Exhaust manifolds - 455

[homebase1]

> From: "Gilbert R. Bethel"
> To: "GMC Chat"
> Subject: GMC: Exhaust manifolds - 455
> Date: Mon, 13 Apr 1998 18:18:27 -0500
> Reply-to: gmcmotorhome

> I have warped exhaust manifolds on my 455. Does anyone know of a repair or
> should I just bite the bullet and buy a set of headers? Your comments are
> requested. Thanks
>
> Russ Bethel
> San Antonio, Texas
>
>
> My suggestion......
Take the manifolds off, take to a machine shop and have them plained.
While you are waiting the two days or so, order a set of ceramic
gaskets from Mondello. Put it together, and as they say around here;
Bob's your uncle !!
MikeB

 

[gilbert r. bethel]

I have warped exhaust manifolds on my 455. Does anyone know of a repair or
should I just bite the bullet and buy a set of headers? Your comments are
requested. Thanks

Russ Bethel
San Antonio, Texas

 

[thomas g. warner]

Russ just went through the same thing. Immediately bought a set of Doug
Thorley manifolds, than decided to see if I could repair the old ones. It
takes a little elbow grease but not hard if they are not really bad. Get a
good metal straight edge and see how much they are warped. I flattened mine
using a 3/8" thick piece of glass and a sheet of garnet paper (silicon
carbide would work OK too), all layed on a flat work bench. Put the glass
on a flat surface contact cement the paper to the glass and start
flattening.Worked for two days on mine, and just before getting done, a
friend (in Rome NY, another GMCer) gave me his old set that had been planed
flat. i installed them and they are fine. I still have my old set, and the
new dough Thorleys. If you are lucky yours are not too bad. Good luck. PS
a good machine shop can plane them also

>I have warped exhaust manifolds on my 455. Does anyone know of a repair or
>should I just bite the bullet and buy a set of headers? Your comments are
>requested. Thanks
>
>Russ Bethel
>San Antonio, Texas
>
>
>

 

[scott woodworth]

Russ,

Here's the dissertation I sent previously to another member of the
list:

Rich,

The GMC motorhome shop manual does not show the use of exhaust
manifold gaskets. There's a good reason not to use them in this
application.

Motorhome service causes a great amount of heat. The engine is
under a moderate to heavy load most of the time. I'd be willing to bet
that the exhaust manifolds can get dull cherry red during long hill
climbs. Using gaskets will insulate the exhaust manifolds from the
heads. That leads to overheating and cracked manifolds.

The best solution is to take them down to a good machine shop and
have the checked for cracks then have them surfaced. Make sure that
they take off only as much metal as necessary.

Reinstall the manifolds without gaskets and torque the center bolts
to 25 foot pounds. Then torque the outside bolts to 15 foot pounds.
That allows the manifolds to expand lengthwise when they get really hot.
That way, they won't crack. Leaving the gaskets out will allow the
water cooled heads to sink more heat from the manifolds. This tip comes
from the book Trailer Life's Rx for Rv Performance and Mileage by John
and Estes, Bill Geraghty. (Out of print)

Note: Do not use galvanized bolts on your manifolds if you ever plan to
remove them. Please use grade 5 or grade 8 bolt and put anti seize on
them or dip them in plain unflavored Milk of Magnesia.

- -Scott Woodworth

P.S. The 403 heads are 4C.