What is the clearance between the valve and piston on a 2003 Hyundai Elantra 2.0.

What exactly do you mean by an "overhaul"? More information please. Which diesel engine is this? The 2.5 litre or the 2.6 litre? What year is your vehicle? Some engines can be different settings depending on the size it is.
I'm assuming this is for the clearance between the valve rockers and the spring end of the valves?
If you're breaking rockers your mechanical timing might not be set correctly risking bent valves or the valves are already bent. Even One tooth out on the timing belt/chain can cause problems. You might need to take the cylinder head off to check the valves.
When you set the clearances the engine needs to be Cold and the cylinder you're working on needs to be at Top Dead Centre (TDC) to make sure the valves are closed. On an inline 4 cylinder engine the cylinders work as pairs, 1 and 4, 2 and 3.
Set cylinder number 1 to TDC. Set the clearances for only cylinders 1 and 4. Using the crank pulley bolt turn cylinder 1 to Bottom Dead Centre, (180' turn clockwise). This should bring cylinders 2 and 3 to TDC so you can set those clearances.
Is the engine burning the oil? (a thicker, more blue exhaust smoke than normal). Burning oil can also be a sign of a blown head gasket or badly worn piston rings.

Interference.

Interference Engines - The Complete List - Your Car Angel

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An interference engine is one that has insufficient clearance between the valves and pistons if the cam stops turning due to a broken timing belt. The result is ...
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Does this car have an interferance motor? - 2005 Hyundai ...

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Mar 22, 2018 - All Hyundai engines are interference design. The only way to test for bent valves *before* replacing the timing belt is to remove the ...
2 answers · Top answer: Yes, it is an Interference engine

There are five safety recalls on the 2003 Elantra. Two are related to a valve in the fuel system, one to a rust-prone suspension control arm, and two to the rear brake lines.

http://www.hyundaiproblems.com/recalls/Elantra/2003

Check for engine codes related to Camshaft Position Sensor or Crankshaft Position sensor. Replace as necessary.

There is no heater control valve on your elantra

Clearance is NOT the issue. The Elantra engine I4 (2.0 lit) is an interference valve-.to-pinston clearance. If the valve timing to piston clearance is exceeded (wether advanced or retarded), the valves will 'kiss' the top of the pistons and get bent. Also verify 'INTAKE' & 'EXHAUST' cams have been installed proprely. Valve timing (timing marks at camshaft sprockets in reference to crank shaft sprocket) is essential. Also,.ensure proper timing belt tension.

The timing belt should be replaced every 60,000 miles, this is very important because this engine is not free wheeling if the belt fails, this means the engines valves hit the pistons. The cost to replace the timing belt is about $300 but varies with local labor rates, the rate where I live is $100.00 per hour.

There are two ways to find the Exact Value Adjustment.

1. Valve adjustments are usually labeled on a sticker in the engine bay.

2. Valve adjustment will also be mentioned in the user manual, if you don't have ask your nearest Hundai service center for details.

As it is a very old version, i have scraped the data.

Thank you

According to the flat rate labor rate for replacing the timing belt it calls for 2.7 hours plus 0.2 hours if the vehicle has a/c. if they didnt line up your timing marks when they done the belt to begin with then they probably ruined other parts internally inside the engine such as valves and pistons.

not usually if it's running and backfiring. A timing belt on certain cars when it breaks can cause major damage because the valves stop and the pistons continue to move up and down can hit the valves bending the valves and even shattering the piston. You need to align the timing marks on the crankshaft and the camshaft for it to run properly