Its been a while since I studied material sci in college but I'll take a stab...


All materials expand when heated and contract when cooled (with the exception of water that expands when frozen). Thus When your engine heats up all the metal parts expand. Ideally, if the entire engine was made of the same metal, it would all expand at the same rate. This of course is not true for 2 reasons:

1. Most engines are made of multiple different metals.
2. The engine does not heat evenly, ie some parts (combustion chamber, exhaust) remain hotter than others.

These two factors contribute to uneven expansion of the engines parts. This poses a problem, how do you ensure that the tolerances stay correct (not too tight, not too loose) when the engine heats up (in order to get good compression etc)? Well, there are a couple of solutions to this for the piston/cylinder fit:

1. Use a ringed piston. The ring is usually spring steel, so it can expand/contract to make up for slight changes in cylinder diameter due to heat expansion. Also, because of the ring, the piston and cylinder never come in direct contact, thus making the expansion of the piston less of an issue.

2. The tapered cylinder used in an ABC/ABN engine. Since the brass liner expands more than the aluminum piston, it is made with a taper so that its very tight at TDC when cold. As the engine heats up, the sleeve expands a bit and loosens up at the top, but not so much that compression is lost. If the liner were made straight, it would open up a gap when hot and you would loose compression.

3. Use an AAC piston/cylinder. Since the sleeve and piston are the same metal, in theory they expand at the same rate and keep compression. I'm believe that AAC sleeves have tapers as well (though probably less than ABC). The disadvantage is that its difficult/expensive to chrome plate aluminum.