Diagnosis and Treatment of a Sticky Valve
Valve sticking is a very common problem in small aircraft engines and although we have a number of things we can do to prevent this, it still happens. It is my hope that this article informs you that there is a relatively easy method to check valve clearances in order to identify tight valves and to repair a sticky or stuck valve without pulling a cylinder. The procedures I am going to detail have been briefly touched upon in the Flyer in the past. Experts say that the likelihood of valves sticking greatly increases around 400 to 500 hours of engine time since overhaul and, not unexpectedly, Lycoming SB 388C (click on link to access) mandates checking valve clearances every 400 hours in our engines while Lycoming SI 1425A outlines procedures for preventing valve sticking. Although it is much more common for exhaust valves to stick, intake valves are not immune from this problem.

Let me give you two scenarios:

Scenario number one: you are at your home field and, for any one of several reasons, are concerned about your valve clearances and are discussing it with your A & P. Scenario number two: you are hundreds of miles from home at an airport with no mechanic, and you are fairly certain you have a stuck valve. To wit, on startup your engine ran rough on either magneto and would not smooth out. You performed the “poor man’s compression test” and after pulling the prop through all the cylinders, it felt like one of them was decidedly weak. You opened the cowling and felt a cold cylinder and, after pulling the top spark plugs and putting your finger over the holes while pulling the prop through, you confirmed the problem in one particular cylinder. You pulled the rocker box cover and saw that one of your valves was stuck in the open position. If it were stuck in the closed position, which is fortunately much less common, the problem would be significantly worse and would involve a bent push rod and tube and would mean having to pull a cylinder.


The procedures for doing this test and the repairs are basically the same for both the single engine and Twin Comanches, although my explanation and photos will be of the IO-320 engine in the Twin.

Scenario number one:


1. Remove the top spark plug and remove the rocker box cover. Push the rocker arm shaft to one side, and the rocker arms will fall off in your hand. Make a note of which rocker arm goes to the exhaust and which one goes to the intake. They are different and have different part numbers on them. You don’t need to remove the push rods unless you have a stuck valve.

Rocker Arm Removed

2. Make sure the piston is at approximate bottom dead center. Stuff almost all of a ½ inch diameter 10 foot rope into the cylinder. Turn the prop again until the piston is firmly pushing the rope against the valves and you can't turn the prop any longer.



Rope In Cylinder

3. Now using a valve spring compressor tool and a pencil magnet, remove the keys which hold the exhaust valve spring seat in place. Put all this (springs, valve spring seat, keys, end caps, etc.) in the rocker box cover for safekeeping. Repeat this process with the intake valve. Note- the exhaust valve stem may have a cap on it in Lycoming engines whereas the intake valve does not.



Valve Spring Compressor Tool In Action

4. Rotate the prop backward and remove the rope from the cylinder. Take the end of the valve between your fingers and wiggle it. There should be a small amount of "play" to the right and left and up and down. If the valve doesn't have any play then it is too tight. (If it really moves around, it is probably too loose and you may need a valve job, but that is a different problem.) Next grab the valve stem near the end and push it into the valve guide and move it in and out. If it moves easily, this is a good sign. This means you don’t have to ream this valve guide. Lycoming has some fancy tools to measure the clearances, but in my experience, your fingers will tell you what is too loose and what is too tight. If neither the exhaust nor the intake valve guides need reaming then you can skip ahead to step number 8.



Checking For Valve Tolerance

If a valve is too tight, you must ream the valve guide. To do this, gently push the valve part way into the cylinder and grasp it with your mechanical fingers. You can visualize it easily if you insert the thin flexible light through the spark plug hole. Now push the valve all the way into the cylinder and set it down gently. CAUTION: do not move the prop with the valve in the cylinder.

5. To ream a valve guide, you must use a valve guide reamer of the proper size. You can find these numbers in the Table of Limits Lycoming SSP 1776 which is also in the back of the Lycoming Engine Overhaul Manual. For my engines (Lycoming IO 320 B1A) the exhaust guides were reamed when new to a finished ID of a minimum of .4985 inch and a maximum of .4995 inch. The intake guides were .4040 min/.4050 max. I will leave it up to you and your IA and/or cylinder shop to determine which reamers you will need for your engine.

Place the reamer in a reamer wrench. Put plenty of bearing grease on the flutes of the reamer to collect the debris. Line the reamer up with the valve guide so that you are going in very straight and ALWAYS turn the tool clockwise ONLY. Slowly turn the reamer and push it into the guide. Even when removing the reamer, you must still be turning clockwise. Turn the reamer until it gets easy to turn and most of it has disappeared into the guide. Now slowly remove the reamer while turning it clockwise. Clean off the reamer with a clean cloth and avoid touching it with your fingers.

Reaming A Valve Guide

6. Once you have finished reaming, put a thin flexible “spaghetti” flashlight into the top spark plug hole to visualize the valve where it is lying on the side wall of the cylinder. It is at this point that you can inspect the valve head and the stem. If you see damage or unusual wear on the valve face, you can stop this process and prepare to remove the cylinder so that you can take it to a cylinder shop for a “valve job” where they will install a new valve and probably a new seat and a new guide. Going through the same hole with a pair of strong mechanical fingers or a powerful pencil magnet, grasp the end of the valve stem and bring it through the spark plug hole after you remove the light. Wrap a piece of safety wire around the valve stem to keep it from falling back into the cylinder. Grasp the valve stem and polish it clean using Scotch-Brite. Don't use sandpaper and don’t scrape it with anything metal. Gently lower the valve back into the cylinder. At this point you can remove the bottom spark plug so that you can pour or squirt some solvent into the top plug hole and through the valve guide to wash out any carbon or metal created by your reaming.

Valve Stem Protruding From Spark Plug Hole Prior To Polishing With Scotch-Brite

7. Using the thin flexible light in the cylinder for visualization, grasp the valve stem in the middle with the mechanical fingers. Introduce the pencil magnet through the valve guide into the cylinder. Unless you have three hands, you can turn loose of the light. Touch the magnet to the end of the valve and pull it toward the valve guide opening. Remove the mechanical fingers gently and introduce your wire hook to support the valve to a position level with the valve guide. Often you can look through the valve guide around the pencil magnet and see the end of the valve stem lining up. Once you can definitely feel the valve advancing into the guide you may encourage it by pushing on the other end of the valve with your hook. This takes a bit of practice, but soon you will be an expert. Push the valve about halfway back through the guide and squirt some oil into the guide and, using the magnet, work the valve in and out to lubricate the guide.



Supporting The Valve With The Magnet and Wire Hook

8. Put the rope back into the cylinder and push it up against the valve to hold it in place while you reattach the valve spring compressor tool and put the springs, the spring seat, and the keys back in place. I can do this by myself, but it is often easier if somebody can hold the prop (and the rope) against the valve while you are using the valve spring tool. Be careful not to pinch your fingers as those springs are very strong if the tool slips off while you are compressing them.

9. Replace the valve stem cap (if the valve has one) and the rocker arms and rocker arm shaft. Sometimes the shaft will go in easily by tapping it lightly with your hammer and sometimes you will need the help of the valve spring compressor tool to relieve the tension on the springs and allow the rocker arms in place. Be careful how much you press on the valve spring tool because you can unseat the keys. After putting on the rocker box cover and inserting the spark plugs, you are finished with that cylinder. Once you get good at it, you can do a cylinder about every thirty minutes.


Scenario number two:

1. If you have an obviously stuck valve, then you are going to do this procedure a little differently than in scenario number one where we are just checking valve clearances with an eye to reaming guides if necessary. A stuck (open) valve is going to require that you push it back in its seat so that you can remove the keys and springs. Sometimes this can take a bit of effort. Don’t hesitate to use some Liquid Wrench or similar solution on the valve stem. Essentially you will have already removed the rocker box cover and spark plug in order to have diagnosed the stuck valve. Now you must use the rope to “unstick” the valve. Refer to steps 1 through 5 in scenario number one and using the “rope trick,” push hard on the prop. When the valve breaks loose, you will usually hear a loud “clank.” This accomplished, you can remove the keys, springs, etc., and then you will have to reverse course and push the valve into the cylinder so that you can ream the guide and polish the stem. Remember that the valve is still very tight and is probably still stuck, but in a different position. It may take some hammering to move the valve if it tightly stuck. You must use either an aluminum dowel or a brass punch put against the valve stem end. You should NEVER hit the valve directly with a hammer.

2. Now with the valve in the cylinder, you can proceed to step 6 in scenario number one, ream the valve guide, polish the valve stem with Scotch-Brite, and finish the rest of the procedures through step 10.

3. At your earliest convenience, you should check the valve clearances on the remainder of your cylinders.


I have made a list of the absolute minimum tools needed to do both a diagnostic check and to ream a valve and where to get them:

1. Valve spring compressor: there are many different models to choose from ranging from $50 to $150 and you can obtain them from aircraft supply houses like Aircraft Spruce. The one pictured is the easiest to use for me and is the most expensive. Many of my mechanic friends have made their own.

2. Valve guide reamers (one for the exhaust and one for the intake guides): I bought mine from a machine shop supply house.

3. Reamer wrench: Mine is a Craftsman medium size reamer handle available from Sears.

4. 10 feet of ½ inch diameter mountain climbing type rope: I think mine was a tie-down rope in another lifetime.

5. Small ball peen hammer: Any tool store.

6. Wire hook for supporting valve in cylinder: I made mine from an old antenna.

7. Mechanical fingers (strong Snap-on two prong flexible type): Snap-On model GA265A. One of the few bargains Snap-On has and you can get free delivery ordering from their website at http://www.snapon.com.

8. Strong pencil type magnet: any tool store

9. Flexible flashlight (long spaghetti type to go in cylinder): This is a Sears Craftsman

10. Spark plug socket and 3/8 inch socket wrench, extension, and ¾ inch open end wrench for removing spark plug : Any tool store

11. Small can of wheel bearing grease: aircraft supply house.

12. Piece of Scotch-Brite for polishing valve stem: any hardware store.

13. Brass punch or aluminum dowel (not pictured): hardware store or Harbor Freight.



Valve Guide Reaming Tools