EngineLabs: How has the traditional double-roller timing chain changed over the past 25 years?
Thompson: We have seen improvements with additional options of billet steel sprockets, adjustability and — in our case — patented tooth profiles. No improvement has had greater impact than the adjustability features. Multiple keyway crank sprockets (see below) first allowed valve timing adjustment in two-degree increments. And then the introduction of the Hex-A-Just offered infinite adjustability at the cam sprocket without having to disassemble the timing set. The continued development of the adjustable cam sprocket focuses on the idea of a one-piece design, with infinite adjustability using the existing attachment bolts for their joint security.
EngineLabs: More aftermarket blocks are being offered with raised cam centerlines. What are the challenges in designing timing sets for these new blocks? And are there any problems with harmonics?
Thompson: The industry has tackled the two mainstream examples of raised cam applications, those being the SBC Rocket and the BBC Merlin. Math is math, so the raised centerline must be such that a given number of sprocket teeth and chain links can be matched to accommodate. It is up to the block manufacturer to insure the math works, or the new block owner will potentially be left with no chain options. Thus far, the current raised block offerings are mostly within acceptable parameters and do not create any additional harmonics. The issue of harmonics is controlled through specific processes and increased product offerings. A more attuned analysis of how sprockets and chains function together, such as the Cloyes custom matching (see below), helps us to eliminate chain whip through center-distance correction and also minimize runout. In addition, we can control harmonics by eliminating problematic OE components, such as spring activated tensioners, and by adding products where applicable, such as a stationary damper block (see below) on the LS3 engine.
EngineLabs: Regarding adjustable timing sets, a skeptic might question the reliability of the mechanism to hold the desired timing in place. What are the limitations of adjustable sets and what extra precautions should the user follow to ensure a secure setting?
Thompson: Keyways and dowel pins are indexing features only. There is no instance where a keyway or dowel pin is relied on to carry torque through the joint. The clamping force of the attachment bolts is the most crucial factor. So, the adjustable sprocket, as with a non-adjustable sprocket, will take you as far as the strength of your joints. When people ask us about this, we tell them you can make the key and dowel from ice, build your engine in a freezer, and as long as you torque the parts up correctly, you’ll never have to worry when the ice melts. The critical steps remain in detailed installation with proper torqueing.
EngineLabs: What are the signals to indicate that a timing set is in need of inspection or replacement, and what are the most common mistakes in installing a timing set?
Thompson: The signals are the same regardless of adjustable option. The easiest and most common indication is excessive chain deflection. If your chain deflection is greater than ¼-inch (for most applications), that would suggest it is time to refresh the timing set. You may also see indications of wear on the sprockets as they will take on a sharp or jagged look. Some of the most common installation mistakes are: improper torqueing of attachment bolts; improper cleaning (leaving debris on the cam/crank nose which leads to seating issues for the sprocket); or just plain rushing through the job and not checking for proper alignment. Another oft-seen issue is the use of improper tools. We often see marks indicating the use of a hammer and chisel (or punch), instead of a sleeve to press on the crank sprocket. This will damage a critical mounting surface for the clamping force joint which will become quite visible at 8,000 rpm when things start coming apart.
EngineLabs: Is there still a market for gear drives?
Thompson: Yes, there is still a market for gear drives, primarily for the weekend car enthusiast, not the racer. Within the parameters of a street application and under a horsepower ceiling, the gear drive functions very consistently and with excellent durability, offering a long-term answer for the car enthusiast who also wants the added benefit of that “blower sound.” In all-out race applications, the flaws of the design become apparent. These flaws include uncontrolled backlash, induced harmonics and timing variation from acceleration to deceleration. Applications are limited to pushrod engines with a timing cover that can hold the idler gears captive. For example, you cannot put this gear drive on a Vortec engine with plastic timing cover.