The Thorley headers are what is known as a Tri-Y design.  Rather than having four equal-length tubes, they are designed as a four- into two into one arrangement, with the first pair being non-consecutive
firing cylinders.  They are supposedly superior in producing low-end torque, but slightly deficient in top-end horsepower.  They should be  the ideal arrangement for our GMC's.  Years ago, they were very  popular, but modern performance freaks are primarily concerned with big horsepower numbers, so they don't sell.  And things that don't sell usually don't stay around long.  Gary Kosier

Group Buys on Headers
 I've arranged a group buy for the Thorley headers for the GMC through our local exhaust guru, Ed Hanson.  He's the one who converted my GMC to a single, rear muffler, 3" system.  (That I'd never part with BTW.)   Anyhow, the P/N for the Chrome Thorley headers for the GMC is  355Y  @ $315.00 plus shipping.  Same for the 403 and yes, you can get them in paint finish but they will be the same price and take a little longer to get. Call for details.

  For a little background information;  Ed Hanson grew up working in his Dad's muffler shop in LA and Doug and Betty Thorley were more than just business acquaintances, they're long-time family friends of the Hanson's.  Give Ed a call at (619)698-7030.

Another source for GMC headers
No information on how good they are.
The best place around that I know of is in Riverside CA.

California Hi-Perforance Warehouse
3230 Motor Circle
Riverside, CA 92504
800-350-7119, 909-688-3687

Last cost that I am aware of was $315 (about 6 months ago). This includes the connector pipe that gets welded to your exhaust pipe in front of the mufflers. If you have OEM type mufflers, they will also
need to be changed to low back pressure type. Best to have a 3" tail  pipe to go with the system. There's a place here in Costa Mesa that has installed the total system from the header (drive in with Headers
installed) all the way with 3" tail pipe for about $350 if you furnish the mufflers. They do a good job......many GMCs.  Duane
Thorleys are also available from Summit Racing at 329.95 with free shipping.You takes your choice Frank Condos

GMC Exhaust Systems

You got-em, smoke-em.  If you have good exhaust manifolds, resurface them, remount with copper gaskets.  Retorque them some time, if you think of it.  They worked for 30 years maybe they will work a while longer.  If you want them to last longer on our crappy gas, put in an A/F gauge and
get your carburetor fixed until the exhaust does not run lean and hot. When the manifolds crack, go to headers.

When you cannot find manifolds, put on Thorley iron headers.  Don't coat them, it cost two times as much, burns off any way, and probably does not do much for reducing the radiated heat, and cannot be proven to do much for horse power unless you have a dyno (which we don't, and the gain is so little who cares).  Put them on with copper gaskets and torque to 35 ft lb--two or three times after running.. Keep your old mufflers if they are ok, you will only gain 10 horse power if you replace them with a high flow muffler and a three inch exhaust anyway, and that is not enough to spit at. Never end a sentence with a preposition and add some fans to draw the heat out of the engine compartment when you stop or put vents in the wheel wells..

When your old mufflers burn out, replace them with straight pipes and cut in a single three inch turbo muffler in the rear (or use two up front doesn't really matter).  Keep your 2.5 inch exhaust pipe until it burns out, you will only gain about 10 horse power if you replace it and that
will come too soon anyway.  It is a little louder with a single muffler but there is little back pressure.

When the 2.5 inch exhaust pipe burns out, replace it with a three inch pipe from the 2.5 inch Y straight pipes to the three inch muffler in the rear. You will help the headers do their job and you will pick up a few horses.

When you have all of the above, you have headers that won't crack, one less muffler to worry about, a full flowing system and you may have picked up 10 to 20 horse power.  Not much here to stress over.  Do what you have to, this is not going to change your life.



I have recently put Thorley headers on my GMC and Al Chernoff and I installed his today.  It took me almost 2 days to do mine but it only took us about 5 hours on Al's and that included putting an O2 sensor into the end of the header stub.

Of course it is quicker with two people, but I had also found out a few things that may make the job easier if anyone is going to do this.

First, we ran a 3/8"x16 thread tap into the holes in the heads so that the new screws will run in easier.  We also put anti-seize onto the threads (molybdenum disulfide).  It is necessary to use the longer original bolt for the front hole on the driver's side as there is a power steering bracket piece that goes under that bolt.

Bolts are to be torqued to 35 ft-lbs but you can't get a torque wrench on one bolt on the passenger side and 2 or 3 on the passenger side.  So we torqued the ones we could and then used a long handled 7/16" wrench on the others after testing one of the torqued ones to see how much pressure we had to put on the wrench.

The gasket set says 35 ft-lbs dry and 25 lft-lbs using moly anti-sieze.

When putting on the driver's side header, leave the lower part of the oil check tube in the block and just drop the header down from the top and it will go right over that tube.  When I did mine alone I had removed that tube first (per Thorley's instructions) and I spent almost 4 hours trying to get it back into the block.  It had to be curved to clear the header tube and then it wouldn't go straight into the block.  With Al's we slipped the header right over the tube and then we just bent the tube out to get it about 1/2" away from the header.

We coated both sides of the gaskets with Permatex Ultra Copper silicone gasket sealer.  This is the highest heat range that Permatex makes (700 deg.).

I am using copper gaskets with out silicone.  I want to be able to remove the header if I have a tranny problem and maybe reuse the gasket.  I am not using silicone, it kills O2 sensors and is hard to clean off.  gene

MrGasket  #7170 - comes with lock washers and is copper
REMFLEX - thick gasket that also works
grade 8 bolts  3/8-16-1.25

When you put the supplied fiberglass heat socks onto the plug wires and onto the speedometer cable, use some small plastic tie straps to secure the ends onto the wires.  This makes a neat installation and keeps the ends of the socks from fraying and the socks from sliding along the wires.

I think you will find that the heat socks over the spark plugs is not necessary on the 455/403 engines.  Headers will not effect the rubber spark plug boots any more than OEM Manifolds.  Also it is not advisable to use the furnished heat socks/sleves over the speedometer cable since as it gets oily, it becomes a fire hazard.  Rap the speedometer cable with thin aluminum material (0.020") & it  will provide a better protection w/o absorbing oil & prevent frying the cable. Duane

I did the insulating of surrounding parts a little different than you. I wrapped my speedometer cable, my oil check tube, my transmission check tube , my transmission shift cable with "Cool Tape". I think this
makes for a much neater look than the wrap tubes that come with the 355Y  headers.  On the speedometer cable and the transmission shift cable I used both the sock that came with the header and the Cool Tape.I wish I had wrapped my transmission fluid cooler lines that run under the headers on the drivers side before I installed the headers. I did get some sock material over these after the headers were on but I don't feel they are really as well protected from the heat as they would havebeen had I wrapped them first.  I may regret it but I did not wrap my plug wires. I pulled them all up and away from the headers as far as I could.I think the nearest one to a header is about 5 inches. I intend to watch these for heat problems and will do something later if I find it necessary. Charles Wersal
Cool Tape

For the wheel well liner on the passenger side, instead of cutting the bottom so that it can't touch the headers, just pull it outward and hook the bottom of the plastic over the top of the shock absorber mount.  It will hold the liner far enough out so that it won't melt and still leave enough plastic to prevent water from being splashed onto the headers and side of the engine.

I think you will find that it is necessary to cut away some of the wheel well material near the header on the right side (similar to left wheel well).  After a long pull up those western mountains, you will see what I mean. Duane

Check all wires and also the two water hoses that lead to the back water heater to be sure that they are tied back away from the headers.

If you are going to use your existing mufflers (or new ones up front) be sure to support the front of the mufflers with hangers.  Don't let the header tubes carry that weight as they might sag when heated up.

I made a "Y" pipe to replace my front mufflers and put on a 3" exhaust and tailpipe and am putting my muffler in the rear.

Here are the pictures


Manny Trovao had a good idea for the tranny lines.  First of all, he replaced them with 3/8" lines rather than with 1/4" lines.  He also (of course) used stainless steel.  Fittings are readily available to fit the transmission for the 3/8" lines.

He routed the lines from the tranny straight to the side frame and then followed the frame forward to the radiator and transmission cooler.  This keep the lines well away from the header pipes.

CAUTION: If you're attempting to do this as a last minute thing. The tranny 5/16 flare fittings also need to be replaced with 3/8 but the problem is finding them. The aftermarket fittings don't have the same thread pattern. These only come with 1/4 mpt but its not what the originals are.  I have found some fittings in Eldo trannies and have been  saving them, so I have a few sets.  In my case, I'm using ss  Compression fittings and had to machine and tap these fittings. Manny


Check with Cory Ure at High Performance Coatings(800-456-4721).  They can
get the headers direct from Thorley without the nickel plating (which must be
removed prior to ceramic coating). Patrick


I do not remember speaking to you directly, but if people let us know it is for a motor home, we let them know this can occur.  The coating itself is virtually bulletproof.  About the only way to get it off is hand grinding it off or spending a couple of days blasting it. One of the only things that will cause a "problem" with the coating is if the pipes get so hot that the pipe expands more than what the coating can, and a crack will occur in the coated surface and eventually, a piece of the coating can come off.  The two
common applications where this happens is the rotary motors where they often run at or close to cherry hot at all times and the older motor homes like the ones you have.  What most people tell us about your type of motorhome is that the pipes almost glow at all times, especially under load and going up
hills.  I would suspect that is what happened to your pipes.

What you have on there is the best thing possible.  There really is not anything that can completely solve all of the problems in your situation, but what you have is the best thing available. Richard



Last week we replaced the old cracked manifolds with new Thorley ceramic coated headers. What a difference!

When we bought the coach, the exhaust aft of the mufflers had to be  replaced. Since we expected to replace the manifolds one day, I had 3" pipe  installed from the mufflers rearward. SO, my comparison is with stock  manifolds, stock mufflers and 3" exhaust. The pickup is noticeable from
about 1500 RPM upwards. Where the coach used to accelerate with reasonable  speed before, it really gets moving now. The distance to 60MPH from a  standstill is about 20% less than the stock configuration with 3" exhaust.

I used Kroil and had no problem removing the old manifolds. Passenger side  manifold fell apart during the removal (along the middle of the manifold).  Now I need to drive it a few hundred miles and retorque.

There have been a few folks who were concerned about noise levels with headers. Here's my tests:

Speed      Before    After
idle           72dB      60dB
45 MPH     85dB      63dB
60 MPH     94dB      71dB

The majority of the noise is wind noise that is coming from my front  windows. Tests were made with a Radio Shack SPL meter held at shoulder  height facing the front then sides of the cab area. Measurements are the  highest of the numbers. The before numbers are with the cracked manifold. I
think that I can get about 5dB out of the cab area just by eliminating the  wind noise - engine noise alone at 3000 RPM makes the cab sit at 64dB.

Tax, title, delivery to my door was $544. It includes the gasket set and a  heat riser assembly to attach to your header. I bought them direct from HPC (and probably paid a bit more) since that eliminated two shipping costs. HPC got the headers without chrome and then coated them.