Modified Mustangs & FordsHow To Engine
Exploring Piston And Ring Technologies
A Better Understanding Of Piston And Ring Technologies Can Lead To A More Potent Engine Build
In the early days of affordable stroker kits, there was much talk about combinations in which the oil ring would intersect the wristpin, potentially resulting in increased oil consumption. In the Ford world, this discussion largely revolved around short deck stroker small-blocks, where the 331 combination that didn't require such a ring package, was at one time considered more street friendly than the 347 stroker, which did. That debate has faded significantly as the modern oil ring rail support has been perfected-essentially an 0.030-inch reinforcing ring that sits at the bottom of the oil ring groove-keeping the oil ring intact and in place. JE's Randy Gillis believes piston stability is the determining factor regarding oil control issues, and hasn't found the intersecting ring/pin to be an issue in current testing. Truth is, there are lots of situations beyond the 331/347 where there is potential for an intersecting oil ring and pin, so lots of effort has gone into making it work. The issue is common on stroker Clevelands and 385-series big-blocks since the top ring is lower on the piston due to the canted valves, as well as Modular strokers. High boost applications can have the same issue because the ring pack is typically moved down on a custom piston for such situations.
To Coat, Or Not...
Aerospace coatings began to enter the automotive world in the late 1980s, with pistons identified as prime candidates. As we spoke with various people in the industry, we found some debate surrounding when piston coatings should be used. It seemed to boil down to three schools of thought: 1) a properly prepared engine shouldn't need coatings, 2) there are situations where coatings are advantageous, and 3) coatings are nearly universally advantageous. We won't weigh in on the discussion, but would encourage you to take up the issue with your particular engine builders. Trust their experience more than your book knowledge.
What we will do is describe what coatings are designed to do. Two types of coatings are applied to pistons. One is a "dry film" anti-scuff skirt coating that is some kind of proprietary blend of molybdenum, graphite, Teflon, and epoxy, which is screened onto the piston skirt and heat cured. The other common piston coating is a metallic ceramic thermal barrier that is applied to the piston top. The idea here is that deflecting heat from combustion helps the piston to live and keeps heat in the chamber where it may result in greater power. Forced induction would seem to be an application ripe for thermal coatings, but the real benefit to both coatings involves increased durability first, and the potential for additional power as second.
Several piston companies offer skirt coatings as standard treatment on particular product lines, while nearly all others can provide coatings as an option. If you're interested in coating pistons you've already purchased, companies like Calico Coatings can do the job. Calico quoted the following prices: $21 per piston for skirt coatings, $26 per piston for thermal top coatings, and $39 per piston for both treatments.
Apples To Apples
Ring thickness measurements may be given in fractions of inches, millimeters, or decimal-inch form. In an effort to help illustrate how thin rings have become, we've converted all the ring sizes we discussed with manufacturers into a common decimal-inch form. Remember, these are rings which could be considered street or street/track worthy depending on your particular priorities. It's worth noting that much thinner rings exist in automotive racing applications.
- 3/16-inch = 0.1875-inch
- 3mm = 0.118-inch
- 5/64-inch = 0.078-inch
- 1/16-inch = 0.0625-inch
- 1.5mm = 0.059-inch
- 1.2mm = 0.047-inch
- 0.043-inch = 0.043-inch