I have just re painted and put new gaiters on the front Bilstein struts from my donor car before fitting them to my Trans Am .
I measured the diameter of the piston and it's 40mm I have done a search but the only struts that Bilstein list now are 36mm

Part number?

There were part numbers stamped into the lower part of the strut but now I have painted them they are not visible [I didn't measure the piston until I had given them a 3rd coat of paint] Also, originally the struts had black gaiters fitted to them which were split.
The standard stuts that I removed had 25mm pistons and collapsed under their own weight .
To say the suspension is firm is an understatement I think I may shake my fillings out