engine Bore Question

[faction]

I'm just looking for some general information, about engines getting bored

I was hoping for some pros and cons, and how often you need to put sleaves in
and any information in general would be nice , thanks

[curtis73]

It depends entirely on the engine in question. Most American V8s can get overbored 2-3 times in any one of several common oversizes. Many import 4-cylinders are aluminum and have to be resleeved almost every time.

But, for the most part, the cost of sleeving a block several times, plus the fact that it probably has exhausted most of its service life means that it becomes an unwise investment compared to just purchasing another engine.

[MagicRat]

Originally Posted by curtis73

It depends entirely on the engine in question. Most American V8s can get overbored 2-3 times in any one of several common oversizes. Many import 4-cylinders are aluminum and have to be resleeved almost every time.

But, for the most part, the cost of sleeving a block several times, plus the fact that it probably has exhausted most of its service life means that it becomes an unwise investment compared to just purchasing another engine.

Just to elaborate on some points here.

Most cast-iron engines have the cylinder bore cut into the cast iron of the block, so the cylinder walls are made of the same material as the rest of the block.
Cylinder bores wear out when they are no longer a perfect cylinder. The action of the piston and piston rings eventually makes the bore slightly tapered (wider at the top) and slightly oval, causing the piston to rattle around in the bore and leak an excessive amount of combustion pressure.

The bore can be re-drilled to a slight oversize, to restore the perfect cylindrical shape, then matching oversize pistons and rings are installed.

Traditionally, most cast iron car and truck engines, (all configurations, not just V8's) are re-bored 0.030 of an inch, in order to remove all cylinder wear.
This means that the first engine rebuild has a cylinder diameter that is 0.030 inch bigger than stock. The second overbore is 0.060. It's rare that engines can be bored larger than this, but some can go as large as 0.080.

The limit here is the thickness of the cast-iron cylinder wall (which can be measured before boring) and the availablility of parts.

Occasionally, for the same engine design, one can find one block which cannot be overbored too much due to lack of material, yet another block of the same design can be overbored more, due to variances in manufacturing tolerances.

The idea here is to bore an engine no larger than is necessary to remove signs of wear. To this end, some engines have oversize pistons in increments of 0.010..... so you can rebore 0.010, 0.020, 0.040, 0.050, etc.

If an iron engine is too badly worn, or has been overbored several times, it can be sleeved to restore the bores to their original size. As noted above, it's usually cheaper to get another block to work with, than to resleeve all the engine bores.

It is useful when only one or two cylinders need sleeves, or the block is a rare or vintage example, where a replacement block is expensive or not available.

Aluminum engines usually have cast-iron sleeves. Sometimes the sleeves are thick enough to permit a rebore, sometimes the sleeve is replaced, in place of reboring.

Sometimes, aluminum engines have a hard-wearing coating that is sprayed onto the aluminum cylinder wall. The piston actually slides on this coating instead of a sleeve. Such engines cannot be rebored or resprayed. Sometimes they can be sleeved, but often are just replaced when worn.

Some small engines (chainsaws, weedwhackers etc) just have the piston riding on an aluminum bore. These engines are never rebored or rebuilt, but are just thrown out when worn.

Reboring any engine does increase its displacement slightly. For example a 350 cubic inch (5.7 liter) Chevy V8, overbored 0.030 inch is now a 355 cubic inch engine.