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Flow Boxes

By Ken Weber

Over the years, I have had occasion to port cylinder heads when I had no prior experience with that particular casting. The results have not always been as good as I would have liked them to be, but with only one head or one pair of heads available to work with, the development opportunities were limited.

I have thought many times that it would be nice to have a junk head to work with before starting on the real part. Flow boxes are widely used in Detroit, but the task of duplicating a particular port and combustion chamber accurately enough to make it useful would be prohibitively time consuming. What is needed is a quick, inexpensive way to duplicate any existing port accurately enough to be useful in flow bench testing, can be machined and altered like metal, and not cost more than can be charged for the whole job.

In his book, Practical Gas Flow, available from Classic Motorbooks, John Dalton describes making a flow box out of plaster. He pours his mold in two pieces so he can separate the two halves and check the shape of the port as he goes. "At last," I sighed in relief, "the answer to my problem."

I visited the local Hobby Lobby and bought some Plaster of Paris. Then I went to Home Depot and bought a sheet of 1/8˝ plexiglass, and started making a flow box. Since I was already making rubber molds of ports, the rest was duck soup. Or so it seemed.

I made a one-piece rubber mold of an intake port and combustion chamber of my project head, a big block Chevy from Canfield. When making the mold, I stuck a valve upside down in the guide, with the tip about even with the valve seat. Later, when the flow box is cast, this will be used to support a spare valve guide that will be cast into the flow box.

I cut up the 1/8˝ plexiglass to make a rectangular box, duct taped the pieces together, and attached the mold of the port and chamber in place with sheet metal screws. The key elements here are to make the base of the box big enough to cover the hole in the head adapter on the flow bench, and since the rubber mold is somewhat flexible, to accurately locate the mold in the box.

The Plaster of Paris was mixed per instructions, poured in the mold and allowed to set up. After the plaster set up, the sides of the box were removed and the rubber mold was pushed out, leaving the plaster flow box with a valve guide firmly in place. Cool!

First problem. The plaster was still damp, and required baking in the oven to dry it out. After the plaster was completely dry, the second problem surfaced. This thing is like a piece of compacted talcum powder. Every time I touched it, I had plaster powder all over me, the bench, in the air, everywhere, and it’s so soft that there would be no chance of drilling any decent holes in it, or cutting a valve seat. Not at all what I was looking for. It became apparent that a different plaster was needed, more like that used for medical casts.

In the search for a better plaster, I was looking at a little plastic paperweight on my desk that had a small globe of the planet imbedded in it. I envisioned a clear plastic flow box where the exterior of the port would be visible as well as the interior, and would be machinable. I remembered seeing some plastic casting resin at Hobby Lobby where I bought the plaster. I bought all they had, three pints, with the hardener, and proceeded to recast the flow box.

Next problem. This particular casting resin was not meant for casting objects of this size, and the heat generated by the curing process ruined the mold. The 1/8˝ plexiglass was just not heavy enough to do the job, and the petroleum jelly I used for a mold release didn’t work. So I had to chisel the remains of the casting box off of the flow box. The result was a flow box that was severely warped – useless.

A visit to a local plastics outlet garnered the correct casting resin and some advice on its use. On the next try, I made the flow box out of one-inch oak, held together with screws. This worked better, but the surfaces of the flow box had wood grain imbedded in them. I poured it in two layers, which caused the heat from the second layer to warp the first layer, and the heat also warped the oak. Back to the plastic shop for more info, resin and a remnant of one-inch thick plexiglass for the next casting box.

Here’s How You Do It
You need Mass Clear Casting Resin ($37.25/gal), catalyst ($2.65); a mold release, such as Partall carnuba paste wax ($4.75); wood, plastic, aluminum, steel, or other material to make the casting box; Blu-Sil type G (approximately $125/gal) for making the rubber port and chamber mold (use petroleum jelly for the mold release); and a valve guide like the one in the head you are working on.

Next, make a mold of the port and the combustion chamber with the Blu-Sil. Install a valve or valve stem in the head as described above. Yes it is expensive, but you can make a lot of molds with a gallon of the stuff.

It helps if you rig a piece of wood to take up some of the volume of the port before you pour the rubber. This uses less rubber, and after you remove the wood, it makes it easier to get the mold out of the port. Be sure to grease the port, valve stem, and wood with petroleum jelly before you pour the rubber. Pour it slowly to avoid trapping air.

Then, fabricate and assemble the casting box out of your chosen material. Keep in mind that you have to put the mold of the port and chamber in it, and when it’s done, it has to fit on your flow bench head adapter. Also consider the volume of resin it will hold. More resin equals more heat during the cure, and costs more. The one used for this article holds two quarts of resin, which costs $20 to $25, not counting the port mold. The casting box can be reused indefinitely, so its cost is negligible.

After assembling the casting box, apply several coats of parting wax to the box and the port mold, then put the port/chamber mold in the box and secure it with some sheet metal screws through the side and bottom. Be sure to locate the chamber so you can get it properly positioned over the head adapter on the flow bench. Locate the end of the port at the right height above the floor and at the proper relationship to the wall and chamber.

Now you’re ready to mix the resin. In a two-quart pour like this, I found that 5cc of catalyst makes a cure time of about 8 hours at 70 degrees. The smaller the casting, the more catalyst you should use. A one-quart mold would probably require around 10cc of catalyst under the same conditions. This will vary with the temperature of the room and the thickness of the mold. A quart of resin spread over a cookie tray will require more catalyst and generate less heat than a quart in a cube.

After the casting box is completely assembled, fill it with water to check for leaks. If it’s okay, pour the water into a measured container to see how much resin you need. Measure the quantity of resin and catalyst into separate containers. Most paint stores sell one and two quart plastic paint buckets that have graduations on them, and the supplier of the resin should have little measuring cups that are graduated by cubic centimeter.

Add the catalyst to the resin and stir for about a minute. If you look closely, you can see when the catalyst has completely blended into the resin. Pour the resin into the mold. Keep the resin close to the mold as you pour, to keep from inducing air into it. Use a disposable funnel if necessary.

After it’s cured, the resin will probably have pulled slightly away from the sides of the box. Disassemble the box and remove the valve and rubber port mold from the flow box. If you want to have a glass-like clear piece, you can wet-sand it with 400 to 600 grit sandpaper. Then buff it with a buffing wheel and some polishing rouge ($5 for the "red brick" of rouge).

And there you have it. It really helps to be able to see the shape of the port from the outside. It’s much easier to see the radius of the short turn and other relationships as you change them. You can take a light surface cut off of the deck to make sure it seals against the head adapter on the flow bench. I cut the valve seat on a Serdi or other similar quality equipment (I haven’t tried grinding the seats). You can drill and tap holes to mount the inlet radius and a valve opening device, and you can port and polish just like the real head. If you polished the inside of the port, you could even do smoke tests on the flow bench and see what’s happening right through the "head."

If you don’t like the results of your modification, just mix a small amount of resin in one of the little cups (I use two parts resin to one part catalyst) and using a small disposable acid brush, paint some resin back on the area you wish to change.

Want different valve seat angles? No problem, just paint some resin around the old seat and recut it with a different cutter. For large additions, I take some of the leftover resin that is cured and using a sanding roll on my porting tool, make a little pile of acrylic dust to mix with fresh resin as a thickening agent. For really big changes, you can dam up the port, mix a batch of resin, and pour it in. Plasticare has clay available for this purpose, but I haven’t used it yet.

Once you get your test port working the way you want it, you can make patterns to use in porting the real piece, or possibly digitize the port and CNC port the real head. And of course, when you’re done, you have a nice acrylic paper weight for your desk, or a physical record of what you did after the real heads are in the hands of your customer.

A word of caution. The polyester resin you are using is basically the same stuff fiberglass parts are made of. Be sure to work in a well-ventilated area, wear rubber gloves, and follow all precautions on the product labels.

When you are porting, wear a particle mask and be careful not to let the cutter get out of control. It’s best to use an old worn out carbide porting tool because the resin fractures with impact, and you could break off a piece you didn’t want to remove. If that does happen, you can glue it back on with some 5-minute epoxy.

Sources:
Mass Clear Casting Resin, catalyst, brick of buffing rouge,
and Partall parting wax:
Plasticare, Inc.
4211 S. Natches Court – Unit K
Englewood, CO 80110
(303) 781-1171.

The resin ships UPS as hazardous material and carries a $20 HAZMAT fee, a $4.25 to $7 package charge (depends on the quantity), plus the regular UPS charge. You would save some cost if you can find a similar product locally, but if not, the people at Plasticare are very helpful.

Blu-Sil molding rubber:
Perma-Flex Mold Co.
1919 East Livingston Ave.
Columbus, OH 43209.
614-252-8035.

The remaining materials are local hardware items.

Ken Weber, who formerly operated a marine engine rebuilding business is an independent technical writer in Denver, CO, covering the high performance engine market.

kweber@engine-builder.com