since 1940

Exhaust valve leakage in Continental O200 cylinder

Continental exhaust valve guide, valve, and seat   photo of Continental 470 exhaust port

see also Exhaust valve leakage in Lycoming or Continental aircraft engines - causes

There are five considerations for long exhaust valve life:

• valve train alignment
• lubrication
• oil cleanliness
• valve cooling
• valve train dynamics -dry tappet clearance, valve spring, hydraulic lifter

Alignment: Seat, guide, rocker arm, valve springs, and rocker bosses should center the valve stem in the guide. The valve stem then moves through the guide with a minimum of side force against the guide. Rapid wear results when the valve stem pushes against the guide. Proper valve alignment occurs when the rocker arm face rests flat against the valve tip. 

 

Rocker face Alignment: The photo of a Lycoming rocker arm shows contact on the edge of the face indicating that the face has been ground at a slight angle.  This pushes the valve stem against the guide resulting in rapid valve guide wear. Grinding the rocker arm face is like putting a filling in a tooth - you don't do it unless absolutely necessary - and then as little as possible. Most shops don't even admit they grind - they call it polishing. Improper face grinding is a frequent problem.   You can check rocker arm alignment by making sure the rocker arm face sits flat on the valve stem. This should be done when the cylinder is installed. Such a check verifies alignment for the rocker shaft, rocker arm, rocker arm face and valve guide. If any one of these are tilted, the rocker face will not rest flat on the valve stem. (get your mechanic in the habit of taking a look after he installs the rocker arms)

This rocker arm wore the valve guides out in 350 hours Another problem associated with grinding the rocker face is the tendency to flatten the curve. A rocker arm face should have a cycloid curve. Anything less or more results in more movement of the rocker face across the valve stem.

Guide Boss Alignment: Poor alignment occurs when a guide boss reams off-center. With the guide boss off-center the guide does not align the valve onto the center of the seat. This is evident if you inspect how the seat grinder grinds the seat face. The seat grinder has a pilot that aligns the grinding stone to the guide. If the center of the guide doesn't coincide with the center of the seat the stone is not positioned onto the center of the seat. The stone cuts the seat more on one side than the other This creates a wide face on one side of the seat and a narrow face on the other side. If the seat grinder ground more on one side of the seat than another it means that most of the contact pressure occurs one side of the seat. The valve will behave in the same way; most of the contact pressure will be on one side of the seat. This uneven pressure pounds the seat face oblong which causes the valve to leak.

Valve guides must be perpendicular to the spring seat. If the guide is not perpendicular, then the springs hold the valve to one side, forcing it against the guide. Check guide perpendicularity by fixing a dial indicator on the guide with the indicating arm indexing from the spring seat. Rotate the dial indicator. Run-out should be less than .005 inch. Guide misalignment occurs due to several reasons. Welding of the guide boss and then not accurately boring the guide boss, Reaming off-center, and forcing the guide in the boss. A crooked spring also forces the valve stem against the guide. Place the valve springs against a straight edge and check for straightness.

Rocker Boss Alignment: Rocker bosses reamed off-center result in poor valve alignment.

New Continental O200 cylinders don't have bushings installed in the rocker bosses. These bosses wear egg-shaped. The rocker shaft bosses on new cylinders don't have bushings installed. These bosses wear egg-shaped and then must be reamed oversize and a brass bushing installed during cylinder repair. It's hard to center the reamer and establish alignment when the hole is egg-shaped. The pilot on the reamer has sufficient up and down movement in the egg-shaped boss to ream at an angle. As a result the rocker shaft, and in turn the rocker arm, isn't held parallel in the rocker bosses. The rocker tip then pushes the valve stem into the side of the guide causing rapid guide wear. If you pass the reamer through the bosses from both sides it will average out any misalignments of the bosses.

Valve Cooling: The heat path for the exhaust valve is through the exhaust seat. The exhaust valve has a large heat receiving surface and requires a wide seat face to conduct heat out the valve and into the cylinder. If the seat has a narrow face, the valve stem overheats and oxidizes oil and forms varnish deposits in the guide boss. These varnish deposits reduce the clearance between the valve stem and guide, forcing the stem against the side of the guide where it either wears the guide, or if using a newer style harder exhaust guide,  the valve sticks.

A guide worn only on one side results from deposit buildups on the opposite side of the guide. Too much clearance between the valve stem and guide causes the valve stem to pivot in the guide. Rapid guide wear results from the high loading at the points of contact. A guide worn on both sides but at opposite ends suggests the valve stem was pivoting in the guide. Valve stem to guide clearance should be kept to a minimum to prevent pivoting.

Solvents, such as Marvel Mystery Oil, used to dissolve deposits, remove the outer layers enough for the valve stem to loosen. The remaining deposits force the valve stem against the side of the guide. 

Valve Assembly Lubrication: If, during the first movements of the valve in the guide it mars the guide, then the surfaces continue to gall. Sufficient wear occurs that the valve flops around in the guide. The valve face then contacts the seat in different spots and widens the seat face oblong and then fails to seal.

Continental, Lycoming, and Volkswagen all recommend a Molybdenum based grease as a valve prelube. Used when assembling the valve in the guide during repair work. We recommend it as well. Moly grease allows the surfaces to seat by burnishing rather than galling. The thin layer of engine oil that remains on the valve stem is not sufficient to prevent metal contact.

Oil Cleanliness: Dirty oil causes guides to wear and valves to stick. Deposits from the dirty oil in the guide causes the valve stem to rub against the guide. The valve either sticks or wears the guide. Using solvents such as Marvel Mystery oil doesn't solve the problem.

 When you check the oil level on the dipstick also look at the color of the oil. The oil should be gray to dark gray. Oil that quickly turns black after an oil change is a sign that combustion gas is blowing by the rings. Blow-by of combustion gas past the piston ring belt turns the oil black. If these combustion deposits circulate in the oil they precipitate out as sticky deposits on the valve guides and ring lands. You should not have black sooty deposits on the exhaust stack. This is a sign of over richness or poor quality fuel. This soot contaminates the oil and gums up the valves and rings. Some poor quality auto fuel produces soot (observation, not opinion)

The Continental 200 cylinder requires more attention to detail then any other cylinder. Many of these cylinders are old, welded and, as a result, distorted. Give these cylinders a proper operating environment; baffling, timing, and cleanliness. With proper care and frequent flying there's no reason these cylinders won't go to the full TBO.

How not to pick up a O-200 cylinder

The traditional C-85, O-200, O-300 cylinder had the push rod housings (the part I'm holding) swaged into the top of the cylinder. When you pick up the cylinder by the push rod housings it loosens the housing in the head causing an oil leak. Newer Continental cylinders use a more traditional rubber seal.
 

Copyright 2003 by Sacramento Sky Ranch Inc. All rights reserved.  Prices subject to change without notice. Not responsible for typographical or misprint.
Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.