Ok, eventually, I promise, this engine is going back together. No, really, I mean it. However, before it does, I need to buy MORE PARTS! This is getting expensive...and I'm not ever going to move away from the 403 Block now. I'm too invested. I had to buy a new intake, so I got a nice Edelbrock Performer 2711 in cast alumimnum. Pricewise, it wasn't outrageous, about $400 with the bolt kit.
It is almost an exact replica of the factory intake, which is nice because I'm a big fan of leaving things factory where I can. The one HUGE different is the weight. This weighs about 15 pounds. The Factory Cast Iron intake is about 60! I know that isn't going to make anything faster, but it sure does make it easier to work with. Why do I need a new intake? Well, when I got my carburetor off I found some bad things.
First off, the base gasket had cracked in two. So, the engine was able to suck air that would be "unmetered" and would cause a lean condition. That problem is easily (and inexpensively) corrected. A new base gasket from Napa takes care of it. More troublesome is that the intake itself had cracked to outside air, worsening an already lean condition. When an engine runs lean, they tend to run hot...and hot is bad. Remember when I said I only thought I had a problem with my radiator? LOL!
All this damage has been caused by heat. Partly by the design of emissions era engines where they would allow the exhaust gases to crossover under the intake to help heat the intake charge (for a more complete burn) and partly by previous owners neglect. I'm the 5th owner of this coach. It is in good shape, but NOT good enough for me yet.
Anyway, we know this engine has been running hot. That also means hot exhaust gasses, right? Make sense. I thought my exhaust manifolds were in good shape, but upon seeing the intake, I figured I should look at them more carefully. Into the sandblaster they go to get the rust and crud off of them. Guess what?
Yup. Those are cracks. This is the RH side manifold. I actually tried to fix it. It didn't go well...The metal is severely compromised. I was just cleaning them up to get ready for welding and the casting exploded.
My LH side manifold looks OK, but not great. No cracks, but the mounting surfaces have some erosion to them from hot exhaust gasses slipping by the gaskets. In an ideal world, I should replace it too.
Exhaust manifolds are not generally something you find used. If you think about it, they aren't a problem until they are...and then they are no good to anyone else. So, unless you have a parts coach sitting around, or you find someone with one, that still has good manifolds, you're out of luck. Yeah, but its a Toranado engine and those are all over Ebay. Well, the GMC Motorhome uses a unique manifold on the RH side (the one I need). Complicating that, I have a 403, not the more common 455 engine. I did find ONE used RH side manifold. Guy wanted $150 + shipping for it. Not a terrible price, but it is a risk. Mine was flaky from the inside out. Using that information and applying it to another, supposed good, manifold, tells me that unless I can inspect, in person, that there is no material degradation, I really won't know what I have. It may not be cracked, but if its thinned like my broken one, its only a matter of time.
Another option is headers. There are a couple different businesses making headers for the GMC. I queried the groups I belong to about the headers. Lots of guys run them. Lots of guys like them. Headers also come with their own drawbacks. They produce a lot of heat in engine bay (because of their tubular nature), there is some noise concern (again, long tubes that can resonate), and finally, they are bent such that they block access to some of the serviceable parts of the transmission. I don't want to pull the header each time I need to replace that part (which can be prone to failure). Ugh...never easy.
In my searching though, I found out that Cinnabar Engineering (https://www.thegmcmotorhomepeople.com/ ) may be making reproduction cast manifolds. I reached out and spoke with a salesman over there. They are making reproductions! Even better, they are using the original pattern designs from 1978 to recreate them. So, its hard for me to even call them reproductions. These should be OEM spare parts.
I debated back and forth on what to do. Go for what nearly everyone recommends and get headers, or get a brand new, cast replacement? I doubled down and gave Cinnabar a whole bunch of my money. I bought both sides, new gaskets and doughnuts. By the time they reach my door, they will have cost me in the neighborhood of 800 dollars. However, headers would have likely been more money and these do not have the drawbacks that I envisioned with headers. Plus, its a 42 year old vehicle. I don't know how much longer people will be making new parts for them. I'm frugal, sometimes to a fault, but this time, I'm all in with both feet!