Note: Allpar does not take responsibility for the veracity of any information or opinions here, does not claim expertise, and is not responsible for any consequences. Please proceed at your own risk.
Many of the articles geared toward Bob’s engine will involve porting. It only makes sense to offer a basic head (and manifold) porting article so that when you work on duplicating these articles with your engine, you have a working knowledge and the skills for what you’re trying to accomplish. No matter how good of a porting idea you learn or come up with on your own, without the basic skills of controlling the proper equipment, you will never be able to do good work.
Unless you are content spending the next few years massaging your ports and combustion chambers with varying grades of sand paper and your fingers, you’ll need some serious equipment to remove material quite a bit faster. Equipment falls into three different categories: motivational force, abrasives and cutters, and measuring equipment.
This would be the grinder itself. There are electric and pneumatic (air) powered grinders. Of the electric options, the cheapest but least recommended is the Dremmel Tool. The reason it is not really recommended is that you can only use the 1/8” shank bits with it. The small bits don’t offer very good control, usually give very irregular surface finishes, and become too easy to take too much material out of one spot. In other words, the Dremmel makes it easy to turn a perfectly good head into junk. If you are trying to remove a bunch of material, the small bits make it difficult to do so uniformly.
A really good all around choice is Eastwood’s Electric Grinder which comes with a flexible shaft in either bench top or hanging model. This is what I personally use in the shop. It has a foot operated speed control that frees up your hands to control your work better. The collet accepts the standard ¼” or Dremmel style 1/8” bits. The hand piece is light weight so you don’t tire your wrists out in the first hour of grinding. (www.Eastwood.com Bench model PN 13305, hanging model PN 13328)
There are a wide range of other electric die grinders available, some with side handles, various weights, some fixed speed, others variable speed. Prices also range from $10 at a yard sale to several hundred for the Mac-Daddy industrial version.
Pneumatic die grinders require a healthy air compressor to operate. This must be factored into your decision. While a basic air grinder starts at $9.95, the compressor to drive it starts at about $500. If you already have a large enough compressor to run a grinder, then this would be an economical way to get started. I used an air grinder for many years (actually went through several over the years as they wore out) before switching to Eastwood’s electric version.
If you are looking to do a considerable amount of porting, familiarize yourself with what’s available. Talk to other porters and find out what they use and why. Then make a purchasing decision based on what seems to suite you best.
Allpar note: be careful what you remove in there; sometimes, there are intentional nubs and bumps that serve a purpose. Doing some basic research before starting is a good precaution.
The way to remove material is with stones, carbide bits, and sanding rolls. The stones are the cheap way to go, but you get what you pay for. I hate stones because they don’t last very long, and they change shape as you use them. Years ago I gouged a port trying to remove a nub. The stone changed shape while I was grinding that nub and took a bunch of material off behind the nub that I didn’t see until it was too late.
Carbide bits are the way to go, hands down. They have the capability to remove a lot of material in a short period of time. To the novice, this can be a detriment as a head can be ruined very quickly. Instead of trying to talk you into spending 10 times the amount of time with the sanding rolls which take much longer, I’m going to try to talk you into practicing on a junk head so you have the skills to maximize the benefits of the carbide bits. Carbides come in different shapes and sizes. Bits for iron have a tight pattern to the cutting faces, while bits for aluminum have widely spaced cutting surfaces. Don’t use the wrong one.
Some bits are short stem, others have long stems. The short stems offer better control for gasket matching and combustion chambers, while the long stems offer reach deep into the ports. I personally find it difficult to use the long stems in easy-to-reach areas because of chatter. I have gone so far as to take a couple of long stem bits and cut about 1” off the end of the stem to shorten them a little.
The sanding rolls can be used on either iron or aluminum. They require a mandrel. The mandrel fits in the chuck of your grinder and has a spiraled end to screw into the center of the sanding roll. The tip extends a ways past the screw threads to offer additional support to the roll. Sanding rolls come in barrel and taper shapes, and grits ranging from 40 to 320. You will get the maximum control of your final shape and finish with sanding rolls, but the material comes off much slower than stones or carbides.
Just going in and removing material by eye will probably offer some improvements over stock, but will be inconsistent from cylinder to cylinder, and will be less than optimal all ways around. You need to know how much material to remove and where, then you need to be able to reproduce that size and shape equally in all of the ports. Furthermore, you’ll need to know the volume of your combustion chambers when you’re done grinding. In other words, you need to be able to measure size and volume.
For size, inside and outside calipers will give you the basic information you need to do a nice job that will perform for you. Outside micrometers are used for measuring valve diameters, while inside gauges tell you width and height of the ports. There are formulas and ratios that you use to plug in this information and know how much material to remove from where.
For volume, a CCing burette is the way to go. It is a tall slender cylinder made of glass or plastic with numbers and lines on the side. It will tell you the volume of your combustion chamber so you can figure out compression ratios. It can be used to get final port volumes as well. Mine has a zero at the top (full) and the numbers go down as the water level goes down. Fill your cylinder and just look at how much water it required. That’s your combustion chamber volume.
Try to find a junk head to practice on before grinding away at your end use head. Practice will give you the skills to control your chosen equipment well enough to achieve a powerful port job. You will have 3 or 4 ports to try and get it right. When you’re satisfied, toss the junk head in the scrap metal pile and begin carefully massaging your prized piece. I will be skipping a few steps that will be further explained in forthcoming articles. For now we’ll just work on the basics.
Don’t forget safety. ALWAYS WEAR EYE PROTECTION! You won’t be able to enjoy your work if you can’t see to drive. Protect your hands and other body parts, and don’t wear loose clothing that could get caught up in the equipment.
Use a gasket and mark the ports. On used heads, there is usually carbon between the gasket outline and the actual port. Check this carbon marking against the gasket you intend to use. Gaskets from different manufacturers will vary in actual size. If the old carbon marks match your new gasket, skip the next step. If your marks don’t match, or you don’t have old marks, then you need to make them so you have a reference for your new size.
I start out by cleaning off the old gasket and carbon with a scuff pad and a drill. This leaves a clean surface. Next mark the area around the port with machinists bluing or magic marker. Go past where the gasket ports end. Next lay the gasket on the head (manifold), line up all the bolt holes, then scribe the gasket size into the bluing/ink. Keep in mind that your new marks will be slightly smaller than the actual size of the gasket.
You’ll want the port size on the intakes a little larger than the manifold ports. This eliminates the collision of the incoming air charge against the mis-matched ports. On the exhaust side, you want the manifold to be a little larger than the ports in the head for the same reason. I shoot for about 1mm all the way around the ports.
Using a short stem carbide bit, work the port opening out to the marks (+/- 1mm depending). After you have established the size of the ports, use your marker and put a ring around the port. This lets you know if you are getting carried away as you move inward. Gradually blend this new size back into the throat of the port. It is acceptable to go from a larger gasket size and funnel down to a slightly smaller choke point. The choke point for most ports is the area right at the turn from port to bowl at the short side radius (SSR).
In the bowl, there will be sharp edges under the seat. These will have to go. Be extra careful not to touch the valve seats with your bits. A slight abrasion mark can most likely be cleaned up with the valve job, but if it is very deep, you’ll need a new seat installed. You want a smooth transition from the valve opening into the rest of the port. If you look around the guides, there will probably be lots of material that impedes flow. Smooth and radius this material around the guides to allow for smooth flow in the direction of travel. In other words, the intake guides should be flowed from port to combustion chamber, and the exhaust guides should flow well from combustion chamber to port.
Practicing these basic steps on one head should offer you the control that you will be able to duplicate the more advanced shapes that will be explained in upcoming articles. Bob’s engine is going to not only scream, but get good fuel economy as well. For you to do it too, you need to practice with the grinder to gain the skills to properly execute the tricks that you’ll learn later.
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