Do You Need New Pistons When Putting that Engine Back Together?
Breaking out the precise measuring tools will help you decide if you need a new set of pistons.
It’s never fun to have to tear down an engine shortly after a rebuild. Regardless of the work being performed, if you have a high power engine, or one with significant mileage, you’re going to want to think twice before shoving those old pistons back in the block.
JE Pistons has a blog which goes into fine detail on many of these points, but on the outset, you can visually inspect each piston for obvious damage. However there’s other signs of trouble you should look for. In fact, they inspect a piston that had minor damage on it from a dropped valve, but otherwise visually checked out fine.
Piston skirts will usually have some minor visual wear, but that’s OK, as it is deep scoring that’s of higher concern. Ring lands can take the brunt of some high-compression and turbocharged engines, which transfers some material from the piston to the ring itself, called micro-welding. Depending on your application, a feeler gauge should be able to show the correct clearance around the entire top ring land. However removal of the ring itself may show evidence of pitting that can’t be seen while it’s on the piston. In this case, you’ll need a new piston and ring set.
Another visual to look at is inside the piston itself. Any unusually dark or purplish coloring will reveal that the engine was running excessively hot. Not only is this going to deteriorate the strength of the piston, but also gives an indication that perhaps air fuel mixtures might not be ideal.
Things that can’t be seen are even more important. Pin bores are an indication of a piston that’s ready to quit. If the pin takes a significant amount of force to extract, its time for that one to go. Taking a measurement of overall roundness will also help. If the engine was professionally built before, you should have pre-build measurements to compare.
Additionally, you can measure the piston skirt to see if it has collapsed slightly. Visually it might look OK, but a proper measurement will be the true test.
So if you’re doing a build and only want to build it once, paying special attention to
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