Thread: Bore / Stroke Measurements???

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I'm new to building engines for other than stock purposes, so let the jokes fly. As long as somewhere in between somebody can help me out.

When someone says, "Bored .030 or .040 over" what does that mean? Also when someone says the stroke is 4.00 for example, what type of measurement is that?

I know the bore is in the cylinders and the stroke I'm assuming is the piston stroke (but you know where assuming leaves you). I want to get the most out of my engine and before I can do that, I need to learn these things. Thanks for the help!

The stock bore on a 350 engine is 4.000". That's the diameter of the cylinder bore. When an engine has some age on it, it can become tapered from the top to the bottom of the bore or egg-shaped due to the piston rings rubbing up and down on the surface of the cylinder bore over time, so when you do a rebuild, you would normally go to the next size larger piston and rings by using a boring tool to enlarge the bore slightly. You would buy the pistons/rings and then have the machine shop bore the block to fit the new pistons. A little bit of clearance is left between the piston and bore, depending on the type of piston used. Cast and hypereutectic pistons are installed with a very tight clearance, for instance maybe 0.001" to 0.002", while forged pistons use a larger clearance, maybe 0.004" to 0.007". So, for instance, if your new cast or hypereutectic pistons measured out on the diameter at 4.030", the machine shop might bore and hone the cylinder to 4.031" or 4.032".

Stroke is the distance the piston travels up and down in the bore. If you start with the piston at top dead center in the bore and then turn the crank to drop the piston to bottom dead center of the bore, the length it traveled would be the stroke of the crankshaft installed in the motor. A 350 uses a stock stroke of 3.480", while a 383 motor uses the stroke of a 400 crank at 3.750". A longer stroke will not only add cubic inches (350 to 383) but will make more torque to pull the vehicle with the longer stroke. Research most any truck motor that has to pull a heavy load and you'll find that the motor is designed with a long stroke as opposed to the "back and forth to the store" passenger car motor.

Hope this helps.

Thanks Techinspector1. It more than helps. It made a previously foggy subject crystal clear. Even if your cylinders are in good shape, is there a benefit to boring? After all, you would be putting in bigger pistons. I suppose I can also deduct that it is of benefit to get a crank with a long stroke if I want to add hp?

"Even if your cylinders are in good shape, is there a benefit to boring?"

If they are not tapered or egg-shaped, you can do a down-and-dirty rebuild by honing the cylinders to remove the glaze which has formed on them and to provide new cross-hatch marks in the bore so that the new rings and the bore can have a chance to wear-in together and putting new rings on the old pistons and re-using them. Putting new rings in the old (smooth) cylinder (not honed) will not allow the rings to wear-in properly and the motor will not seal up properly. You have to determine how much the cylinder is worn when doing this, because if it is worn too badly, you'll have excessive piston-to-cylinder-wall clearance and the pistons will rattle in the bore because they are "loose". Boring the cylinders larger and using larger (oversize) pistons provides a brand-new surface for the rings to run on and insures that the bore is not tapered or egg-shaped and increases cubic inches at the same time.

"I suppose I can also deduct that it is of benefit to get a crank with a long stroke if I want to add hp?"

yes, to add cubic inches and thus more power from the same motor.

Originally posted by techinspector1


"I suppose I can also deduct that it is of benefit to get a crank with a long stroke if I want to add hp?"

yes, to add cubic inches and thus more power from the same motor.

a long stroke isn't neccisarily the way to get the power you want.... the reason honda's are so fast for little 4 cylinders, is because they have a very short stroke... the shorter stroke... the more RPMs you can push, less distance for the piston to travel, less time for each stroke...

dont get me wrong i despise honda cars, but the short stroke thing was just something that should be mentioned.... different applications need different builds