Here's the truth as I know it. And I worked my way through college boring motorcycle cylinders...

► It starts with measurements. To do it properly, you need to "map" the bore by taking 9 separate readings: top, middle, bottom straight on. Then the same 3 readings 15° to the Left, and again 15° to the Right. Cylinders tend to wear tapered with the larger dia at the top, but they also wear oval because of the front-to-rear slap of the pistons. The mapping is advisable due to this weird wear pattern.

Originally Posted by Allan G
Unless there’s some damage to the bore I find that the old piston and some feeler gauges give me a reasonable idea. The side of the piston gets little wear, you can measure with feeler gauges against the side of the piston (left/right where there is no thrust wear)...

► This is VERY misleading. No wear accumulates on the sides of the piston because the piston is not round, it's oval. The ovality of the piston is not known and so taking a measurement there is useless. And what will you do with pistons that have no sides ?

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► You can most probably only bore the cylinder from 10-over to 20-over if you use a Sunnen hone. The problem with boring bars (cyl boring machines) is that some machines bore the cylinder from the bottom, and others from the top. These 2 surfaces on the cylinder are not parallel, thus a bore job from the opposite (wrong) end will hardly ever align the hole with the previous. The answer then is go to the next piston size and try again.

I highly agree that all cylinders are bored to the pistons. Piston sizes vary from maker to maker. So you either need to use a shop that has the pistons in stock, or supply +20 and +30 piston sets to your machinist. This because there is typically no charge to get the cylinder taken two sizes bigger... as long as the cylinder remains in the boring bar. If it has to come out because you need 4 weeks to get the second set, then all the setup has to be done a second time. You may incur a second charge for the next larger hole. Thus, when working with a machinist, supplying both piston sizes up front is the way to go.

Hope this helps.