11/1/2003
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Coatings Can Help Restore Power to Tired Engine Components
By David Vizard
If you’re building even just a few performance/race engines I’m sure you have come across the situation where the engine’s bores have worn just enough to warrant a honing job to clean them up. This, of course, leaves the pistons a little looser than you would like and the consequences are either new pistons or accept a compression/power loss.
NASCAR Winston Cup teams see the worn bore situation continually and have circumvented the need to install new pistons when the block is honed for nothing more than a cleanup. The technique is as follows:
Start with a virgin piston or a piston with only a crown coating. After the piston has seen a race or an extended test session the engine is torn down and the block re-honed as required to clean up (usually about a thousandth).
To go with this, the piston skirts and lower face of the ring grooves are coated. This achieves several things. On the skirt it not only brings the piston back up to size but also cuts oil drag without necessarily reducing the oil film thickness. The coating on the lower faces of the ring groove, which must have good heat conduction qualities, also serves two functions. It cuts friction between the ring and piston, and, due to its conformability improves the seal between the lower face of the ring and groove.
Another advantage of reconditioning and reusing the pistons is that everything in the bottom end is dimensionally a known factor. Unlike installing new pistons there are no piston/block height issues to deal with. This means a faster and less costly rebuild.
I was donated a set of race pistons that had been in a 5,000-mile old street/strip nitrous motor that had a head gasket let go (it happens when you don’t use top quality bolts!) and had not been stripped until three months later. With the rust and other stuff on them these looked like a disaster but proved dimensionally sound. This meant they were good candidates for restoring to better than new by the use of application specific coatings. After stripping the pistons of rings and removing them from the pistons they were given a cursory clean and dimensionally checked. The crown was then machined to reduce the compression ratio (CR). There is not space to go into the theoretical reasons why, but for what it’s worth, with typical components, the optimum CR with nitrous is about 10:1.
With zero ring groove wear and near zero skirt wear they were good to reuse. To clean the carbon and other deposits off I recommend a popular type of oven cleaner. It works on normally stubborn carbon deposits almost right now bringing the piston to near as new appearance. Because the pistons were going to be used in another nitrous application, this time with up to 400 hp jetting, I honed out the pin bores to give a little extra clearance. The alloy that was used in this style piston has a tendency to gall if stock pin clearances are used. When this is opened up to .0007˝ to .001˝ the problem goes away at any sane power levels for a stock blocked SB Chevy. After that the pistons were shipped to be coated.
What came back was actually a better piston than that same piston as new. The economics are that the coatings generally run no more than 80 percent of even the least expensive quality piston. If we are talking big budget race pistons the coatings only run to 30-40 percent of the cost of a new piston.
For a nitrous motor the piston crown’s ability to deal with heat is crucial to the overall reliability of the engine. Here is an interesting statistic concerning piston crown coating. Since using them (from about 1982) I have never lost a nitrous motor. To demonstrate the reliability to a concerned friend for whom I built a very expensive street 480 inch BB Chevy, I ran an almost full 80 lb. bottle through the motor, with 400 hp jetting, in one go! The engine produced well over 1,100 observed hp for more than two minutes and did not drop below that mark until the bottle was almost empty.
By the end of the run the headers were red almost to the collectors and I had hundreds of gallons of boiling water. At the inspection teardown the pistons, and indeed all the innards, including the coated heads and valves were in perfect order. You can be sure this is not a test I would have undertaken without the benefit of coated pistons.
From The Wizard...
The company I used for this project was Calico Coatings located on the far outskirts of Charlotte, NC. This is the company that did the coatings for most of the front-runners, including winner Joe Sherman, in the Popular Hot Rodding Engine Masters contest.
Just for the record I have also successfully used coatings from all of these suppliers: Techline (Riverside, CA – 909-304-0498), Polymer Dynamics (Houston, TX – 713-694-3296), Swaintech (New York area – 716-889-2786) and Calico Coatings (Charlotte, NC – 704-483-2202). |
Ex-aerospace engineer David Vizard, with over 3,500 published articles and 29 books (5 publisher’s best sellers) is one of the world’s most widely published automotive writers. He is also a university lecturer, holds numerous patents and is a winning engine/car builder. In his best year he amassed a combined 169 track records, first places and championship wins, from just 8 engines. Technology he has developed has been used on everything from F1 across the board to drag race and dirt cars.