Hi all,
this is regarding some cavitations that have appeared on the crown of a Piston, the head is not a flat one, rather it has a inward curvature profile to induce better fuel mixure/swirel i belive. anyway the cavitations appear on the curvy surface. the dia is approx 500 mm and the temperature subjected to is in the range of 900 centigrade.
the maximum cavitation depth that i have noticed is in the range of 12- 15mm
i would be greatful if someone could enlighten me on a proceedure/ rules of thumb or any useful info.
cheers
syda
note- i work for a ship repair/ building yard in Colombo, Sri Lanka
You may try the automotive thread for an answer. There are several knowledgable people who post their from time to time. It may be for a ship, but it's still an internal combustion motor.
You would probably be best to get a new piston I would think. The cavitations are going to form hot spots on your pistons. Knowing the application of this piston we could probaly give you a better answer. If it is a motor that is used hard and at the upper edge of it's rpm band those hot spots will lead to detonation in the combustion chamber=blown hg at the least. I have been told that welding the imperfections could still cause hotspots due to material differences and you would have to rebalance the motor due to possible weight difference on the repaired piston. if not the bearings will prematurely wear.
do you know what the piston is made of or have any mtr's , manufacturer's info. can you send picture with a reference measurement on the picture.
darren
What is that piston out of? If it is from a ship and huge I can't help.
I used to race 2 cycle motorcycles in a prior life. If the timing was advanced just a teeney bit too much a the heat would melt a hole in the piston top.
do you have a scrap one ?
try burning different rods on it and see what works
you might have to put it in a fire and let it burn of any contamination.
i will assume the ship is worth a lot of money a new piston should not be too big a cost to keep it under way
darren
I see 500 mm (about 20 inches) in your post and assume that to be the cylinder diameter. I also assume that these cavities are the result of use rather than manufacturing defect. I further assume the material to be aluminum and that the engine is a diesel. My experience is with much smaller Caterpiller engines but the appearance of this sort of defect in worn pistons on these engines was not unusual. In America the cost of repair would have far exceeded the replacement cost so pistons with this sort of defect were discarded. If this is not a cross head engine the wear on the pressure surfaces of the piston probably is also substantial by this time. If you attempt to repair this expect the metal around the defect to be badly heat checked and also contaminated with sulfur, carbon, unburned and partially burned fuel... so you will probably first have to machine out the defect before you weld. Be careful to avoid distortion when welding. When finished you will have to check the diameter and roundness of the piston and the straightness of the piston ring grooves. If a piston ring is not free in its groove you will have to remachine the groove and either obtain a wider ring or run a spacer in the groove. If distortion has reduced the diameter of the piston or made it out of round you may have to restore it to it's original dimensions. All in all not a trivial repair and I've probably missed a lot.
Bill
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