This flathead porting exercise is the hands-on work from a full-length magazine how-to I've been working on. It's not the entire story but it covers the essential steps for prepping a flathead block for a stout street motor. All I'm omitting here are aspects we've already covered at one time or another.
We've covered "good-flatmotor/bad-flatmotor" in exhausting detail on the HAMB so we needn't go there again to learn what to look for and how to qualify a block. And we know why we're doing the work, this old-timey precursor to blueprinting that was called "porting and relieving."

And, we're not going to get into high-po racemotor porting because:
1. Even the giants are still in major disagreement about what works and what doesn't.
2. Ryan would round-file the project long before it swelled to 10 pages, which is not quite halfway home . . . and still there'd be no agreement possible.
3. I'm not the guy to guide you into super-trickery because I'm still a struggling student of this interesting but rather arcane art.

What I will do is take you through the steps I follow for a street/performance "port and relief" job. It takes care of most of the "blueprinting" that can be done to a flatmotor block outside the machine shop.


When stock Ford camshafts are reground to alter valve timing and increase valve opening durations, the height of the lobes and the diameter of the base circle may be reduced by something approaching 1/4 inch. The adjuster bolt in an adjustable lifter, takes up the resulting slack when it's time to open the valve, but the lifter body sits lower in its bore in the block. During valve adjustment, when the lifter is sitting on the heel of the cam, the lifter is at its lowest position, and in this position the top of the adjuster bolt is often flush with the top of the lifter bore, making it difficult to fit a wrench to the adjuster bolt.

The solution is to cut a 1/4-inch relief into the top of each lifter bore to expose the adjuster bolt head enough so it can be reached with a wrench to adjust valve clearance. Then, a 1/8-inch hole is drilled into each lifter bore, just above the camshaft oil gallery tube, so that a length of steel rod can be inserted to prevent the lifter from turning when the adjuster bolt is turned.

This job isn't essential. In fact, thousands of hi-po flatheads have been built and run and raced and serviced and tuned without their lifter bores being modified as described. Most folks who have struggled with Johnson tappet "wrenches" and carved and ground-down custom nut turners for working inside the lifter valley will recognize this approach as a pretty good idea, however.

A long (10-12 inches) centerpunch and an equally long 1/8-inch drill are well worth the price if you are doing only one block. Think about getting your paws down inside the lifter valley to hold a regular centerpunch and then striking it with a hammer with enough force to create a real dimple to guide a drill. The long drill needs no explanation.

A 1/8-inch hole is drilled into each lifter bore, just above the camshaft oil gallery tube.

To cut a 1/4-inch relief into the top of each lifter bore, use a cylinder burr with a radiused end and a short (2-1/2-inch) shank so you can bear down a little.

Then, use a ball-shaped stone in an air die grinder to radius the relief in the lifter bore, both inside and out, and . . .

. . . you have lifter bores that take all the hassle out of valve adjustment.

Let the porting begin.

I've shown you the tools and the end result and now it's your turn to get your hands dirty!
Use a short-shank burr, push it all the way into the collet and cinch it down like you plan to leave it there forever. Guide the nose of the grinder with your offhand and start cutting from one end of the relief to the other, increasing downward pressure as you approach the center and then ease off as you move to the other end of the relief, creating a crescent shape. A half-dozen passes should do the trick.

And remember: We get to do "do-overs" and little bits of extra fussing with these small pieces of work, to make them pleasing as well as functional.

Stay tuned for the second installment which, like the first one, cleverly puts off the actual traditional port-and-relief how-to work from the past as we cover a super-important mod that the old guys usually overlooked in their narratives.

Episode Two . . .

Before we launch into the stuff everyone's expecting we have to cover some ground that few early how-to authors covered . . .


For a complete flathead porting job, the center outlets for the siamesed ports should be enlarged and the outlets for the end ports enlarged and recontoured. Enlarging the center ports doesn't involve much more than enlarging the opening 1/16-1/8 inch all around and blending the machined area that was milled into the block when it was manufactured. The tall rectangular passage between the center cylinders is actually rather generous because while the inside exhaust ports share it, they don't share it at the same time.

Exhaust gases flowing out of the end ports, however, run smack into a flat wall in each corner of the block before turning 90 degrees, first one way and then another, to get out of the block and into the exhaust manifold or header. The path in the block can be greatly improved by shaping the passage at the outlet into a gentle curve back up into the exhaust tract. This is done by grinding away the abrupt shoulder inside the outlet, closest to the corner of the block, and shaping it into a smooth, progressive outlet port leading to the header tube. Here, too, the outlet should be opened up 1/16-1/8 inch all around.

Unlike the intake ports and manifold, where the runners are matched from one piece to the other, it's preferable to have the outlet from the block a little smaller than the inlet to the exhaust manifold or header. This "mismatch" creates an anti-reversion dam that inhibits the exhaust gas from trying to sneak back into the cylinder when the intake valve begins to open as the exhaust valve is just closing.

Carbide steel burrs work better than stones for the initial work in the exhaust outlets because they remove a lot of material quickly. Also, the work in this area doesn't require the finessing that the intake ports and bowls do. For the initial cut, to grind away the shoulder inside the port, my favorite is a short-shank 1/2-inch burr with a rounded end.

It takes only a few minutes to get the big obstruction out of the way, and then change to an oval- or flame-shape burr with a 4-inch shank to blend the outlet back up into the exhaust tract. Because of its shape, this burr will cut behind the maximum diameter of the cutting surface as well as in front of it, allow you to blend the outlet in both directions. It's like being able to get inside the port and work back toward the opening.

Then, refine the surfaces of the outlets with a stone dressed into an oval or flame-shape, and finally put some shine on the outlest with either 100-grit cartridge rolls or 100-grit cloth in a flapper stick.

Here's your objective -- this ledge inside the end exhaust outlets is a big impediment to exhaust gas flow.

Here are your tools -- these two burrs and the dressed stone will let you turn an obstructed end outlet into a good-flowing tract in about a quarter hour. BTW, long burrs like this one are the poster children of "DON'T FREE-SPIN THIS TOOL!" Have the burr inside the outlet, in light contact with the surface, before punching the button.

A few minutes work will knock down the ledge, blend it with the inside of the passage, open up the exhaust outlet.

Finesse the surface with the stone, blending it inside to outside.

Then, change to sanding rolls or sanding cloth in a flapper stick for a bit of smoothing in the outlets aids flow and reduces carbon buildup.

The center exhaust outlets have a little milled ridge that's removed . . . .


. . . when the outlet is opened up. This one gets some extra smoothing as well.

Finally, for this installment, here's something that I should have covered in the beginning -- your health.

None of the work described here should be done without the protection of a respirator -- not even once, and if this is going to be your very onliest flatmotor port and relieve project ever. This one job won't irreparably harm you, probably not, but you're going to have your snout closer to grinding dust and particles than probably any task you've done before. And while your body will eventually slough off the residue, you're miles ahead by not letting into you lungs in the first place.

A quality respirator like this will set you back less than thirty bucks. Its value is incalcuable in terms of your health.

This is it, boys and girls -- intake porting and port matching and top-end exhaust port work, plus a little work in the bowls beneath the valves.
In addition to an electric and an air-powered die grinder with a 1/4-inch collet, some quality ruby aluminum-oxide stones and a dressing stick, you'll need that respirator and eye protection, plus layout dye, a sharp scribe, and a quality intake gasket, like a FELPRO -- either new or one that has been salvaged intact from a recently running motor, or better yet from the motor you are working on.

You can put your carbide burrs back in their tubes unless you have the experience and confidence using them to safely rough-cut some of the work at the top of the bowls. An aggressive burr can get away from you before you realize it and cause real mischief. Personally, I use my fatigue level as a guide for using burrs. After 4-5 hours of steady porting and block work I'm inclined to do tasks that require stones and leave the burrs parked until the next day when I'm rested. There aren't many do-overs in this work that don't involve some expensive repairs. If you're uncertain, stay with the stones for the work up top.

This is probably a good time to point out that this isn't a six-pack job -- unless it's a six-pack of caffeinated soda.

An accurate gasket makes a handy template for scribing the outline of the ports on the manifold deck and on the underside of the intake manifold so that when both are opened up to the scribe lines there will be a continuous flow path from manifold to block, with no sharp "steps" to inhibit flow. Just make certain the gasket is for standard ports and not for large, high-performance ports.

The initial work opens the port up to the scribe line, removes any knobs, warts, and casting or rust texture, and enlarges it ever so slightly at the roof where it enters the bowl -- on the short radius. Not only is there nothing to be gained by enlarging the bottom of the port -- the long radius -- but flowbench tests have shown that this actually degrades flow in a Ford/Mercury flathead.

One of the most productive things to do to improve intake port flow is blending the roof of the port, the short radius, where it enters the bowl. Reach into an unmodified intake port from the bowl and you can feel the abrupt edge, at the bottom of the valve seat, that's a definite flow inhibitor. Knock this down a little with the 1/2-inch stone and blend it into a smooth curve from the intake port to the valve seat and you've done a significant wakeup of intake flow.

So, let's see what all this work looks like .

These runners are typical of what you're likely to find. The one on the right has normal tool marks, while the one on the left shows signs of the block having been parked with water in it.

The ridge at the end of the milling is normal, and one of the things you want to eliminate.

The large "brow" beneath the valve seat on the bowl-side of the intake ports is a flow impediment.

The exhaust bowls have a similiar obstruction that needs some whittling.

Lay down some layout dye around each of the intake runners on the deck.

Set the intake gasket on the deck and locate it with with the dowels or with a couple of 3/8-inch bolts if there are no dowels in your block.

Use the gasket as a template to mark the area on each runner where material must be removed.

The stones I use are ruby aluminum oxide, 3/4 x 1-inch straight with 1/4 x 4-1/2- or 6-inch shank, and 1/2 x 3/4-inch straight, 1/4 x 4-1/2- 6-inch shank. The carborundum dressing stick is used to shape the stones like those shown. The radius at the back of each stone prevents it from grooving the port. On the larger stone it helps ensure a correct contour at the beginning of the runner rather than leaving a hump downstream of the deck. More on this later . . .

Finally, some real visible progress . . .

Work from the inside of the runner toward the entry, in a semi-circular motion from one side of the port to the other. Don't be too quick to cut to the scribe line. It's very easy to fool yourself into believing you've done a match when you actually created a hump in the runner just beyond the deck. So, cut upward, toward the deck until the cut is straight.

A pair of inside calipers/dividers are helpful in keeping the runners uniform with one another. I check at 3-4 locations, horizontally and vertically. It's not as good as a flowbench, of course, but it beats eyeballin' 'em.

These runners are just about done. After they're all at this stage I'll go back and do a final touchup on all of them one last time. I find that this helps me with uniformity. The looked bellied out but it's really camera distortion.

This is typical of the texture in finished runners.

The exhaust ports can be enlarged quite a bit very easily. I also open up the top of the bowl, where it dumps into the runner to create a generous flow path.

Here the bowls have been blended with the valve seats. I work from the bottom of the bowl outward, using a stone with a generous radius on the back end so it doesn't knick the valve seats. Remove as little material as necessary to create a uniform shape and surface.

Compare this intake at the bowl to the stock one shown earlier and you'll see how much material can be removed from the short radius to "straighten" the intake tract.

We're getting close to the end, doing a street relief, port matching the manifold, and sanitizing the block inside and out.

Plug old valve assemblies into both of the holes of the cylinder you're relieving to protect the valve seats.

Do a simple blend and softening of the flycut valve pockets. No need to remove any material from the deck.

A nice burnish with sanding rolls aids flow and removes potential glow points. The fly-cutter chatter-marks on the valve pockets are visually amplified by the polishing, and while they aren't very pretty, they're harmless enough and really not worth the time required to make all the flycuts look perfect.

Remove all bits of flash and casting "anttrails" in the lifter valley, timing case, and crank chamber to reduce the potential for little crumbs breaking loose and finding their way into the lube system.

Use the intake gasket to mark the manifold for material removal.

An inexpensive "offshore" stone works fine on an aluminum intake manifold. Just be sure to dress the stone with grinder's grease so it won't load up and can be easily cleaned when your're done. When the runner contours are correct, blend them with a flapwheel.

Tidying up some of the external warts isn't essential to a good-running flatmotor, but it doesn't take a lot of time, and the visual reward is well worth the effort.

Twenty hours later and this block is ready for final cleaning and assembly. (It's already bored, honed, and decked.)


The obvious question is: How much of an increase does all this work yield? The short answer is: It just depends.

On an otherwise box-stock flatmotor it might be worth 2-5 horsepower, depending on how good or bad the original casting and machine work was.

The real value in this work is in the potential it provides -- what it permits other modifications to do, such changes as higher compression, camshaft and valvetrain improvements, increased fuel delivery, and improved ignition. It's not unreasonable to attribute 12-15 horsepower to the port work on a fully modified motor.

At the very least, it can't hurt so long as it's done sensibly. And it is a helluva lot of satisfying fun if you're even just the slightest bit fussy about your hardware. I have another one to do next week and I'm jazzed just thinking about it! And it's not even mine!


The roof of the runner is potentially a bit tricky, and I won't use anything other than a stone, worked in constantly moving in-and-out strokes with a slight circular pattern. It's a bit like rubbing your stomach while patting your head, but once you develop the motion it works very well with zero risk of finding water, and it yields some wonderful shapes on the short radius.
Carl . . .
Based on what Bing has shown me, I wouldn't take any more off the deck in the transfer area than I had to. I would take some material out of the head to soften the compression and gain some transfer flow potential.


Got a PM last night from someone porting his block per my instructions, and he pointed out a problem that he had at the beginning involving tool control with an air die grinder.
I find it helpful to have a small pressure regulator valve on the grinder to control tool speed and keep it constant. This permits me to maintain a firm grip on the grinder, with the trigger all the way down. Also, I position my forward hand as close to the working end of the tool as possible and use my thumb or a finger for a steady-bridge against the block.

To avoid awshit breakthroughs in the roof I don't use a burr from the deck side of the runner, only on the bowl side. Here I do the initial cut on the bottom of the valve guide area with a little blending down onto the bowl sides to remove the "cheeks" where the runner and bowl meet.

Since I've been using the CHA (Cylinder Head Abrasives) stones I have been able to do the roof and short radius very quickly without danger of hitting water. The 1/2-inch aluminum oxide, dressed with an egg shape, is quick, thorough, and safe.

In response to an earlier question you had, during my mentoring I was taught that there was little or no benefit in dropping the floor of the runner, and in fact Ed Binggeli learned long ago, doing flow testing, that it can actually degrade flow. Opening up the sides of the transition of the runner to the bowl appears to help equalize the area of the runner.

BTW, I neglected to provide contact info for the good stones:

3322 Luyung Drive
Rancho Cordova, CA 95742

1-916-638-1212 Local calls
1-916-638-0510 FAX


The stones run from $2.00 for 1/2-inch, 4-1/2-inch shank, to $2.60 for 3/4-inch, 6-inch shank. FWIW, the 4-1/2-inch shank is really long enough to reach any place you need to in a flatmotor; I usually end up cutting 6-inchers down -- to about 4-1/2 inches!

When you order, ask them for a catalog. They have pieces I've found nowhere else. Just as important as the range of their porting tools and supplies, the quality of theirstones is excellent, easily the best I've used.

Flatdog . . .
Thanks for the explanation on the use of the "tool." I can see the application now, for blending the grinding work and getting rid of ridges.
The high-quality ruby aluminum-oxide stones, with long shanks, available today sure make the transition work from one end of the port to the other a lot easier than it was in the wayback. I use a combination of side-to-side, back-and-forth, and circular movements of the stone in the port to develop a shape and a uniform surface. The final circular movement, which I learned from Kent Fuller, levels bumps, ridges, and troughs as if by magic! Fuller learned this from Don Clark at C&T Engineering in the early '50s, when Fuller was a young guy.

I want to add a recommendation I missed earlier, and that is buy SA (Standadard Abrasives) products when offered a choice. Swap-meet offshore rolls and cloths and flapwheels are no bargain at all. Bargain sanding rolls blow up almost immediately while SA rolls just keep on working, no matter how hard you lean on 'em. Best of all, SA straight cartridge rolls wear into very good tapered rolls with use, and provide you with some really neat capability for finessing shapes and polished surfaces. The bargain stuff never gets this far.

Who'd have thought?

Mike Bishop

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