Motorcycle piston ring end gap is one of those measurements that if not done right can result in some pretty catastrophic results. Is getting it right rocket science? Absolutely not. In fact, versus valve measurements and motorcycle cylinder piston clearance, it is probably the easiest of the three to take. All you really need is a feeler gauge.
That said, piston rings perform a lot of critical functions during engine operation and the measuring of motorcycle piston ring end gap should be taken very seriously. Remember that piston rings expand during operation, so don’t just eye-ball things – get your service manual and know what the end gap should be before even getting started.
The end-gap specification is designed to minimize the escape of combustion pressure (i.e. by having the minimum gap possible once in place), but also so that as things heat up and expand the ends of the piston ring don’t butt against each other.
The latter situation (where the ring ends hit each other during operation) can result in a ring breaking inside the combustion chamber and this is a true nightmare.
Measuring Motorcycle Piston Ring End Gap
To actually measure piston ring end gap, place the ring inside the cylinder and square it up (make sure it isn’t in there at an angle). The squaring up can be done using the piston. Insert the piston in the cylinder and push the ring an inch or so along the cylinder diameter.
Remove the piston and the ring should be sitting there squared up inside the cylinder in the position it will be in during engine operation. Grab your feeler gauge and measure the space between the ends of the piston ring.
If the gap is too big to begin with, you need to buy the right size rings.
If the gap is too small, you can carefully file the ends a little bit at a time until the gap is within specification. Do a very small amount of filing at at time, reinsert into cylinder, and check the gap again. Try not to file at an angle, but rather so that when you compress the ring in your hand, the two ends come together cleanly.
Other than that, there really isn’t too much to motorcycle piston ring end gap. I told you it wasn’t overly complicated.
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