Tips on how to file a Piston Ring

Filing your piston rings

The gap in a piston ring allows it to expand into the bore of the cylinder, whilst the engine heats up. It does so without the ends of the ring touching each other. The catch here is, if there is too much gap between the ring, the ring will push against itself when it's hot. This will distort the ring.

By contrast, if the gap is too big, it will allow blow-by from the cylinder chamber due to there being a gap between the edges of the ring. This will severely impact the cylinders performance.

As it happens., the ring gap is so important and the tolerance so precise that the gap itself is measured respective to the rings bore. This ensures the most accurate fit possible, rather than just fitting a set of generic rings that are straight off the shelf made to manufacturers tolerances.

The ring gap is one of the few things you have complete control over when putting together an engine. Most other tolerances and clearances are set by the machine shop, but the ring gap you can yourself control. It's completely your responsibility to get right.

Not Enough Gap

Without enough gap between the ring ends, the ring pushes against itself when heated. The ring expands to the point where it touches itself and then has no where to go. With continued pressure and heat it expands to the point of distorting itself, bending the shape of the ring. When this occurs, the seal with the side wall is broken and combustion pressure lost.

Measure Twice, Cut Once

You'll be measuring over and over for each ring that you file. It's something you must be very careful with and as such you'll do repeatedly. You must be careful with both the setting of the ring and actually taking it's measurement.

To take the measurement, you must put the ring in the bore of the cylinder. Make sure you don't twist it when you're putting the pressure onto it to get it into the bore. If you push too hard you can bend it or break it. At the very least you'll deform it, thus making it useless. Put a bit of oil down the sides of the bore to help the ring fit in carefully.

Once it's in there, make sure it's oriented properly within the bore. This is important because firstly it'll be the spot the ring actually sits in when the engine is together but also because you'll be pulling the ring out and remeasuring multiple times, so consistent location is key.

Accurate Measurements

Make sure your rings are squared with the bore each time you go to measure them. Do this either with a ring squaring tool or just an inverted piston itself.

Sometimes people will use a torque plate when settings the gaps. This helps gets a more accurate cylinder bore dimension as it simulates actually having the head on the engine. This has been seen to help, but only to the tune of about 0.001 inch of a gap. So it really depends on whether that gap difference is worth it to your tuning.

When you decide what your final gap should be you can usually use the guide that comes with the rings. It shows a ratio in thousandths of an inch per inch of bore size. When making your decision it's always useful to use a measurement that's too big rather than too small, as it will avoid the ring butting up against itself.

It's important to be aware of the different gap specifications between first and second ring. Typically the first ring has a smaller gap than the second. This is because it's important to allow any blow by that might get past the first ring, to easily find a way out past the second ring.

The last ring you need to look at is the oil control ring. This ring is only to support the amount of oil flowing around, so the gap doesn't matter so much. Generally you don't have to file these down, so if you find you do it may mean you have the wrong oil control ring in your package.

File Carefully

The usual rules apply when filing your ring. Only file in one direction, make sure it's properly supported, make sure the ring end is square to the file and only file one side of the ring.

You can always take off more, you can't put any back

You can measure to your hearts content, but if you can't alter the dimensions, it won't mean anything! Filing a ring might sound simple enough but all the tiny tricks can make a huge difference in the end.

A big trick is to be sure to identify which ring is which. The top ring is always made out of a harder material, thus it will be slightly harder to file than the second ring.

This can be particularly tricky when you get into a rhythm of filing. If you're filing the second ring which is softer, you'll suddenly move to the top ring which is harder and is then less receptive to your filing efforts. Conversely, you might move from top to bottom and suddenly take off more ring than you should!

You need also to be sure not to file off the ring coating, as most of todays modern rings are coated with special material which you should not remove.

Dress the rings

Once you've finished filing it down to sure, use a jewellers file to remove any tiny burrs that may have been left behind.

Always keep your ring square to your file wheel and don't let it flutter. Any slight misalignments or movements will leave you with a useless ring.

Following these tips, working slowly and carefully, while measuring often, you can file your own rings like a pro, and will be sure to get the most out of your freshly rebuilt engine.