2000 Windstar 3.8L,, If the timing belt goes, is it no clearance between the valves and pistons?

you don't change the piston to valve clearance
the valve timing does that as it opens and closes the valves so that the piston doesn't hit it
all the details for timing the valves are in the workshop manual as you have not bothered to include the make, model ,year and engine type in your post

IF the belt slipped or broke there may be damage. to check for valve damage remove valve tappet cover and check valve clearance ,if the clearance is excessive there is damage . bent valves and damaged pistons .

If you Blew off the timing belt then need Head work also since it is an interference Motor
meaning that the Valve train and Pistons Have no Clearance so U are going to need a lot more work than just a timing belt to run

First of all, you must state WHY you are changing the timing belt. If you are changing the belt due to a scheduled maintenance, then we would have to go from there, but if you are replacing a belt that has broken---well my friend------I am here to tell you that you have what is called an "Interference Engine". An Interference Engine is an engine that does not have adequate clearance between the pistons and the valves in case of a timing belt failure. Lacking this adequate clearance, in the case of a timing belt failure, the pistons "Interfere" with the valves, causing them to bend. This "Knocking" noise that you hear is more than likely the pistons banging against bent or broken valves. This can be diagnosed by doing a compression test. However, when seeking advice, next time , give the history of the vehicle prior to the parts exchange. It can help narrow your situation to the problem by 90 + %.
MR. VINCENT. ASE MASTER TECH

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.

You have a zero clearance engine, meaning there is no room for clearance between the pistons when they're up and the valves are closed. If the belt broke there is a high probability that the valve(s) came in contact with the pistons and caused significant damage to the valve, valves, or cylinder head.

This engine has been identified as an interference engine in which the possibility of valve to piston damage in the event of timing belt failure is most likely to occur, when repairing I would also replace water pump, due to pump is timing belt driven. On an 2002 VW Jetta TDI 1.9L engine, VW recommends timing belt replacement every 80,000 miles.

Yes it is as bad as the belt snapping, if your lucky there will be cutaways in the pistons so the valves may not have touched, its a bit tricky to check until you replace the belt and can check valve clearances and cylnder compressions,
or remove the head and check.
If you replace the belt it is often recommented to replace any bearings and pulleys as well, as a tight bearing may be the initial cause.

you should be ok.. i have never seen valve damage on one of these engines after belt breaking..just retime everything and install a new belt..make sure you check water pump and tensioner bearing for looseness/wear