Valve stem seals

[madras]

Hi - anyone ever replaced any or had them fail, do you need engine out?

[NSXGB]

Shouldn't need to remove the engine. Worst case scenario is maybe cylinder head off. I don't know if it's possible on the NSX engine but I have replaced them on other cars with the head on, using a special tool which compresses the valve spring from the top allowing you to remove the collets and remove the hat & valve spring to get at the seal. Have to make sure cyl is at TDC otherwise you lose the valve into the cylinder; then your head will have to come off...
Not a cheap job unless your cam belt is due.