Would you re-use this piston?
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Looking at some photos is essentially meaningless unless a crack is visible, a piece is broken off or serious detonation has caused melting. A machinist can measure how badly worn the ring lands are as well as checking the OD of the piston.
A lot of things are at play here. Why was this engine dismantled? Has a machinist checked the top, middle and bottom of the bore for sizing to see how out of round it is and how much taper is there?
If the cylinder must be bored, then your question is moot.
If you spend the money to bore the cylinder, then a new rod is wise from a metal fatigue perspective. If the crank has to be turned 10 or 20 under, then a new rod is necessary. It's a slippery slope. If you intend to keep this tractor long term, then you need to face up to spending whatever is needed to do a complete rebuild. Decking the top of the block to make sure it is dead flat, having the camshaft profiled to make sure it is lifting the valves high enough, either replacing or scrapping the balance gears, valves, seats, spring, guides, new bearings, planing the head to remove warpage and so forth. Those who don't do a total rebuild often wonder why their engine blew apart months later.
A lot of things are at play here. Why was this engine dismantled? Has a machinist checked the top, middle and bottom of the bore for sizing to see how out of round it is and how much taper is there?
If the cylinder must be bored, then your question is moot.
If you spend the money to bore the cylinder, then a new rod is wise from a metal fatigue perspective. If the crank has to be turned 10 or 20 under, then a new rod is necessary. It's a slippery slope. If you intend to keep this tractor long term, then you need to face up to spending whatever is needed to do a complete rebuild. Decking the top of the block to make sure it is dead flat, having the camshaft profiled to make sure it is lifting the valves high enough, either replacing or scrapping the balance gears, valves, seats, spring, guides, new bearings, planing the head to remove warpage and so forth. Those who don't do a total rebuild often wonder why their engine blew apart months later.
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12 Posts
If there's oil on the top of a piston, then this means that the piston and rings are badly worn. Piston rings operate in a particular manner. The top ring holds the compression. The 2nd ring is the oil scraper. It flexes as it travels up and down in the cylinder, and glides over the oil on its way up and scrapes the oil on it's way down. If the 2nd ring came with an expander, install it under the ring. It helps to stabilize the piston in the cylinder. And the oil ring assembly is the lubricator. It lubricates the cylinder wall and other rings so they'll last longer. All new or unworn piston rings have a square edge. If the piston skirt becomes worn in a cylinder, this will cause the square edge on the rings to become "rounded" and then the 2nd ring won't be able to scrape the oil on its way down. Instead, it brings the oil to the top of the piston (past the gap of the top ring).
The piston in this post is obviously too worn for reuse. The skirt on all pistons is supposed to make contact with the cylinder wall. The ring lands are not. That particular piston is badly worn. The ring lands made contact with the cylinder wall. When this happened, the piston "wobbled" in the cylinder, causing the square edge on the rings to become "rounded," allowing oil to bypass them. A new STD size piston & rings should be installed with good results.
The piston in this post is obviously too worn for reuse. The skirt on all pistons is supposed to make contact with the cylinder wall. The ring lands are not. That particular piston is badly worn. The ring lands made contact with the cylinder wall. When this happened, the piston "wobbled" in the cylinder, causing the square edge on the rings to become "rounded," allowing oil to bypass them. A new STD size piston & rings should be installed with good results.
Whoa....... slow down there Brian.
Are you suggesting that he just pop a new piston and rings in that engine and bolt it all back up?
If the piston and rings are that badly worn, what does that say about the bore? And how about the rest of the internals, since they all have the same number of hours on them?
Not enough info from you. :sidelaugh:
Are you suggesting that he just pop a new piston and rings in that engine and bolt it all back up?
If the piston and rings are that badly worn, what does that say about the bore? And how about the rest of the internals, since they all have the same number of hours on them?
Not enough info from you. :sidelaugh:
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2,052 Posts
if the cylinder wall must be bored then a new piston will be needed. That makes that piston in the picture a paper weight now.
If the cylinder wall isnt too bad then maybe just a good hone will work and if, and I say IF, the piston isnt cracked or abnormally worn then it may be reused with NEW rings if you are on a tight budget. Personally, I would just replace it all with new parts since you have already got to this point in the rebuild. Do it once and do it right. Ya it will cost more but at least you will know that it will last for a long time if the work was done correctly.
If the cylinder wall isnt too bad then maybe just a good hone will work and if, and I say IF, the piston isnt cracked or abnormally worn then it may be reused with NEW rings if you are on a tight budget. Personally, I would just replace it all with new parts since you have already got to this point in the rebuild. Do it once and do it right. Ya it will cost more but at least you will know that it will last for a long time if the work was done correctly.
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