Casting Call

How to restore a cracked cast-aluminum intake manifold with a finish that looks like it came from the foundry.

By Jeff and Teresa Lilly

Photos by Teresa Lilly

This article is originally from the Aug. 1993 Classic Auto Restorer magazine

Picture, if you will, a scene: You’re walking alone through what appears to be an old salvage yard. Rusted iron motor parts are scattered everywhere. You seem to be drawn by some force toward a fenced area of the yard, where a sign on the gate reads NO B.O.S. (Bogus Old Stock). Trembling with excitement, you push open the gate and are blinded by the light illuminating a perfectly restored Corvette intake, perched on what appear to be storm drains and bolted atop a mammoth hunk of reciprocating engine displacing 427 cubic inches and producing 435 horsepower, bone stock.

You have entered the Casting Zone.

Here you will learn the secret ritual of restoring a cracked and welded aluminum Tri-Power manifold to the appearance of an original casting—magic that can also be practiced on similar manifolds and other cast aluminum parts once its mystery has been revealed.

1 The first step with this 1968 Corvette 427 Tri-Power intake, or any manifold, is to check for warping with a straightedge and feeler gauge. A gap of more than .008-inch means the gasket may not seal properly, so the intake should be milled at a good machine shop.

2 This intake had a crack in one of the bolt flanges, which was repaired with a MIG welder using aluminum wire. The finished weld is thorough but ugly, so it’s necessary to do some cosmetic reconstruction. (If you repair it yourself, use a welder that has infinite heat control and a timer, such as Century’s 11090. This will enable even a beginner to make deep, tight welds. Otherwise take the job to a competent shop and make sure they get penetration through the full thickness of the metal.)

3 Tools you will need include a variable-speed electric drill; an eye shield; round and cone-shaped burrs; a knotted-brush attachment; 80-grit sandpaper; ScotchBrite 7447 scuff pads; a tap and die set; a tub big enough to hold the intake; muriatic acid; Ditzler metal prep acid; a spray bottle; an air-impact hammer with needle scaler attachment; and some baking soda.

4 Set the drill at about 600 rpm and use the round burr tool to cut down the metal of the weld. Aluminum cuts down quickly at this speed, so be careful and keep the burr moving slowly back and forth, to avoid making ant grooves in the metal.

5 Finish with the cone-shaped burr tool, smoothing the edges and blending in the welded area as well as you can.

6 If the weld has penetrated effectively it may have left dimples like this one inside the port. File them down.

7 Mount the knotted brush on your drill and go over the work lightly at 1500 rpm to clean up the aluminum. Blend any marks left by the burr tools and smooth any ridges left by the cutting action.

8 Use 80-grit sandpaper to smooth out the wire brush marks. The intake should now be shaped correctly, with some scratches left by the sandpaper.

9 Tap out all the existing holes in the intake and make sure all the threads are clean. If not, repair them with a Heli-coil. Or if you’re a perfectionist, weld the hole solid and then drill and tap a hole the correct size.

10 Put the intake into the large tub. If the water and vacuum nipples are good, pour muriatic acid over them to remove rust quickly. If the nipples are bad, replace them.

11 Mix the metal conditioner (one part to two parts water), pour it into the spray bottle and squirt it onto the intake. After it’s been in the intake for 10 minutes, take a ScotchBrite pad and agitate the acid with a circular scrubbing motion. Then spray the intake again and let it sit for about 15 minutes before rinsing off.

12 Remove the needles from your needle scaler and grind them individually almost to a point. (Note that this profile is suitable for duplicating the casting texture common to all GM Tri-Power intakes. The profile of the needle scaler tips may vary depending on the original texture found on different make vehicles.) The flat needle on the left has not been ground, while the one on the right has. Note how the ground needle is rounded, not sharp. Once you have the profile right, reassemble the needle scaler and place the intake in your workbench.

At this point, STOP. It is very important to turn the intake upside-down so that you can practice making imitation casting marks at a spot on the intake where they will not be seen. Only after you have practiced with various hand movements and air pressures should you move on to the top surface. When you’re satisfied with the texture you are achieving, turn the intake back over to expose the area to be restored.

13 Use the needle scaler at 25-35 psi, moving it around like Woody Woodpecker with 10 beaks. Don’t press hard; let the air hammer do the work.

This is how your imitation casting marks should look.

14 Now return the intake to the tub and blend the restored area with the original manifold surface by pouring full-strength muriatic acid over the needle-scaled section. After 15-20 seconds it will bubble. The acid eats the aluminum rapidly, smoothing out the marks you made with the needle scaler and blending them with the original casting marks. Rinse off the acid and scuff the area again with a ScotchBrite pad. If it does not match the original texture, use acid again to blend it more.

15 Because muriatic acid is strong, it will keep eating away at the surface unless it is neutralized. Put the intake into a tun full of water (about four gallons) and add about a quarter of a large box of baking soda. It will bubble for five to 10 minutes as the acid is being neutralized. Halfway through, turn the intake over so that both sides are equally exposed. Once the bubbling has subsided you can empty the tub, then refill it with clean water and rinse the intake

16 Eagle One mag wheel cleaner will finish off the intake by giving it the correct finish as it came from the factory. Spray an even pattern over the whole intake and let it stand for five minutes at room temperature. The white boiling action of the wheel cleaner’s phosphoric acid will deep-clean the surface. Note that it is not necessary to neutralize this acid. All you need to do is rinse it in clear, cool water and blow the intake dry with compressed air.

17 Spray the intake thoroughly with lacquer thinner, inside and out. This will remove any remnants of contamination and give it a consistent finish. Again, blow it dry.

A painting note for purists: The Tri-Power intake was bolted to the motor at the factory, then a mask was dropped over the intake to cover all but its outside edges before the motor was sprayed Chevy orange. When the mask was removed, the intake was left paint-free except for a strip around the edge. The intake can also be painted this way off the motor, if you touch up the bolt heads after it’s attached.

18 To duplicate the original paint pattern, fabricate a tent-shaped cardboard mask that covers the entire intake except for the edges, then apply Chevy orange paint (acrylic enamel or urethane with hardener) to the edges. When the mask is removed after spraying, your intake is cosmetically as good as new, and it can leave the Casting Zone.