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Well, it's all down to money. With a 79mm bore in an A15 block, & you can find a way to keep the gasket from blowing between the end pairs of cylinders you would have 1608cc

Now if we offset grind the crank & just use 1mm undersize bearings, we can incrase the stroke out to 83mm. If dished pistons are used, we might just get away with a 1/2 mm skim off the rim & the total displacement would be 1627cc, but this would be pretty much pushing it unless special rods &/or pistons are used to allow a welded stroker, & even then, you start running into crankcase clearance problems.

So there you have it, 1608cc with just a boring bar &1627cc if the crank is stroked. I'd say that is probably very close to the practical/cost effective maximum in an A15.