Help figure out what engine displacement is
 

I purchased a 1983 911sc that has no paperwork with it. It has a replacement 1978 (verified by SN number) engine that was supposedly built with oversized pistons and 964 cams. Is there an easy way of verifying what it really is? Any markings i can look for? I have an endoscope if that helps. Not a big deal but it would be nice to know.
I tried to search for information here and elsewhere but couldn’t find anything other than stripping down the engine.
Thanks for any suggestions.

Stripping down the engine is really about the only way to know everything but you can collect a few clues.

You might run an endoscope into one of the cylinders to see piston shape crown shape (CIS or something better).

You can remove a chain cover and try to read the number off the end of the cam.

Pull the fan off the alternator and peer at the top of the cylinder where the cylinder meets the case. You might see some cylinder ID there.

Post pics of the motor and you'll get comments on the induction system.

I've read about a thing called a whistler that will tell you cylinder volume acoustically but I've never seen it used. No idea how common they are.

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No real way to know short of taking off the head, removing a cylinder, measuring stroke and bore, and running it through the usual calculation:

(0.7854 * (Bore^2) * Stroke * Number of Cylinders) / 1000

Some racing series for which displacement enforcement is critical have a method to check displacement. I think it’s fluid based?

You could also examine your cylinders externally and do a bore scope to see if pistons are stock. Stock pistons would suggest a stock configuration.

A Whistler is for checking compression ratio. A cubic inch/displacement checker was commonly referred to as a "Bubble checker." Neither require tearing down the engine to perform the checks (other than removing the rockers with the displacement checker), but the tools are expensive. The laziest way would be to find a racer with access to both tools.

Short of that, use your borescope tool to peek inside the spark plug hole. If the piston looks stock, it is likely a stock displacement. Most aftermarket sized pistons have piston tops that do not look factory. If the piston looks aftermarket, it could be a different displacement.

Cams, on the other hand, will require checking at least max valve lift with a dial indicator.

Thanks everyone. I'll start with the endoscope and see what the pistons look like. I'll try to take pics since I wouldn't know stock from aftermarket. The car has quite a few mods that would suggest the PO was telling the truth. SSI's, 964 calipers with braided lines, turbo tie rods and torsion bars, front strut brace, etc..

You can also run it on a dyno. Torque level and shape of the curve would give you an idea of both displacement and cam, certainly with very low precision.

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Good idea Mixed76 but living at 5280 feet, I'm not sure I want to know how little power it's making. :)