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This is a very high level, conceptual question: it seems to me like every engine is designed so that pistons are extracted from the top, which requires removing cylinder heads, a huuuge pain in the rear end.

My question is, why aren't they designed to enable extraction through the bottom, after the rods are detached from the crankshaft and then the crankshaft slightly rotated to clear the exit path for the piston being pulled down by the rod? It seems to me that removing the (upper) oil pan, which this would requires, would be a lot less pain than both the oil pan AND the cylinder head(s). Are there any engines that make this possible for the convenience of maintenance?

amphibient
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Possible on some engines once the crankshaft is removed - the balance masses can take up a lot of space.

Also depends on the design of the engine and crankshaft as an inline 8 cylinder can have a lot more room than a 60 degree v6.

Solar Mike
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If you imply to take them out the bottom, this also would assume you are trying to stick them back in the same way. Even if you can get them out the bottom, though in most cases you'd need to remove the crankshaft to do it, you still need a flat surface engage against in order to get the piston rings to go up into the bore. Without the flat surface (like the deck on the top side), trying to get them in from an uneven surface would most likely cause you to break a ring or land.

Trying to do all of this through the bottom would on the outside seem like it'd be easier, but the thing is it would be extremely hard to make it happen. The idea of removing pistons from a block is nearly synonymous with rebuilding the engine. To rebuild the engine, you start from the bottom with a short block and move your way up. I don't see an easier way for this to happen than by removing the heads.

Pᴀᴜʟsᴛᴇʀ2
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In some engines it is possible. I restored once a 1974 Skoda and the pistons could be extracted from below without removing the crankshaft. However I think it is useless: very complex to compress the rings to put them back; if pistons are to be removed it is more likely it is because an engine restoration, so for instance, if you just want to change rings you still need to hone the cylinders and in that case you will need to carefully wash everything.

Aram Alvarez
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