Changed the intake manifold, now there's an exhaust leak between manifolds.

[timk225]

I did my first performance upgrade on my 1973 Duster 225. I got a factory iron (would have preferred aluminum) Super Six intake from a '78-'79 Aspen. I cleaned it well, primed and painted it factory blue.

Then I very carefully installed the carburetor adapter plate to bolt on a Holley 2300 350 cfm 2 barrel. A good but not excessive layer of Permatex #2 non hardening sealer on the gaskets below the adapter, and just the carburetor and its supplied gasket on top.

First surprise came after I pulled the factory manifolds, took off the 1 bbl unit, did some mild porting on the exhaust outlet, and reassembled the new intake to the old exhaust. Thanks to the adapter plate being larger, you can no longer put on the nut on the cylinder head side of the carburetor, there's no room. The through bolt itself barely made it past the adapter plate.

Then, when taking off the outer 2 thinner bolts between the manifolds, one snapped off, but the other came out easily. And surprise surprise, when I went to put the one bolt back down through the intake to the exhaust, the adapter plate was in the way. And no way was I going to ruin my excellent sealing job! So I got a 4 inch long all thread bolt, cut it to about 3-1/4", and threaded it up through the exhaust and put a nut and lock washer on the pull the intake and exhaust manifolds together. One bolt out of 3 ought to be enough to seal it, right?

Well, I guess not.

On initial fire up of the new carburetor, the engine cranked for 8 seconds and started up all by itself. From an empty fuel bowl to running. No pumping the gas or anything. Idle is about 1400 rpm so it needs tuned. It has #63 jets in the Holley 350.

But I had a loud exhaust leak between the manifolds. Of course something had to be wrong!

So after looking at it, I snipped open the larger inside bolt hole off the old metal gasket, loosened up the manifolds and the one outer bolt, and spread them apart enough to slip the second metal gasket in there. Surely that'll take care of any gaps and seal it, right? I lined out the outer 2 small gasket holes, so I can be sure the inside will be positioned right.

Nope. Still a noisy leak. I put my hand down there with the engine running and I can feel the exhaust gas blowing out the rear side of the manifold. Annoying.

So I have my handyman over there today while I'm at work, pulling the manifolds loose again, just enough to lift the intake off the exhaust. I'll use my Dremel grinder to carve a notch in the adapter plate to get a nut on the larger inner bolt. And my handyman seems to think he can get the rusted broken bolt out of the exhaust manifold. Go for it. I don't think we need to, surely 2 out of 3 bolts will be enough to seal it, but whatever.

But no matter what, that adapter plate is NOT coming off that intake manifold, that is one of the most excellent sealing jobs I ever did!

For 2 machined flat surfaces, I didn't think getting them to seal would be such an issue. Something had to go wrong.

 

[halfafish]

Those are some nice upgrades, you will like the results. Now on to your leaking issues. I realize if you did a bang-up job sealing your adapter plate you might not be in a hurry to re-do it. However, it is likely in your best interest to do so. That said.........I would not plan on one or two of the three mounting bolts to make a secure connection. I suggest getting the broken bolt out, then chase the threads as they get crapped up with age - that's a big factor in why did the bolt break. I would then secure the two manifolds together, and lastly install the carb adapter. When bolting the manifolds together I put both flat faces on a flat surface to make sure the two are plumb with each other then tighten things down. A test fit is a good idea first so you don't use up your gasket should you need to disassemble and do some adjustment. When I get done I re-check the faces with a straight edge to make sure I really got them true to each other.

If you haven't used the Remflex gaskets yet I can't recommend them enough. Yes, you can get a good seal with standard stuff if the stars align for you. Remflex gets it right every time. They are expensive and a one time only use, but well worth it.

All the above advice is worth exactly what you paid for it, feel free to follow or ignore as you wish.

 

[Dartsun]

I'll second Halfafish suggestion on the remflex gaskets. I love them so far well worth the money.

 

[timk225]

I looked up those Remflex gaskets. It might be the right move but I'd have to wait for them to get here. I'll decide by tomorrow. My plan was to try again with the replacement steel gasket and a good layer of Permatex #2 non hardening sealer on each side. But I will keep those Remflex gaskets in mind for the next time I need them.

 

[69conv]

Remflex

 

[Slantsix64]

I also say remflex

 

[timk225]

I found the Remflex website and ordered a gasket for my Slant 6. But for now I'll use the steel gasket with high temperature RTV and carefully align and then tighten the manifolds together off the car.

I'm planning engine work or at least a good head porting / rebuild job later this year, at which point I'll use the Remflex gasket during that reassembly.

$23 for the gasket, but $17 for UPS shipping!

I used my Dremel grinder with a coarse tooth carbide cutter today to dig out the side of the adapter plate enough to get the inner bigger bolt in and tightened down. I'll clean up the steel gasket and manifolds and reassemble them with a heavy layer of RTV, and carefully tighten them together. I wasn't putting enough importance on the security of that joint. I had to clean the whole bottom of my newly painted intake to get all the black carbon off of it, it was leaking about 3/4 of the way around.

I saw that Remflex offers uncut rectangular pieces of their material. I half considered getting one of those and trimming it to size, but leave the center area solid, to keep some heat off the bottom of the intake. I think a thin piece of sheet metal might be better for that.

 

[slantsixdan]

Your problems here are:

• Not a good choice of gaskets, especially at the intake-exhaust junction. That stamped-metal gasket often won't seal worth a crap with used manifolds, and stacking two of them is a surefire recipe for major leakage here.

• Very improper manifold installation procedure. You can't just tighten whatever bolts in whatever order to whatever torque and expect things to go well.

Take the manifolds off — all the way off — and start over again. First, do what is necessary to ensure access to that inboard intake-exhaust stud/nut. You may have to drill, slot, or enlarge holes in the carb adapter plate. The nut needs to bear (via an appropriate flat washer) on the intake manifold surface, not on the adapter plate.

It is very much worth your while to get the good gaskets for the manifolds-to-head and intake-to-exhaust junctions.

Follow the install procedure and torque specs described here, except the Remflex gaskets call for less-than-factory torque on the manifold-to-head nuts, so follow that spec.

 

[timk225]

I did take it all apart and redo it. The factory steel gasket was clean in the corner near the 1 of 3 bolts that was actually installed, it was blowing carbon out all around the other 3/4 of the gasket.

I got my Dremel grinder and ground out a good notch in the carburetor adapter plate to clear the inner nut.

I cleaned everything, got a tube of the copper high-temp RTV, gave the gasket surfaces a good heavy layer, reinstalled the single new gasket, and carefully reassembled it with mild torque on all 3 nuts. I didn't have a torque wrench. The RTV said 24 hours to cure, so I left it in the trunk of my car for a day. I wanted to crank the bolts harder, but the larger inner bolt was rusted down to about 3/4 of its original thickness right at the exhaust manifold, and I didn't want to snap it off and get into drilling and tapping for a new bolt.

I reassembled it onto the engine today, and all the leaks were fixed. This Holley 350 is a new carburetor, it is strange to crank the engine for only a few seconds and have it start right up, no pedal pumps or anything.

I need to tune the Holley 350 now, I'll make a separate post for that.

I'll also have to get a shorter V-8 gas pedal cable, as my Slant 6 cable is way too long and is bent around pretty hard. I have to take off and slightly bend the lower kickdown rod and fabricate a new upper one to match the Holley linkage arm. I'm not using a Lokar cable, I don't think it is needed.