Low Tension Rings for Box Stock
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TUESDAY, APRIL 2, 2013
The newest hot setup for Box Stock Class engines is to install low tension piston rings. There are a few tricks you should know when using these rings, but if you feel comfortable replacing piston rings, it is easy.
For reasons I'll explain below, I recommend buying the DJ-1290LT ring set and the DJ-1290TR +.005" top ring for a fresh standard bore or a DJ-1291TR +.010" top ring for a bore that is not fresh. What is outlined below should pass tech in the box stock class as of today.
My method for Box Stock is to start with a standard bore block (preferably new) and hone the block until you have .004" piston-to-wall clearance. -Pay close attention to what your rules allow on bore size- This means you are going to hone approximately .002" from a new cylinder. You really need to have this honing done with professional equipment. It is necessary to use a torque plate, and the engine's side cover should be installed when honing. With these items installed, you use a dial-bore gauge to try to get the finished bore within .0002" of being round and straight from top to bottom. Piston-to-wall clearance is obtained by measuring the bore with a dial bore gauge and measuring the piston skirt with an outside micrometer. You measure the piston skirt 90° to the wrist pin, halfway up the skirt. Subtract the piston measurement from the bore measurement to get your piston-to-wall clearance.
The low tension ring set is simply a standard bore ring set that has been specially heat treated in order to take some of the spring out of the metal and/or shrink the ring. It will come with a treated top compression ring (a thick ring with a chrome outer face), a treated middle scraper ring (a thick ring that is all black), a treated expander ring (looks like the center layer of a corrugated box), and two thin rails.
Install the expander ring first in the bottom ring land of the piston. Then install the thin rails in the same ring land so the expander is sandwiched between the two rails. The gaps of these rings should not be on top of each other.
Next, install the middle scraper ring (You are using ring pliers, right?)...by the way, the letters near the gap of the ring go up.
Finally, throw away the compression ring that comes in the low tension ring set and install the DJ-1290TR... again, the letters on the ring go up, and don't put the compression ring gap directly over the scraper ring's gap.
If your project begins with re-honing a used engine's cylinder, use the DJ-1291TR top ring and file-fit it. Your ring gap on the top ring when installed squarely in the cylinder should be no less than .006".
It is always best to install the piston into the cylinder with aring compressor.
The reason I chunk the low tension compression ring is because clone compression rings lack any sort of gas-porting assistance to drive the ring into the cylinder wall during high compression and combustion pressures. It is worth mentioning that the Honda compression ring does utilize gas porting technology. Anyway, lack of tension in the compression ring simply does not give me the sealing force that I want. You also MUST have a significantly better seal at the compression ring over that of the scraper ring or you will get "ring flutter." All you need to know about ring flutter right now is that it will rob you of about 10% of your engine's potential horsepower, or about 1 HP on a Box Stock engine. So ring flutter is bad, 'nuff said. Also, about 80% of the ring drag reduction comes from the installation of the low tension scraper and oil ring pack, so the better setup is to go with a good top ring and low tension lower rings.
Some final words...If you attempt this, YOU are the engine builder. If it gets screwed up or doesn't pass tech, the blame is in the mirror, NOT on the phone.