I have had several trucks that hauled heavy loads. Every one of them eventually developed cracks in the exhaust manifold(s). I have a 2003 W20 with the stock exhaust manifolds and think cracked manifolds is a matter of
WHEN not
IF I will have a cracked manifold.
Reading on this forum and my own research it seems that replacement manifolds and exhaust headers for the W20 have become rare as hen's teeth. I decided that I would get some headers if possible for insurance against a future manifold crack.
I researched header availability and find that ONLY Doug Thorley still makes headers for the W series Workhorse. However, they are out of stock and only schedule a production run when they have enough orders to justify the setup. They accept your order and place you on a waiting list. They will charge your credit card when the headers ship. Don't order directly, contact them at:
info@dougthorleyheaders.com. Tell them you would like to get on the waiting list for a set of THY-312 headers for a 2001-2011 W series Workhorse motorhome. They will probably respond quickly telling you how to get your headers on the list.
Banks still makes the headers for non-EGR applications. In other words for 2004 and newer W series chassis. However the Banks headers are ~$400 more.
I've ordered Doug Thorley headers. I expect to see them in 4 to 8 months! That's OK, I don't yet have a cracked manifold and can wait. With the headers no cracked manifolds and as a bonus more performance for sure and maybe slightly better fuel economy (NOT).
The more that are ordered the sooner we will all get our headers.