What do you think about a 3.754" - 3.755" bore? I would like to leave it instead of boring. All cylinders seem to be round within .001".
What size pistons are you planning to use? The bore size can be anything so long as you have pistons and rings that fit with acceptable clearance. Obviously you can get pistons custom made to any dimension, but it is lots cheaper and easier and quicker to just use off the shelf sizes and bore to match.
Does your statement "3.754-3.755 bore" refer to cylinder taper or the range of variance between the 4 cylinders?
I think you are referring to taper. In any case, the numbers you give (.001 in both cases) would be of no concern for either taper or out of round.
However, since the stock bore is 3.750, and clearance is usually determined by, or part of the finished dimensions of the pistons you would have excessive clearance with stock standard pistons. In fact, I'm a little puzzled at how the bore size could have increased by 4-5 thousandths so uniformly.
There are sections of the bore which never experience any wear whatsoever, regardless of the engine design. That would be the section below ring travel and in line with (parallel to) the crankshaft. Did you check there? If that section measures 3.754-3.755 I would be wondering what happened to give the extra 4-5 thousandths clearance.
Something doesn't sound quite right here. I agree with Royce regarding boring vs custom pistons. Much cheaper to bore the block for some standard oversize pistons than to make some pistons to fit your bore.
One last thing, you would definitely not want to use original cast iron pistons in the bores with the dimensions you are describing. The pistons depend on contact with the bore to transfer the heat of combustion away from them. Stock cast iron pistons would have at least 6-7 thousandths clearance and would have less contact with the cylinder wall for heat transfer. There is good reason aluminum replaced cast iron for aluminum in piston manufacture long ago. The thermal properties of cast iron is good for stoves, not for pistons.
My statement, "There are sections of the bore which never experience any wear whatsoever, regardless of the engine design. That would be the section below ring travel and in line with (parallel to) the crankshaft."
I am referring to the section of the bore that when checking, your inside micrometer would be parallel to the crankshaft center line and below ring travel.
I don't know why these dimensions are as they are but I double checked and they are 3.754" - 3.755".
I was hoping to hear this would not be too large for standard aluminum pistons. This engine had cast iron pistons, standard size valve stems, with the old 2 piece valves, standard size solid push rods. I checked hole size for valve stems and push rods, only worn slightly oversize.
check you cast iron pistons for wear and try to reuse them with new rings new valves and new adjustable push rods. You will be pleased with it.
I did it a few years ago with a stuck engine that was for several years out side. Cleaned it out honed the cylinders and put it back together. Till now it is the easiest and best running model T engine I did.
Here a few photos of the engine during the rebuild.
I think aluminum pistons, standard size would probably work just fine. My rationale comes from one experience in particular:
When I worked at PM Auto Parts in Dallas in the 70's I bored many Model A blocks along with a wide variety of other makes. One of the local Model A club members brought in the block that had come from a running Model A. In fact I had heard it run myself prior to the teardown.
We had a Rottler boring bar which indexed off of the pan rail, rather than sitting on top of the block surface as portable bars do. The first step was to check your setup. We did this by setting the cutting tool to the currently existing bore size and running the bar down one hole to make sure everything was straight. When you did this you could hear this very faint chipping sound since it was impossible to center the bar in a worn cylinder. As long as that sound stayed consistent and the faint marks down the bore stayed straight then you could conclude the set-up was fine.
On the Model A block I was referring to the bar didn't travel more than an inch down before it was cutting all the way around. I pulled the bar up and adjusted the tool in by about .005 thousandths and started again. About another inch down the bore and it's cutting full circle again. At that point I measured down towards the bottom of the bore in the area I described above where there would be no wear and found the bore to be standard. I had set the tool to around .030 over on my first pass when it started cutting full circle.
I checked the other 3 bores and found to my amazement that this block had around .030-.035 thousandths wear taper in the bores. I had never in my entire time working there found that much bore wear in any engine. And again, this was the block from a car I had heard run prior to it's removal.
On closer inspection I could also see that each bore in this engine had more than one ridge in the cylinders. It looked like stair steps in the top 1/32 of an inch of the bore. This was the result, over the years of periodic and successive adjustments to the rod clearance via shim removal which of course slightly shortens the rods by the thickness of the shim removed each time.
The owner of this A had supplied a set of .040 over pistons for this job and it turned out to be a rather tedious and involved effort to center the boring bar for these pistons. In the worst case, I only had about .003" to work with to find the center of the wear so that the bores would clean up.
Sorry for the long story, but that experience is why I think you will be fine to run aluminum pistons in the approx .005 over bores you have in your T.