hi
could any1 give me some help? After the completion of a cylinder head overhaul what checks I need to carry out before start up?, while running? and when stopped ?
cylinder head checks
cylinder head checks
Naka
-
Big Pete
- Engineering Mentor
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- Joined: Fri Apr 24, 2009 11:18 pm
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Re: cylinder head checks
Hi Naka,
Welcome to the Diesel Duck.
After dismantling and cleaning, all individual components should be checked before assembly, against the Maker's wear limits, and also checked for signs of visible damage. In particular: - valve guides, valve seats, valve springs and valve retainers and valves,( also Valve cages if used) All worn or damaged parts should be replaced.
Any drilled or cast passage ways for oil, fuel or water, should be proved clear and cleaned if neccessary.
If Rotocaps are fitted, they should be stripped down, cleaned and inspected, if there is any sign of indentation on the running tracks they are scrap. Some manufacurers supply spares, most say that if the Rotocaps stop turning at the correct speed, or show any signs of indentation, they should be scrapped.
If there are indentations in the track and the spring plate, then it is most important that all the springs are renewed and that all the balls (or rollers) are thoroughly checked to ensure that they are still perfectly round, as well as renewing the track and spring washer. ( basically the only parts you won't renew are the spring retaining plate and the circlip, it is just not worth the hassle, cheaper and easier to replace the complete Rotocap)
Certain highly stressed parts, in particular the combustion face of the cylinder head should be crack detected.
The cylinder head should be stripped down and hydraulically pressure tested for leaks, paying particular attention to the combustion face, injector pocket, water cooled valve seats, valve cage pockets, and the inside of the air and exhaust passages. If any leaks are found, the cylinder head is probably beyond repair on board but it may be possible to have it overhauled by specialists ashore.
The cylinder head valve seats should be machined to the correct angle in accordance with the maker's instructions, the valves should be machined to the correct angle (slightly different to that of the seats) again according to the maker's instructions. Usually using Cris Marin pneumatic valve grinding machines or similar.
To avoid confusion each of the valves and corresponding seats should be marked with a marker pen or paint. NEVER STAMP OR ENGRAVE ANYTHING ONTO THE VALVE OR THE COMBUSTION FACE OF THE CYLINDER HEAD. These are highly stressed areas and can fail prematurely because someone stamped I.D./reference numbers on them.
The old oil seal "O" rings in the valve guides should be removed and the inlet and exhaust valves assembled in the head and checked with Engineer's Blue/Prussian Blue/Micrometer Blue, there should be a continuous line of contact between the faces.
If not the valves should be lightly lapped in with grinding paste. Ideally there should be the narrowest continuous seat possible, no more than 2 mm wide. I f you get a seat more than 2 mm wide, the machining angle for the valve or seat is probably wrong or one or the other is worn beyond limits, or possibly there is excessive play in the valve guide.
In the distant past it used to be common to grind valves in, untill the entire surface of the valve was in contact with the seat, this is now strictly prohibited by nearly all engine makers
Grinding the valve until there is full surface contact, makes it more likely to fail, because there is a wider seat for debris to be trapped in, leading to valve burnout.
The spring force, closing the valve, is divided over a larger contact area, so the SEALING PRESSURE, between the faces is less, so there is more chance of leakage.
Grinding in a valve to give full surface contact at ambient conditions does not neccessarily mean that there will be full contact at engine running temperatures because the shape of the valve and seat will change as they heat up.
The more metal that is ground off the valve and seat at each overhaul, the fewer overhauls you will get out of the components, so you are wasting spares, as well as your time, by doing this.
Final test of tightness of the valves, after assembling them with the valve springs, is by putting Kerosene, Paraffin, Penetrating oil or similar very light, low viscosity oil on top of the valve and seeing if it leaks past, this is most easily detected if the area around the underneath of the valve is coated in white chalk, before the oil is added. Any moisture will turn it from sparkling white to dull grey.
After satisfactory completion of the leak test, the valves should be dissassembled, cleaned, new "O" ring seals fitted in the valve guides, the valve spindles lightly lubricated with engine oil, both to avoid damage to the "O" ring on assembly and to prevent corrosion while the overhauled head is in storage.
Do not forget to check that the sealing face to the cylinder liner is in perfect condition and lap it in with the special makers tool. If you do not have one, you can make one from the top of an old cylinder liner.
After completion of all the overhaul work, and testing, make sure the cylinder head and all it's subcomponents are clean and dry, coat everything lightly with oil to prevent corrossion, and stand the head on a block of wood so that the sealing face can not become damaged, also make sure that the wood can not become wet and cause corrosion underneath the cylinder head.
Cover the cylinder head with a porous cover, such as unpainted canvas, so that it can breathe, plastic or painted canvas can trap condensation and cause corrossion to start.
When the cylinder head is to be used, it should be cleaned and visually inspected, before being fitted to the engine in accordance with the Maker's instructions.
On completion, the cooling water, L.O., fuel etc will be connected and the pumps run up, and the engine checked for leakage.
The cylinder head should be inspected for adequate flow of lubrication through the valve gear. Some engines for instance feed L.O. through the rocker gear brackets, and if you miss out the tiny "O" ring between the cylinder head and the valve bracket, the rocker gear is starved of L.O. and will seize.
Once the engine has been preheated to a safe temperature, the engine is usually started, the normal procedure of turning with the turning gear and slow blowing with the indicator cocks open will show if any leaks have developed in the head since it was overhauled, or been started in another head by the cooling and heating cycle.
When the engine is running, another thorough examination ( eyes, ears and hands) should be made of the outside of the engine and the valve gear for any abnormalities. Also check the funnel exhaust and the engine instrumentation.
The engine should be closely monitored the first time it is run on load.
However, so long as all the tests and checks during the head overhaul are carried out correctly, and everything is done according to the makers instructions, there shouldn't be any problems.
I hope this gives you the basics. I am sure a few more people will add their comments.
I hope the cylinder head overhaul goes well.
BP.
Welcome to the Diesel Duck.
After dismantling and cleaning, all individual components should be checked before assembly, against the Maker's wear limits, and also checked for signs of visible damage. In particular: - valve guides, valve seats, valve springs and valve retainers and valves,( also Valve cages if used) All worn or damaged parts should be replaced.
Any drilled or cast passage ways for oil, fuel or water, should be proved clear and cleaned if neccessary.
If Rotocaps are fitted, they should be stripped down, cleaned and inspected, if there is any sign of indentation on the running tracks they are scrap. Some manufacurers supply spares, most say that if the Rotocaps stop turning at the correct speed, or show any signs of indentation, they should be scrapped.
If there are indentations in the track and the spring plate, then it is most important that all the springs are renewed and that all the balls (or rollers) are thoroughly checked to ensure that they are still perfectly round, as well as renewing the track and spring washer. ( basically the only parts you won't renew are the spring retaining plate and the circlip, it is just not worth the hassle, cheaper and easier to replace the complete Rotocap)
Certain highly stressed parts, in particular the combustion face of the cylinder head should be crack detected.
The cylinder head should be stripped down and hydraulically pressure tested for leaks, paying particular attention to the combustion face, injector pocket, water cooled valve seats, valve cage pockets, and the inside of the air and exhaust passages. If any leaks are found, the cylinder head is probably beyond repair on board but it may be possible to have it overhauled by specialists ashore.
The cylinder head valve seats should be machined to the correct angle in accordance with the maker's instructions, the valves should be machined to the correct angle (slightly different to that of the seats) again according to the maker's instructions. Usually using Cris Marin pneumatic valve grinding machines or similar.
To avoid confusion each of the valves and corresponding seats should be marked with a marker pen or paint. NEVER STAMP OR ENGRAVE ANYTHING ONTO THE VALVE OR THE COMBUSTION FACE OF THE CYLINDER HEAD. These are highly stressed areas and can fail prematurely because someone stamped I.D./reference numbers on them.
The old oil seal "O" rings in the valve guides should be removed and the inlet and exhaust valves assembled in the head and checked with Engineer's Blue/Prussian Blue/Micrometer Blue, there should be a continuous line of contact between the faces.
If not the valves should be lightly lapped in with grinding paste. Ideally there should be the narrowest continuous seat possible, no more than 2 mm wide. I f you get a seat more than 2 mm wide, the machining angle for the valve or seat is probably wrong or one or the other is worn beyond limits, or possibly there is excessive play in the valve guide.
In the distant past it used to be common to grind valves in, untill the entire surface of the valve was in contact with the seat, this is now strictly prohibited by nearly all engine makers
Grinding the valve until there is full surface contact, makes it more likely to fail, because there is a wider seat for debris to be trapped in, leading to valve burnout.
The spring force, closing the valve, is divided over a larger contact area, so the SEALING PRESSURE, between the faces is less, so there is more chance of leakage.
Grinding in a valve to give full surface contact at ambient conditions does not neccessarily mean that there will be full contact at engine running temperatures because the shape of the valve and seat will change as they heat up.
The more metal that is ground off the valve and seat at each overhaul, the fewer overhauls you will get out of the components, so you are wasting spares, as well as your time, by doing this.
Final test of tightness of the valves, after assembling them with the valve springs, is by putting Kerosene, Paraffin, Penetrating oil or similar very light, low viscosity oil on top of the valve and seeing if it leaks past, this is most easily detected if the area around the underneath of the valve is coated in white chalk, before the oil is added. Any moisture will turn it from sparkling white to dull grey.
After satisfactory completion of the leak test, the valves should be dissassembled, cleaned, new "O" ring seals fitted in the valve guides, the valve spindles lightly lubricated with engine oil, both to avoid damage to the "O" ring on assembly and to prevent corrosion while the overhauled head is in storage.
Do not forget to check that the sealing face to the cylinder liner is in perfect condition and lap it in with the special makers tool. If you do not have one, you can make one from the top of an old cylinder liner.
After completion of all the overhaul work, and testing, make sure the cylinder head and all it's subcomponents are clean and dry, coat everything lightly with oil to prevent corrossion, and stand the head on a block of wood so that the sealing face can not become damaged, also make sure that the wood can not become wet and cause corrosion underneath the cylinder head.
Cover the cylinder head with a porous cover, such as unpainted canvas, so that it can breathe, plastic or painted canvas can trap condensation and cause corrossion to start.
When the cylinder head is to be used, it should be cleaned and visually inspected, before being fitted to the engine in accordance with the Maker's instructions.
On completion, the cooling water, L.O., fuel etc will be connected and the pumps run up, and the engine checked for leakage.
The cylinder head should be inspected for adequate flow of lubrication through the valve gear. Some engines for instance feed L.O. through the rocker gear brackets, and if you miss out the tiny "O" ring between the cylinder head and the valve bracket, the rocker gear is starved of L.O. and will seize.
Once the engine has been preheated to a safe temperature, the engine is usually started, the normal procedure of turning with the turning gear and slow blowing with the indicator cocks open will show if any leaks have developed in the head since it was overhauled, or been started in another head by the cooling and heating cycle.
When the engine is running, another thorough examination ( eyes, ears and hands) should be made of the outside of the engine and the valve gear for any abnormalities. Also check the funnel exhaust and the engine instrumentation.
The engine should be closely monitored the first time it is run on load.
However, so long as all the tests and checks during the head overhaul are carried out correctly, and everything is done according to the makers instructions, there shouldn't be any problems.
I hope this gives you the basics. I am sure a few more people will add their comments.
I hope the cylinder head overhaul goes well.
BP.
It is always better to ask a stupid question than to do a stupid thing.