Well for over a year now we have been documenting failures in the 4b11 engine. In our research we have found most all of them failing for the same reason each time. In analyzing the components after the failure of these engines we have come to a back up our initial interpretation with case after case of symptoms, all of which were following the same path.



We are not certain if there is any rhyme or reason that these failures are happening to a given set of cars build date span, if there has been manufacturing or quality control changes along the way however the facts will remain that a sizable portion of the Evo X community is at risk of a critical flaw in the 4b11 engine building process at Mitsubishi.



What we have found time and time again is that the top compression ring on these engines at the very least have ring end gaps WAY TOO SMALL for a turbocharged performance vehicle. In removing the rings and measuring the gaps in the cylinders, what we found was a shocking .007" gaps.

Most piston ring manufacturers recommend a minimum end gap of .004 inches times the bore diameter for the top piston compression ring. So for a 4 inch bore, the standard end gap would be .016 inches.

86mm stock bore = 3.38582677"
.013-.014 recommended ring gap



For a modified street performance engine that generates more horsepower and heat, the end gap should be opened up a bit to compensate for increased thermal expansion. The recommendation would be a minimum top compression piston ring end gap of.0045 to .005 inches times the bore diameter. For a 4 inch bore, the ring end gap on the top ring should be increased to .018 to .020 inches.

86mm stock bore = 3.38582677"
.017 recommended ring gap for modified street performance



For a nitrous or turbo racing engine, the top ring end gap should be opened up to as much as .006 or .007 inches times the bore diameter. Now we're looking at a 2nd ring end gap of .024 to .028 inches in an engine with 4 inch bores.

86mm stock bore = 3.38582677"
.023-.024 recommended ring gap

NOTE: These recommendations are rules of thumb only. Always follow the end gap specifications recommended by the piston ring supplier or engine manufacturer.

Basically, Mitsubishi has failed horribly at correctly setting up proper ring end gaps for many of the Evo X 4b11 engines. What this means is there is no room for ring and piston expansion. It does not take much heat and boost to have the piston ring ends in cylinder #4 to expand and touch, until one of the ends breaks off and cracks off a piece of the piston top and ring land. From the photo's below you will see that where the ring ends collide, one ring side breaks the uppermost deck of the piston and the other end of the ring breaks the lower land.

From the piston chunk usually disintegrates, however the heavy duty piston ring piece that just broke off is very strong and will not just melt away. It will batter the piston, spark plug, valves and head as it bounces back and forth. In many cases gets ejected into the intake manifold and sucked back into another cylinder to do more damage. If that was not enough, once it finally gets sucked out of the exhaust valve it will most certainly contact and destroy the turbine blades in the turbocharger, adding insult to injury and racking up an even higher repair bill.





Feel free to take a look at some of the pics we have documented along the way from information we have collected and documented over the last year or two of seeing the same occurance in many Evo X's across the country.