Measuring Cylinder Bore

I pulled the engine (Kohler K341) out of my garden tractor to rebuild before spring. It burned enough oil to keep the mosquitos down and leaked from about every gasket. 

I measured the cylinder bore with a caliper to see if I need to get it bored for an OS piston, the standard bore is 3.75 and I measured 3.745. There is a slight ridge at the top of the cylinder, I can feel it with my fingernail but I don't know if I would say it "catches" my fingernail. This is my first full engine rebuild so my fingernails aren't calibrated. 

I know the ridge will affect the measurement when using a caliper. Is it worth spending $50ish for an actual cylinder bore gauge or should I just go with my original plan and use a ball hone and put a standard size piston in? This was supposed to be the parts tractor but I haven't figured out the governor issue on the other one so I have been using it for mowing. 

You are measuring the unworn part of the cylinder.   Wear occurs  down the cylinder a ways.

You need to get rid of that ridge no matter how minor.

Do you have a ring compressor ?

  Short block might be a better way.

If you anticipate doing more engine work down the road a bore gauge is a good investment.

Otherwise, an engine shop could measure the bore for you.

do you want me to bring a cylinder bore by?

My go to would be a telescoping gauge set like this:

But if you have access to a bore gauge that would probably be easier.

clean the cylinder measure the bore at the top with what ever you have. Then take a ring and install it at the top of the bore where tge cylinder is not worn keeping it square to the bore with a piston that has the rings removed and measure the gap in ring with a feeler gauge. Then slide the ring down the bore with the piston and measure the gap at the top where you can see ware and at the middle and the bottom. The variance in the gap from your initial gap is the ware on the cylinder. This is by no means perfect and will not tell you if it I out of round but it is a good starting point. I have used the for many many rebuilds of lawn mowers as well as SBCs. Far from perfect but way better than nothing.

The difference in the gap size is gap is equal to the wear times Pi.

So if the gap increases by 0.00314", then the wear is about 0.001".

Rough rule of thumb, gap increase divided by three = wear.

See, high school math IS good for something after all!