Checking Compression Ring Gaps - Engine Builder Magazine

Checking Compression Ring Gaps

Minimum gap tolerances must be observed in order to prevent the ring ends from butting together as the ring expands when the engine approaches operating temperature. It is recommended that you use a minimum gap of .0035? per inch of cylinder diameter.

Example: 4? (bore) x .0035 = .014 minimum gap.

Maximum ring gap is an important part of ring performance in that too much gap results in lost compression, power loss and ultimately poor oil control.

The chart in Figure 1 indicates the specifications for compression ring gap as outlined by the SAE (Society of Automotive Engineers) as standards for the automotive piston ring manufacturers.

An important fact to remember is that the manufacturers rigidly adhere to these tolerances and that the ring gaps are inspected in gauges accurate to .0001? at the cylinder diameter the ring is manufactured for. Any increase in the diameter of the cylinder is being used in, over the designated size, results in an increase of approximately .003? in ring gap for each .001? increase in cylinder diameter.

In order to check ring gap, the rings should be placed at the lowest possible part of the cylinder (below ring travel area) as this is the portion of the cylinder which is unworn and which the ring is sized for. Checking ring gap in the worn portion of the cylinder will show a gap increase in direct relation to the amount of cylinder wear present. The illustration in Figure 2 shows the effect of cylinder wear on ring gap.

Figure 2 graphically illustrates the effect on ring gap that cylinder wear has. You will notice that the tapered cylinder has .012? wear at the top portion of ring travel which increases the ring gap .036?. Therefore, it is entirely possible for the ring to have .061? end gap in that portion of the cylinder and be within recommended manufacturing tolerance.

For this reason, it is recommended that you use a maximum of .003? wear per inch of cylinder diameter, not to exceed .012? for successful reringing. If a cylinder is worn in excess of this it should be rebored and the proper oversize ring installed.

Checking ring gaps can give an approximation of cylinder wear as well as keep you from installing wrong sized rings for the application being rerung.

–Tech Tip courtesy of Hastings Piston Ringsfigure 1 indicates the specifications for compression ring gap as outlined by the sae.Figure 2 shows effect of .012? cylinder taper on ring gap in 4-inch bore.

You May Also Like

Jesel Certified Performance Rebuilds

Engine components are serious investments for any racer and maintaining that investment could be the difference between winning a championship and losing it.

Now that the race season is over, it’s a great time to freshen up your engine. Many shops are calling on their race customers to get a jump on that rebuild work before things get crazy again quickly in early 2023. One rebuild service folks with Jesel components might want to consider is taking advantage of Jesel’s CPR department to inspect, update and rebuild your Jesel rockers, lifters and followers.

Going the Extra Mile with Cylinder Head Porting

It’s not just the port work alone that creates spectacular cylinder head performance. The most critical areas of a cylinder head are those which pass the most air at the highest speed and for the longest duration. Your bowl area, the valve job, the throat diameter, and combustion chamber are all crucial parts. 

Tight Tolerances and Building Power

As you ascend Mt. Everest, you reach an area called the death zone. Once you climb high enough, the margin of error becomes perilously thin. That death zone also applies to engines. As the horsepower per cubic inch and rpm increase, the margin of error decreases. 

CNC Update: Features and Automation

Precision is key when it comes to automotive parts; the complex designs of connecting rods, pistons and rings, blocks, cylinder heads, and other parts require super tight tolerances that are getting more and more difficult to be met by hand or with other machining processes outside of CNC.

All Things Media Blasting

Engine building is a segment of the automotive industry that has always been ahead of the curve in media blasting, and no matter the engine shop, cleaning equipment is a common bond.

Other Posts

Engine & Hub Dynos: Necessary Tools and Additional Revenue

Being able to see the horsepower and the direct correlation to what is lost in the driveline is invaluable – dynos offer a myriad of benefits for the modern engine shop.

November 2022 Shop Solutions

November tricks and tips for the shop!

Shop Solutions October 2022

When machining on the CNC mill, it’s necessary to blow the flood coolant and chips off the parts for inspection. I tried a tool holder mounted fan, but it wouldn’t get all the chips and coolant out of the deeper areas.

Could Engine Oil Soon Contain No Oil?

The trend towards ever thinner engine oils is an effort to reduce fuel consumption and emissions. How far can it go?