Gapping Rings

Update by Alan Cesar to the Subaru Impreza WRX project car
Jun 19, 2013

Get a quality set of feeler gauges. If your spouse is patient enough to let you reappropriate the three seasons room to rebuild your engine, thank her profusely.
After you install the ring into the bore, line it up square by using a piston with a ring installed in the oil ring groove. Then take your reading.
A ring file is a special tool you probably won't use again, but you need to spend the $50 on one if you plan to rebuild an engine.
Use a kitchen whetstone (about $5 on Amazon) to deburr your rings after filing. Then check your gap and file again if necessary.

One critical step in assembling your new engine is setting the proper ring gap. The ring is a single piece of metal sized to fit precisely into your engine’s bore. As the ring heats up when the engine is running, the gap at the end gets smaller because the ring expands.

A properly sized gap is critical. If the gap is too small, the ends will butt together—that’s bad news. In that situation, the ring might buckle or even break. If the gap is too big, you’ll have lower compression and more blow-by. Obviously, too small is worse than too big.

You’ll need a good set of feeler gauges, a ring file and a whetstone. The ring file is the most expensive tool here, so it’s a shame it has such a singular purpose. But it’s what you need, so suck it up and buy one already. You can use a regular hand file, but the ring file makes it much easier to keep it square and really speeds up the process.

If you clean up the whetstone well enough afterward, you can use it to sharpen knives in your kitchen. This is the process we used to gap the Total Seal rings on our Subaru WRX engine.

[photos by Zoe Bowman]

Wicked awesome Scooby Rexes in your treeware mailbox. Subscribe to Grassroots Motorsports now.

Join Free Join our community to easily find more project updates.
View comments on the GRM forums
Sponsored by


Cooper Tires

Our Preferred Partners