Can I put an 8bolt crankshaft from a 4.6 into a 4.6 that has a 6bolt crankshaft
Yes you can, but then you will need the proper flywheel or flexplate. The Ford Modular 4.6 engines are internally balanced.
I have the 8bt flywheel and the 4.6 motor with a 6bolt flywheel so I need an 8 bolt crankshaft right
http://www.pera.org/articles/eb10324.htm Good information at the link. Not sure if you have a Romeo or Windsor engine. I failed to notice you were referring to a 4.6 in an F-150. My fault.
http://www.enginebuildermag.com/2006/03/rebuilding-the-ford-4-6l/ Romeo and Windsor There were two cranks used in these motors from ’99 through ’04 with a couple of variations. There’s a light one and a heavy one, depending on the year, and they had six or eight bolt crank flanges, depending on the application and the year. The Romeos always had a six-bolt flange, but the Windsors came both ways, so check the application charts to see when and where they were used.
Looking for a Used F-150 in your area?
CarGurus has 208,988 nationwide F-150 listings starting at $1,200.
Search Ford F-150 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Ford F-150 Experts