Poly 318 Intake and Exhaust Manifold Installation

(also applicable to LA 273, 318, 340, and 360 engines)

Intake Manifold Installation

As part of my larger engine-building series, I want to cover installing intake and exhaust manifolds. After installing intakes a couple times, the processes becomes simple, but for those with less experience there are many steps that often are not covered when someone asks online. I usually read two-sentence responses for instructions like, “Cover the ports with RTV. Install the intake.”

The first thing to sort out is what type of intake gasket to use on the A318. Most suppliers including local parts stores carry only the OEM-style sheet metal gaskets that can be difficult to seal on aging heads/intakes with mating surfaces not as true as they were originally. The problem compounds if the head/intake/block has been machined and has wider clearances. Leaks can lead to oil migrating from the valley into the intake runners, vacuum leaks, and coolant leaks. The solution to the leaky metal gaskets is to either order fiber gaskets made by BEST brand or make intake gaskets out of a sheet of gasket material. Another benefit here is that gasket sheets come in different thicknesses. I either plan ahead and order the BEST gaskets, or if I am in a bind make my own to avoid any issues with the metal gaskets.

Making Intake Gaskets

First, I clean the intake mating surface of old sealant/oil/debris. Working at the bench, I place the gasket sheet against the intake surface, poke through the end bolt hole, push through a bolt and hand-tighten a nut, and chase the gasket to the other side until I have bolts in all the holes to keep things aligned and to locate the bolt holes. I take a sharp X-Acto knife and cut out the intake runners, water jacket, exhaust crossover, and trim up the perimeter. I block off the exhaust crossovers with sheet metal plugs but always cut the gasket anyway since I suspect that the hot exhaust would eventually degrade the gasket material to where it could flake off. Once finished and the gasket is removed, I blow out/off the intake with compressed air to remove any gasket debris.

Intake Installation

  1. Cover the lifter valley with clean paper towels or rags to catch any debris. Lightly plug the head intake runners and exhaust/coolant ports with crumpled paper towels, but make sure you are careful not to leave a bunch of fibers to suck into the chambers.
  2. Using a sharp gasket scraper and/or razorblade, scrape off all residual sealant, paint, grease, etc. from the sealing surfaces. I keep a paper towel in one hand to clean off the blade regularly to keep less debris from falling into the engine.
  3. Carefully remove all the head rags followed by the valley rags being careful not to drop debris into the engine.
  4. Clean the intake manifold surfaces of all debris if you have not already.
  5. I take a single-cut file or a confirmed flat sanding block with 80 grit paper and face the thermostat surface on both the intake and housing to true it up since they often corrode.
  6. Blow off the intake well with compressed air.
  7. Before I apply sealant and gaskets, I saturate the corner of a rag with lacquer thinner (brake clean works too), wrap it around my finger, and wipe all the sealing surfaces and the corners where the head and block meet to remove any grease/oil. Sealant does not stick to oil, so I go this extra step for a very clean surface.
  8. Doublecheck the runners/ports and valley carefully for any debris.
  9. For sealant and placement, there is plenty of debate. Unless the gasket manufacturer specifies installing the gasket dry, which is not the case with my homemade gaskets, I seal around the water jackets, exhaust crossover port, in the corners where the head and block meet, and on the end rails. I do not seal around the intake runners. I use Permatex Ultra Grey RTV silicone 82194, although when I started my old man taught me to use 3M weatherstrip adhesive or Gasgacinch (I miss the pinup girl with dirt champ car on the can, thanks Edelbrock).
  10. Before final installation, I dry fit the intake so I can check the end-rail gap. To do this, I place a dab of Permatex High Tack 80062 on each end of the cylinder head (a dab of RTV will work too). I lay the gasket onto the head and lower the intake manifold onto the gasket. I measure the gap between the block and the intake at the front rail. If the gap is more than 1/8” with the intake not torqued down, I cut end-rail gaskets out of a sheet of cork using the intake manifold as a template. If the gap is less than 1/8”, I plan on using RTV. Lift off the intake manifold, lift off the gaskets, and wipe off any residual sealant from the temporary dabs.
  11. Working on one head at a time, I squeeze and dab a thin (maybe 1/16” thick) 1/4″-wide line of RTV around the water jacket and exhaust port on the head and use my nitrile-gloved index finger to even it out if necessary. I put on enough RTV to make a good seal but not enough to heavily ooze out into the jackets once torqued. Keep this line of sealant back about 1/16” from the edge when possible to allow room for the sealant to squeeze out without blocking the runners/ports.
  12. Squeeze a pea-sized glob of RTV into the corners where the head and block meet.
  13. Align the gasket at the bottom corners, push it into the glob of RTV, and gently swing it up onto the head surface. Align the gasket with the runners, and lightly press the gasket into the sealant on the head.
  14. Repeat on the other head.
  15. At this point, I would install the cork end-rail gasket if my dry fit found I needed it. I coat the block rail with RTV making sure to squeeze into the corners overlapping the side gaskets and then gently press the cork into the RTV. Leave it to setup and continue onto the next step.
  16. Using the same process for the side gaskets, I place a bead of RTV around the water/exhaust jackets/ports on the intake manifold. This is when I install the exhaust crossover block-off plates I cut out of 20-gauge sheet metal using dabs of RTV to hold them in place on the intake.
  17. If not using cork end-rail gaskets, I lay down a 1/4″-diameter bead of RTV on the rear and front end rails of the block making sure to press well into each of the corners overlapping the intake gasket. If I already installed the cork gasket, I now run a thin 1/16” thick bead of RTV over the top of the cork making sure to press into the corners. Be aware that the A318 end-rail foam/rubber gaskets that come in the kits will usually slip out of place during torquing and/or leak, so do not use them—hench all the work using RTV or cork.
  18. Position the intake directly over the valley and slowly lower it into place aligning it as best as possible before making contact with the gaskets. As with many things, I am sure having two people would be helpful at this step with an iron intake, but it is doable solo.
  19. To clear up confusion, no intake bolts go through to coolant on an A-block head. Some are dead bosses, and others go through to the intake valley where there is potential of oil seeping up the threads and pooling on the intake. It is easy enough to seal them, so I always do. However, a lot of people I see online use RTV or Teflon tape, which I hate for these bolts. Cured RTV is an irritating pain to clean off threads and out of threaded bosses, and removing the bolt and reinstalling almost always drops crumbs of cured RTV into the engine that are practically impossible to clean out. Teflon tape shreds all over the place and can fall into the engine too when removing. I use RectorSeal T Plus 2, which is a PTFE-enriched paste sealant that I also use on the exhaust bolts that go through into the water jacket. It is designed for water and steam pipefitting, works well at high temps against oil and water seepage, and stays pliable for future removal/installation.
  20. I start all the bolts by hand and tighten evenly in three different passes using the sequence in Figure 1. I tighten by hand to where I feel the gasket seat, but the factory iron spec is 30 ft. lbs. for anyone who wants to attempt (and likely fail) to get a torque wrench to fit around all the bolts.
  21. Some people wipe the silicone that pushes out from the front rail, but I was taught to leave it alone and let it form a “C” around the ridge to help lock it in place. If I use a 1/4″-diamter bead, it is clean with no excessive amount flooding out.
  22. I let intake RTV cure for 24 hours in weather above 60°F and 48 hours below before filling the coolant and firing the engine, especially with RTV end rails. Silicone activates via oxygen, so leaving the valve covers off or ajar and the water jackets empty will help speed up the process.
  23. Now is a good time to adjust valves if needed since the covers are off and I have to wait for the RTV to cure (see my article on valve adjustment). Otherwise, I would install the carburetor, accessories, and whatever else I had to remove. Connect any sensors/fittings/hoses that go into water jackets, but leave one hose disconnected from the intake or timing cover to use later.
  24. Wipe on a coat of RTV around the thermostat opening on the intake, center the thermostat, spring-down, and gently press on the gasket.
  25. Wipe a coat of RTV on the thermostat housing surface, and set the thermostat housing onto the manifold. I use a small dab of anti-seize compound on the thermostat bolts in case coolant ever weeps into the bosses. If using stainless-steel bolts in an aluminum intake, anti-seize is imperative to block galvanic corrosion. Hand tighten with a snub or 6” 3/8 ratchet until I feel the gasket seat—likely no more than 20 in. lbs.
  26. Install the upper radiator hose.
  27. After the 24 – 48-hour cure, I fill the radiator until I see coolant appear near the top of the heater hose fitting I left open in the intake/timing cover. Install the heater hose or plug and fill the rest of the radiator. Using this method removes most air pockets from the engine.
  28. Install the valve covers and any other parts required to fire the engine.
  29. After a few heat cycles driving the vehicle, retorque the intake manifold bolts since they will likely be a little loose after the gaskets are fully seated.

Mopar A-block poly intake tightening sequence
Figure 1: Intake Tightening Sequence (editor J. Lotspeich)

Exhaust Manifold Installation

Note on Permatex Ultra Copper 81878 RTV: In some applications that may leak, using a thin 1/16” coat of Ultra Copper RTV is acceptable. In some situations, particularly on a race engine when I expect to remove the headers multiple times, I have used Ultra Copper RTV in place of the gasket without sealing issues. Most situations should not require RTV, however.

  1. Lightly plug the exhaust runners with crumpled paper towels, but make sure not to leave a bunch of fibers to suck into the chamber via backpressure.
  2. Using a sharp gasket scraper and/or razorblade, scrape off all residual gasket, paint, debris from the sealing surface.
  3. If using Ultra Copper RTV, wipe off the sealing surface with the corner of a rag saturated in lacquer thinner or brake clean.
  4. Remove all the rags being careful not to drop debris into the engine. Verify there is no debris in the runners.
  5. Clean the exhaust manifold/header mating surface in the same way.
  6. Blow off/out the exhaust manifold/header with compressed air to flush out any debris.
  7. The lower row of threaded bosses goes through to coolant, while the top row are blind holes. For the same reasons I explain regarding intake bolts, I discourage the use of RTV on the exhaust bolts. I use RectorSeal T Plus 2, which is a PTFE-enriched paste sealant designed for water and steam pipefitting, works well at high temps, and stays pliable for future removal/installation. Coat the stud or bolt threads. If using studs, install them now until they seat tightly. 
  8. For studs, slide the gaskets onto the studs, slide on the manifold/header, and start the nuts.
  9. For bolts, slide one bolt through the manifold/header on each end, slide the gasket over the protruding threads, lift the manifold/header into place, and start the two bolts but do not tighten. Using a finger to jostle the gasket up/down/sideways, start the other bolts.
  10. Run down all the bolts/nuts and tighten using a three-step sequence of Figure 2 until the gasket seats. Do not overtighten since doing so can crack/break off the cast-iron ears.
  11. If Ultra Copper RTV is used, allow 24 hours (longer in temperatures below 60°F) for cure before starting the engine.
  12. After a few heat cycles of driving, retorque to bolts since they will likely be loose once the gaskets fully seat.

Mopar A-block poly exhaust tightening sequence
Figure 2: Exhaust Tightening Sequence (editor J. Lotspeich)