Drilling and Tapping a Prop for Puller Holes

By Tomas Iglesias, Yacht service diver, South Florida. Reviewed by Dana Whitcombe, technical reviewer.

Propeller Removal & Repair | Hull & Bottom Cleaning Last Tuesday I got a call from a manager at Pier 66. A 42-foot Ferretti had a starboard prop that wouldn't budge off the taper. The owner's pet mechanic had already tried a three-arm puller with the arms hooked onto the blade edges. It popped off twice. I could see the scoring on the tailcone from the second pop. Right then I knew: we needed to drill and tap puller holes into the hub. That's a common situation down here. Lots of props, especially reconditioned ones, come back without factory puller holes. Or the holes are stripped from years of corrosion and heavy pulls. You can't get a clean pull without a solid flat surface for the puller's bolts. So you make your own. It's a standard field job, and if you follow a few rules, it's not risky. Ignore them and you can drill right into the hollow shaft. This is one of those jobs that looks dead simple in a manual but takes practice to do without ruining a $3,000 wheel. Let's go through it.

First, Do You Even Need the Holes?

Most new props from Michigan Wheel or Veem come with puller holes already tapped in the hub face. But reconditioned ones, or older stuff, often don't. And sometimes the original holes are stripped or bell-mouth from too many pulls with the wrong bolt. You can try pulling with the blades, but that's a hack. It bends the pitch and you'll be chasing the prop afterward. The only clean way to get a prop off a straight taper is to push from the hub. So decide if you really need to drill. If the prop has holes, just clean them with a tap and move on. If not, you've got to cut new ones. And that means you're committing to a job that takes about twenty minutes on a good day. But don't rush it. Drilling into a paid-for prop is irreversible. There's also the question of whether the prop hub has enough meat. On smaller outboard props, the hub might be too thin to safely tap. On those, you're better off using a puller that grabs the inner hub behind the prop, or a tapered wedge. Depends on the boat. Give or take, you need at least a half-inch of solid bronze on the hub face for two nice 3/8-16 threads.

Drilling the Hub

First, pull the nose cone and any remnants of the shaft nut. Clean the hub face with a brass brush and brake cleaner. You need a true read on where your hole locations are going. Mark the center of the hub face with a marker. Use a pair of calipers to find the mating surface. The holes should sit on a bolt circle about three-quarters of the way from the shaft axis to the hub's outer edge. That leaves enough meat to tap threads without cracking the lip. On a typical 20-inch wheel, that means a 1-1/2 inch bolt circle for two holes at 180 degrees. Center-punch your two marks. I'll drill a 1/8-inch pilot first, then go to the tap drill. For a 3/8-16 thread, that's an F drill (0.257-inch), or a 1/4-inch if you're in a pinch on the boat. If you're going 1/2-13, use 27/64. For a prop with a hub face that's a little over an inch thick, you don't want to go deeper than 3/4 inch. Put a piece of tape on the drill bit at that depth. Once you're through the pilot, step up and drill with the tap drill. Go slow, use cutting oil, and back off to clear chips. Now, here's where I've seen guys screw up: they angle the drill. The hole has to be square to the face, and parallel to the shaft axis. If the drill wanders, the tap will bind and snap. I use a drill press when the prop is in the shop, but on the boat I use a right-angle drill with a good centering setup. Sometimes I'll use a portable magnetic drill if the hub is steel, but that's overkill for bronze. And here's the digression: a few years back I had to drill holes in a Rolla stainless prop for an 80-foot sportfisher. That thing was harder than a wedding dickens. I went through three high-speed steel bits before one of the old-timers handed me a cobalt split-point bit. That did the trick, but it taught me to know your metal. Bronze drills like butter, stainless fights you the whole way.

Tapping the Threads

After the holes are drilled, clean out the chips and give them a shot of Tap Magic or any thread cutting fluid. Start with a taper tap to get the thread going, then switch to a plug tap to finish. Turn it a quarter turn in, half turn back to break the chip. It's monotonous, but it's the way. If you try to crank it all the way through, the tap will jam and you'll be extracting broken taps from a bronze hub at 30 bucks an hour. A lot of these hubs are nickel-aluminum-bronze, which machines nicely. But if you've got a high-strength stainless prop — maybe a Rolla or a custom piece — use a cobalt tap and go slower. Stainless will eat a standard high-speed steel tap for breakfast. And clean the threads with a bottoming tap only if you want the bolt to seat deep. For a puller hole, you don't need full threads to the bottom. Just enough for the puller bolt to bite and hold under load. One more thing: don't blow compressed air into the holes while the prop is still on the boat. You'll send chips right into the shaft seal. Use a magnet or a vacuum if you have one.

Pulling the Prop Without Damaging It

With the holes tapped, grab a puller that fits. My go-to is a standard two-arm puller with a crossbar up to six inches. I screw the puller bolts into the hub, using a washer under each bolt head so you're not loading the threads unexpectedly. Then I tighten the central screw onto the shaft end. On most yachts, the shaft has a dimple in the center, so the screw seats right. Give it a bit of pressure, give the hub a light rap with a brass hammer, and it'll usually pop free. If it still fights, you can apply heat around the hub with a torch. But careful: heat up the hub, not the shaft, and don't get the stuffing box too hot. I've seen guys melt a drip-less seal doing that. Also, never pull on the prop while the key is still in the shaft; it'll hang up. So drive the key out first from the backside. Once it's off, chase the threads with a tap and put anti-seize on them. Next time that wheel comes off, you'll be glad you did.

Frequently Asked Questions

Can I drill puller holes with the prop still on the boat? Yes, I do it all the time. But you have to be careful to keep the drill square to the hub face and use a depth stop so you don't hit the shaft. Clean the shavings out before you pull so they don't get into the seal. What size puller holes should I drill? It depends on the prop size. For most yachts up to about 40 feet, 3/8-16 is plenty. Larger wheels sometimes use 1/2-13. Match the tap drill to the thread: F drill for 3/8, 27/64 for 1/2. What if I drill too deep and hit the shaft? That's bad news. The shaft is probably hardened and you'll dull your bit, and you may have pushed metal chips into the coupling. Best to use a tape depth stop and, even better, check the hub thickness with a caliper first. Do I need a bottoming tap for puller holes? No. A standard taper or plug tap works because you only need a few threads to hold the puller bolts. A bottoming tap just gives you more thread depth, but that's not necessary for a pull.