wiped down I sprayed both the valve and chamber with a liberal amount of carburetor cleaner to rinse away any remaining compound paying special attention to the valve guides and valve shafts.
As you can see in the picture the intake seat on the right is flat and the exhaust seat is shiny. This is a result of lapping the intake valve.
With the valve lapped and the spring and keepers clean I installed the valve in to the head
I did not use assembly lube or install the valve seals, as I will do that later. I know it means more work as the valves will once again need to be removed and reinstalled, but I chose to do it this way so that I have the opportunity to clean each chamber again and insure that no contaminants from the lapping process end up in the chamber or valve guides.
As with the intake valve the exhaust valve cleaning process started at the wire wheel using the same techniques as above.
As you can see above, the exhaust valves were in worse shape than the intake valves, which is to be expected. There is a lot more work involved in lapping the exhaust valves due to the pitting. The process is identical to what was used on the intake valve above however I started with the course compound. Also as the valve was being lapped I periodically pushed the valve about a 1/2" out of the head and wiped of the compound on part of it to check my progress.
If the pitting was still visible I applied fresh compound to the section I had just cleaned and continued the process. Once all the pits have been removed the process can be repeated using the fine compound.
Lapping a pitted exhaust valve by hand can take anywhere from fifteen minutes to an hour. This is why I chose to cheat and use a power drill.
Lapping With a Drill
The technique for lapping a valve with a drill is basically the same as doing it by hand with a few exceptions. First YOU MUST use a variable speed drill. Lapping valves with a drill that only has two speeds: full speed or off, is not a good idea. The drill is there to reduce effort more than speed up the process and when using the drill you want to keep the speed of the drill at 1/3” to 1/2” of max. Even at slower drill speeds the drill will be spinning the valve much faster than you ever would by hand so you must oil the valve shaft and valve guide. As I was lapping each exhaust valve I would add a drop of oil to the shaft through the exhaust port.
As with hand lapping, the valve will need to be worked in and out of the seat to continually freshen up the compound. Once again you should be able to here a change in sound as the compound brakes down. As I was lapping each valve I would stop periodically and feel the face of the valve and the valve shaft. Each time, I was checking for heat. If there is any change in valve temperature you are running the drill to fast. Throughout the full lapping process the valve should not heat up at all. After each exhaust valve was lapped using a drill and the course compound I would lap them by hand using the fine compound. Weather lapping by hand or using a drill the over all process repeats it self as you lap and install each valve. The final result should be a fully lapped and assembled head ready to be install on the motor.
Today
Most of those pits were just color change on the surface and came out rather quickly with lapping. Also the valve pictured was the worst of the eight, and I used it to show what pitting can look like. As far as the seats there was not a single pit in any of them. After reassembling the heads I tested each chamber for leaks by putting in a spark plug and filling the chamber with paint thinner and none of them leaked. When I pull the heads apart to add assembly lube and the valve seals I will check for concave surfaces. Thanks for the tip!!!
