Gear Up For Accurate Timing With Milodon Gear Drive Systems

Hot-rodders and racers have a number of options available to them for timing sets for their American V8s: the simple timing chain, belt-drive, and a couple of types of gear drives. The timing chain drive is quick to install and relatively cheap, so why consider a gear drive? And what are the differences between three-gear and four-gear drives?

“The big advantage to running a three-gear Milodon gear drive is consistent cam timing, as well as consistent ignition timing for distributor ignitions,” says Steve Morrison, owner of the Simi Valley, California, company, which he joined in 1975. In contrast, timing chains and even Kevlar belt drives have deflections that affect accuracy, he says.

“Anything that’s going around a circle like that reacts to centrifugal forces,” he explains. “A gear can’t change; it’s billet steel with a Rockwell 50 hardness, so, there’s no flex; it can’t change shape. When you set up the .003-inch of gear lash, it’s always .003 inch, whether it’s on the engine stand or at 10,000 rpm.”

As components wear, slack in a chain or belt can cause cam timing and ignition timing to become retarded, making what was a crisply tuned engine now “idling rough and sluggish,” Morrison says. “A gear drive doesn’t really do anything miraculous, like increase compression or valve lift. It just turns the camshaft. But, from day one to day 1,000, you can have dyno-fresh, brand-new timing.”

Chris Marin, owner of turbo Buick specialist Black Arts Automotive in Palmdale, California, uses a Milodon gear drive on his 281ci Stage 2 Buick V6 using an on-center 153 block punched out to a 4.061-inch bore, with a 3.625-inch stroke and 6.500-inch rods. The combination runs a 230/230 solid roller and TA Performance SE heads that he ported and flowed. A strip of newsprint is used to set the proper .003-.0035-inch backlash of the gears.

Less is More with Three-Gear Drives

Milodon gear drives use a fixed-idler setup with one gear bolted to the block either through the supplied timing cover or on a steel plate, instead of a floating two-gear idler. This makes installation a little more involved for a Milodon drive, but it also provides a number of benefits. Those with bad experiences with a four-gear drive should consider the differences, Morrison says, including the misconception that gear drives lose horsepower.

“Robbing horsepower is possible with the four-gear drive. What keeps the idler gear from falling off the block is that it’s wedged between the crank gear and the cam gear,” Morrison says. “The harder you drive the cam, the more that gear tries to drive between those two gears — which is like a wedge — and creates drag, therefore losing power. But if you can fix that idler, then it doesn’t create that drag, and it only gives the benefit of improved timing stability.”

When things are making noise, they’re usually not happy and there’s something going wrong. — Steve Morrison, Milodon

The four-gear-type is easier to install because it installs as a basic timing chain set would be, by lining up the crank and cam sprocket dots. But that comes at a cost because it’s installed a bit loose to accommodate multiple possibilities, such as an align-bored block.

“But that’s really not a good scenario, because timing will be bouncing around. When you’re on and off the throttle banging gears, that dual idler gear setup is flying all over the place,” Morrison explains. “I know, it’s 150 bucks. I mean, go figure… You’re going to make three billet steel heat-treated gears for $150 retail? You have to wonder what you’re really getting. Ours, of course, are U.S.-made, and we’ve always made those gears in-house since 1967.”

Noisy Gears Are Not Happy Gears

The four-gear-types have earned a reputation for being noisy, even if one doesn’t select the noisier gear drive option from the manufacturer. In contrast, set up properly, Morrison says, the Milodon gear drive will still have some noise, but not the “obnoxious, power-steering-out-of-fluid sound.”

“The gears will make some noise because they’re straight-cut. It’s like an M22 tranny in a ‘69 Camaro. When you hear an engine with our gear drive, people will say, ‘That thing sounds bad-ass.’ I mean, you can always bolt on loud mufflers, but with the gear drive, it’s got that little whine to it that gives it a little bit of intrigue coming down the street.”

Some buyers have asked about the availability of a noisier option, but that’s, in essence, a defective design, Morrison says.

“When things are making noise, they’re usually not happy and there’s something going wrong. We cut all our gear teeth in-house on what is called a gear shaper. It only takes two gears at a time and shaves off about .010-inch at a time, as opposed to a gear hobber, which puts them on what is like a barbecue skewer. They have about 40 gears on this clamping device, and they cut them all at once.”

While hobbing works fine for industrial and commercial applications, it falls short of the finish imparted by a gear shaper, he says, which means Milodon gears have a very smooth finish. Because of its design, Morrison says, the Milodon gear drive can be used with most EFI systems using a knock sensor. The drive won’t be making enough noise to cause the computer to retard ignition timing. He also says it’s an “old wives’ tale” that a properly set up gear drive will transfer undesirable harmonics into the valvetrain.

A diamond wheel spinning at 10,000 rpm grinds the inside diameter of a cam gear (left) to a runout tolerance of .0005 inch, and a gear shaper (right) cuts a pair of gear-drive cam gears. It's a slower process than gear-hobbing, but it yields a much higher-quality and more-precise part, Milodon's Steve Morrison says.

What’s a Good Candidate for a Gear Drive?

Practically any American-made V8 can use a Milodon gear drive, Morrison said, from the weekend street car, to late-model dirt-track cars, to boats or drag-racing. Some designs allow for an accessory to be driven off the nose of the cam, such as fuel pumps or dry-sump oil pumps. It’s a durable setup, proven even in extreme conditions.

“Sometimes, customers drive a five-stage oil pump off of it, and you can imagine the drag on that. That’s driving the whole oiling system, the scavenge, and pressure pumps, so that’s huge. And then they started running dual fuel pumps for the blown cars. Additionally, the force needed to turn the cam when opening valve springs with 750-800 pounds of open pressure strains the timing system, even though it is often the last part to get attention from some engine builders,” Morrison says.

“The customer will do camshafts and port the heads, and do all this trick stuff, but then when it comes to the timing system, they go, ‘Well, a chain is good enough.’ You just did 10 hours’ worth of porting on your head, but it was good enough in the beginning. So you just did an extra 10 hours for nothing? People don’t realize how much loss there will be, and it’s so simple to prevent.”

Installation and Degreeing Requires a Few Extra Steps

Complete installation instructions come with each gear drive set, and there’s even a DVD available (P/N: 14900, which sells for about $30 retail) that goes into detail about what’s required to install a gear drive, belt drive, or timing chain, and how to degree the cam.

Installation will vary slightly by whether it is for a full-cover design, such as for the small-block Chevy, big-block Chevy and big-block Chrysler, or the kit that fits under the stock timing cover, as found in 302-460 Fords, 340-360 small-block Chryslers, early Chrysler Hemi, Pontiac, Olds, and AMC engines.

Full instructions are included in each kit, so any description here is simply to give an idea of the process. Essentially, a three-piece thrust bearing assembly and hub are installed on the cam, followed by the cam hub, cam gear, and crank gear. A strip of newsprint 1/2-inch wide by six inches long is wrapped around the idler gear, between the cam gear, idler gear, and crank gear to set the proper .003-.0035-inch backlash as the idler is centered between the cam and crank gear and pushed into place.

Then, for the full-cover designs, there are drill bushings permanently installed in the timing cover, and holes are drilled through these bushings into the block. Then the cover is removed and hardened dowel pins are installed in the block. For the under-cover design, installation is much the same, except the idler plate is held in place while holes are drilled for the mounting bolts. If the block should need to be align-bored later on, the idler assembly can be installed in the new, slightly higher location (such as .005-inch higher for a block align-bored .010 inch) by removing the dowel pins, plugging them, and drilling and installing new dowel pins.

John Garland, owner of AMC specialist Garland Performance, in Sibley, Mo., shows how a piece of paper (newsprint is recommended) can be used to set the gears' backlash to .003- to .0035-inch. This engine is a 360 AMC with a 401 crank, for a final 393 cubic inches.

We won’t go into detail here (check out other EngineLabs articles and videos for details), but for the next step, you’ll need to degree the cam, which will require a degree wheel and a dial indicator. “Milodon doesn’t include dots on the gears, as crankshaft keyways and camshaft dowel pins are often inaccurate,” Morrison says, matter-of-factly.

“If you set it perfectly as the cam card says, the thing’s going to run awesome,” says Morrison. “If it’s off for some reason, it could be pinging because it’s too advanced, or it’s going to be sluggish because it’s retarded. So we’re going to throw out the factory markings, as we can’t assume every keyway and every dowel is in the exactly correct position. Even if everything is off, the setup for the gear drive will compensate and give you the exact “straight-up” cam position, just like the cam card says.”

For the degreeing procedure, the cam hub, idler gear, and crank gear are in place. But, the cam gear will be removed for the time being. Then the procedure has you set the cam in the “straight-up” position with 0 degrees of advance or retard. The cam gear’s vernier (asymmetric) bolt pattern allows the gear to be installed in one of seven different positions, with two teeth locations for each bolt hole position. This allows the cam to be advanced or retarded as much as 12 degrees. At this point, the cam gear is aligned with the cam hub bolts holes and is installed.

Milodon's adjustable vernier cam bolt pattern (left) allows a large number of easy cam degree changes accessible through the removable cam cover, as seen on this big-block Chevy (Mark IV) flat-cover-style gear drive (right).

“It all takes a little extra time and effort to install and set up, but it pays off in the long run,” Morrison says. “Even if you switch it between future engines, you’re going to go through a couple of cars before you see any signs of wear, let alone needing to replace one. The Indy Light series, which ran the Buick V6 exclusively, all used a Milodon gear drive. They ran over a million race miles without any gear drive breakage. So to say they pretty much last indefinitely is not too far off.”

So for the racer or engine-builder looking for a heavy-duty, accurate timing drive, Milodon provides one option to seriously consider.

Article Sources

About the author

Jay Sicht

Since childhood, Jay has been fascinated by planes, trains, and automobiles, and all things mechanical. He's been in the automotive aftermarket for 25 years, having written about it for 15 of those years.
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