you can add/change valve seals with the engine still in the vehicle, same as an OHV/pushrod engine, but it's a pain...same as OHV/pushrod engine
. i don't believe you can add "Factory" seals to the exhaust side of 90 heads without machining but you can maybe use another seal as long as the O.D. of the seal isn't too big & get into the I.D. of the valve spring. i use U.S. Seal stuff when things get tight, they have seal's with really thin wall/jacket designs that can help out a ton when stuff gets tight. i never build engines ever without exhaust valve seals. i'd like to add them to my 90 too
