Piston slap is much more common in the B230F engines than the B21/23 engines.

I retired a 1982 245, B21 (non turbo), with 350k miles. No piston slap.

Wife's '91 740, B230F (225k miles), slaps so loud it wakes me in the morning. But once it warms up, the slap is gone. It started this noise around 175k miles.

This is caused by:
1) A newly designed piston (for the B230) that is shorter, lighter. This allows it to "rock" in the bore if the clearance is loose.
2) The aluminum used in the piston. Aluminum expands thermally twice as much as steel. The aluminum piston must be designed small so the fit becomes correct once it warms up.

This small (when cold), short piston can rock or wobble until it expands to fit the bore. The B21/23 pistons are longer so the skirts prevent this rocking or wobbling that causes the audible "slap."

I'd guess the noise from your B21FT engine is something else, possible a worn bearing or wristpin (though this is uncommon). It is possible that you have a defective piston that is rocking, of course, but the B21/23 design is not known for this problem (in sharp contrast to the B230F).
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Don Foster (near Cape Cod, MA)