In this chapter I discuss the three most commonly used methods of camshaft drive (the manner in which the camshaft is driven by the crankshaft). These are chain drive, belt drive, and gear drive. Chain drives are the norm for most traditional OHV V-type engines in the performance arena. Many late-model OHC and DOHC OEM engines have belt drives. You commonly find gear drives in specialty applications or race engines. Each style offers distinctive design and benefits.
This Tech Tip is From the Full Book, MODERN ENGINE BLUEPRINTING TECHNIQUES: A PRACTICAL GUIDE TO PRECISION ENGINE BUILDING. For a comprehensive guide on this entire subject you can visit this link:
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Chain drives are offered in singleand double-tooth sprocket designs; double rollers are the preferred choice for any high-performance application. Relatively less expensive OEM systems often used aluminum or composite cam sprockets with nylon teeth to reduce noise. The nylon teeth, exposed to age and abuse (or both), can become very brittle and can fail, sending sharp bits of nylon into the oil pan sump. Basically, the nylon tooth setups are worthless for any engine that’s spitting out some respectable power.
Timing chains are made in two basic formats: links and bushings. The bushings eventually wear and create sloppy cam timing. Roller chains with rollers provide superior contact with the sprocket.
Both single-roller and double-roller setups are available with crank and cam sprockets featuring single- or doubletooth rows. Double rollers offer more strength, less stretch, and more consistent timing. In some cases, you’re limited to using a single-roller chain due to timing cover clearance.
If you’re planning to use a chain drive to run your cam, go with a double roller. But be aware that it’s the quality of the materials and the design that counts. Roller chains may be made with seamed or seamless rollers. Seamless offer greater concentricity and greater durability. Avoid bargain-priced overseas junk and stick with a major brand such as Melling, Cloyes, Comp, etc. Cam sprockets may be made from cast iron, powdered metal, or billet steel. For the street, and for engines making fewer than 300 hp, the higher cost of a billet gear simply isn’t necessary. PM is stronger than you might think, and is used in many OEM high-horsepower engines. But if you’re dealing with a reasonably powerful build (500-plus horsepower), paying a few extra bucks for a billet setup makes sense. If nothing else, it provides more peace of mind. Always buy the highest quality that you can afford.
Swapping from a timing chain to a belt drive provides the possibility to increase power. A belt drive has reduced frictional loss, theoretically more precise timing, and smoother valvetrain motion. A dry belt system also eliminates windage variables (compared to a timing chain and gears running in oil). A belt drive does a better job of isolating crankshaft torsional vibrations. It can be viewed as a second harmonic dampener for the rotating assembly because it absorbs a degree of operating harmonics.
You might assume that a belt stretches over time, but this isn’t a concern. Performance drive belts have substantial reinforcement to prevent stretch. If a belt fails, it is likely due to age.
Belt drive systems offer rugged performance, accurate timing, quiet operation, and help to dampen harmonics. Aside from the relatively higher price tag (depending on brand and application, around $800 to $1,200), there’s really no downside to going with a quality belt drive system. Even Pro Stock racers use belt drives, and that is an endorsement.
Installing a Belt Drive
Here I use a Jesel belt drive installation on a big-block Chevy application as an example. I installed this drive system onto a 632-ci build with a Dart Big M block.
The first step is to install a stock-type big-block Chevy timing cover gasket to the block face. I smeared a thin coat of RTV on both sides of a Victor-Reinz gasket for insurance (habit). Then I installed the Jesel aluminum cover housing. This cover is secured with six 1/4-inch x 20 socket-head cap screws, which I tightened to 50 inch-pounds.
Jesel supplies a handy spanner wrench to hold the cam steady during bolt tightening. Three 5/16-inch x 18 torx-drive bolts were supplied, as well as a handy T-45 torx bit. Jesel recommends applying a light coat of RTV to the three bolt threads. These torx bolts were tightened to 30 ft-lbs (Jesel’s range is 28 to 30).
Next I oiled a brass thrust washer (on both sides) and slipped it over the cam nose. This was followed by the cam nose adapter. For sealing purposes, a light coat of RTV was applied to the rear face of the adapter where it meets the cam nose face. Jesel supplies a handy spanner wrench to hold the cam steady during bolt tightening. Three 5/16-inch x 18 torx-drive bolts were supplied, as well as a handy T-45 torx bit. Jesel recommends applying a light coat of RTV to the three bolt threads. These torx bolts were tightened to 30 ft-lbs (Jesel’s range is 28 to 30).
Next I installed another brass thrust washer to the outside perimeter face of the adapter (oiled on both sides). The next step was to install the shim pack supplied in the kit. I installed all three shims (these are located over the six 1/4-inch x 20 studs on the face of the cover), followed by the seal-fitted shim cover flange. In order to avoid damage to the rubber seal, I first removed the small key from the cam nose adapter.
According to Jesel’s instructions, camshaft endplay should be .010 to .015 inch. The supplied shims must be added or removed in order to achieve a cam endplay of .010 to .012 inch. When I set up my dial indicator, I found endplay to be .017 inch. I called Jesel to ask their advice, and one of the techs assured me that .017 inch is acceptable.
With cam endplay verified, I removed the thrust flange and the three shims. I lightly coated all shims and the rear face of the flange with RTV and reassembled, using the supplied 1/4-inch x 20 nyloc jam nuts, tightening them to 50 in-lbs. With the shim pack and flange installed, I re-installed the small key to the cam nose adapter.
Next I installed the crank gear drive (I installed a pair of keys measuring .185 inch thick by .735 inch long and .300 inch high into the snout’s key slots). This is an interference fit, requiring an aluminum driver that slips over the crank snout driver, which was supplied in the kit. The crank snout had a slight step up in diameter. When the gear was slipped onto the crank and stopped with hand pressure it was then driven on another .384 inch (in this case) until it seated.
At this time, I rotated the crank to place the crank gear’s timing dot at 12 o’clock. The cam adjuster plate dropped over the four studs on the black cam gear and was assembled with the cam timing mark at the top, aligning with the center (zero) timing mark on the adjuster. I installed the cam gear and adjuster plate onto the keyed cam-nose adapter.
Next I installed the cam locking flange using the left-hand-thread bolt provided. It was snugged just enough to keep the cam gear keyed in place. With the four adjuster plate to cam gear nuts installed with about 11 ⁄2 threads, I tilted the top of the cam gear forward to slip the timing belt into place.
This took a few minutes because I needed to keep the timing dot on the bottom of the cam gear aligned with the top dot on the crank gear. The belt was a tight fit, so patience was definitely required. Once the belt was in place, I snugged the four 12-point nuts to secure the adjuster plate to the cam gear.
Once the timing belt was in place, with both dots aligned (cam gear dot at 6 o’clock and crank gear dot at 12 o’clock) and with the cam adjuster marks set at zero, I tightened the cam gear’s center left-hand-thread bolt to 70 ft-lbs (I needed a 12-point, 5/8-inch socket for this bolt).
I was then ready to degree the cam. Although the job seemed daunting at first glance, it actually wasn’t that big of a deal. Because the oil was sealed by the rear cover, the belt drive was exposed, making cam adjustment easy.
The theory behind a gear drive is simple. Crank motion is directly transferred to the cam via a set of meshed gears. With no chains to stretch or wear out, gear drives provide superior and consistent cam timing. Gear drives are also stronger and last longer than the average timing chain set. Those are the pros.
The cons include potentially greater harmonics transmitted to the cam and (in most cases) a notable whine that results from gear engagement. The whine is somewhat similar to the sound created by a supercharger blower, which can be really cool or really annoying, depending on your frame of mind.
If you want a quiet engine (aside from the exhaust note, of course), you probably won’t like a gear drive. However, some gear drive manufacturers now offer a “quiet” version that produces less whine. Gear drives and related parts are available from Edelbrock, Milodon, BHJ, and others.
You can replace a stock timing chain with Edelbrock’s Accu-Drive camshaft gear drive kit. This gear drive system transmits power from the crankshaft gear to a full-floating main idler, which drives the camshaft gear. A unique feature of this system is the ability of the main idler gear to float to an optimum position between the crankshaft and camshaft gears, ensuring absolutely equal load sharing between them.
The gears are made from billet SAE- 1144 steel with induction-hardened teeth. Gear teeth are shaved for precision operation. The set has hardened and ground idler pins from billet steel.
To install, just replace the stock crank and camshaft sprockets with AccuDrive gears and slip in the idler assembly. Most Accu-Drives require no modifications to the engine block, although some fitting of the axles and front cover may be required.
The gear drive system from Milodon has a three-gear setup with easy cam timing adjustment. All Milodon gear drives are three-gear, “fixed idler” models. The idler gear mounts solidly to the block (under cover) or to the cover (full cover). This system does not rob any power from the engine and (more important) does not allow cam timing to vary. The full-cover drive uses an adjustable cam gear and hub assembly to set cam timing, accessible through the removable cam cover. The cam itself is bolted to the hub.
The Milodon gear drive uses the cam gear and hub to advance or retard the timing. You simply unbolt the hub and turn it until the indicator mark is lined up with one of the seven bolt positions on the cam gear, with no offset bushings or keyways to mess with. The cam must be installed straight up (no advance or retard) as follows:
With no cam gear in place, locate TDC for the number-one cylinder using a degree wheel. Rotate the crankshaft until the degree wheel indicates the intake valve opening per the cam manufacturer’s specs.
Place a dial indicator on the intake valve and rotate the crank until the valve opens to the recommended checking clearance (usually .050 inch of lift).
At this point, the camshaft is now installed straight up with no advance or retard. Install the cam gear. Adjust for your desired advance or retard. Place a mark on the cam hub next to any cam gear bolt hole position and label that hole number-1. In a clockwise travel, label the remaining bolt holes two through seven.
You’re now ready to set cam timing. Each bolt hole has gear-tooth positions for advance and retard. The chart in the provided instructions shows the settings for each hole. To advance the cam, turn it clockwise until the indicator mark on the hub is lined up with the proper bolt. To retard the cam, turn it counterclockwise until the mark lines up with the hole you want. Tighten the hub bolts and your cam timing is set.
BHJ offers a timing set length gauge (TSG-1) that’s adjusted to the desired center-to-center distance using a micrometer and the setup pins or an optional setup standard. The dial indicator, included with the gauge, is then zeroed at the desired length. Timing sets can then be dropped onto the appropriate mounting plugs.
The TSG-1 requires 100 psi minimum air supply. Activating the air switch applies exactly 100 pounds of tension to the chain, as specified by leading chain manufacturers. The timing set length is read as a plus or minus dimension from the desired length on the dial indicator.
This precision measuring fixture eliminates the need to special order custom-length timing sets or waste time with tedious trial-and-error methods of obtaining proper timing chain fit.
TSG-1 accepts timing sets with center distances ranging from 4.125 to 6.125 inches, using the appropriate mounting plugs. The gauge includes a precision dial indicator, setup pins, and one pair of mounting plugs.
Also available is a timing set gauge made specifically for the Mopar R5 or the small-block Chevrolet using the Jesel belt drive. Additional mounting plugs and optional setup standards are available for all engines.
Ford small-block cam gears have one cam alignment dowel. Considering today’s high-lift cams and high valvespring pressures used with roller cams, this timing gear design really needs an upgrade for better timing control. BHJ makes a timing-set drilling fixture (PN TSDF-FOS) that allows you to add a second alignment dowel placed 180 degrees from the existing dowel location. This requires a simple three-step process that can easily be handled with a bench vise and hand-held drill (or on a drill press or milling machine).
The kit includes a fixture plate, end mill, drill, finishing reamer, drill bushings guides, and mounting hardware. This is a worthwhile and easy-to-perform upgrade if you’re running a nasty cam in your smallblock Ford and plan to use a chain drive.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks