Get Some Release

Installing Compression Releases

Installation of the releases requires removal of the heads, which Eric Bennett of Bennett's Performance, in Signal Hill, California, accomplished in no time.

Here, we got a good look at the compression release in the closed or operating position. When the cap (between Bennett's left thumb and forefinger) is pulled away from the body, the engine can operate normally.

When the compression release is open, pressure can flow through the valve. This allows the starter to bring the engine up to operating speed before the valves close and normal compression is realized.

Just as Bennett finished removing the heads, his old pal and chopper builder, Kelly McKernan, of Full Bore Kustom Choppers of Tucson, Arizona, showed up and volunteered to walk us through the machining steps that would be performed on the heads. After checking the specs on the heads, McKernan determined the head would need to be tilted to 35 degrees on the adjustable angle plate. After he secured the head and locked the plate, he visually confirmed the 35-degree measurement would place the compression release parallel with the spark plug.

Next, Kelly clamped a dial indicator to the mill's spindle. This allowed him to rotate the indicator by hand around the plug in order to position the spindle directly over the plug. Once satisfied with the machine's position, he cranked the table inward (along the Y-axis) a total of 1 inch. This placed the compression release in its correct position -- far enough from the spark plug, but still well inside the combustion chamber.

In order from left to right, all the tools needed to make certain the compression release is installed properly. At the far left is the compression release, then, 1/2-inch end mill, center drill, 1/8-inch twist drill, 3/4-inch end mill, countersink, 0.354-inch twist drill, M10X1 plug tap.

McKernan began with a 1/2-inch end mill to create a flat surface for...

...a center drill to start a hole.

From there, a 1/8-inch twist drill spinning at 1000 rpm was used to drill a hole approximately 1-3/4-inch deep. McKernan knew he'd gone far enough when the drill broke through to the combustion chamber.

Next up was a 3/4-inch end mill used to create a smooth surface for the sealing washer to mate against. This bore also creates clearance to allow a 7mm deep socket to install the compression release in the head.

A 0.354-inch tap drill is used to drill a 0.650-inch deep hole for introduction of a M10X1 tap -- that's right, a metric thread.

Before the tap was introduced to the hole, McKernan gently broke the edges at the top of the hole with a countersink. This allows the tap to start more easily.

The tap was started from a position in the chuck...

From there, a T-handle was used to be certain the threads made it all the way down to the bottom of the hole without any damage.

Before the head was separated from the angle plate, McKernan test fit the compression release to be certain everything fit just right. It did -- one down, one to go.

This is what the finished head looks like from the top.

And this is what it looks like from the combustion chamber. As you can see, the hole is small enough to keep the head's structural integrity without much effect on the geometry of the combustion chamber, and, at the same time, it stays far enough away from the spark plug, valves, and cylinder walls.

With both heads modified, Bennett wasted no time reassembling the top end.

Just to prove how much of a difference the compression releases make, we asked Bennett to perform a compression test in both the closed and open positions. With the releases closed and the plug wires removed, the engine was cranked and the 170-pound reading was recorded.

A second reading was taken while the compression release was held open. This time a maximum pressure of 115 pounds was observed. By adding the release, cranking pressure was reduced by almost a third.

All buttoned up and ready to run. Just depress the cap on the release before you start the bike -- you'll be glad you did.

Starter stall: It's that annoying sound emanating from your motorcycle, and you've heard it before -- you hit the starter button and hear arrrrrra. You think to yourself, is my battery going belly-up? Or maybe it's the starter. You let off the button, and you wait a second to try it again; this time the engine rotates slowly at first, then gets up to speed before firing. You let out a sigh of relief and wonder what you can do to eliminate this problem.

Just what causes starter stall? There are different elements that combine to put serious strain on your starting system. Things like getting the engine components moving quickly and, of course, compression.

Something can be done to help alleviate the last one, compression. That's the reason for the noise you hear as your starter stalls. The engine parts start to rotate, but then slow down as the compression begins to build.

Compression is one of the main items contributing to battery and starter failure; and compounding this problem is the proliferation of high compression, big inch engines.

A compression release (they are installed in pairs, one per head) is a small valve which threads into the head from the outside. At the bottom of its threaded mounting port is a 0.125-inch hole that enters the combustion chamber near the spark plug hole.

Inside the release is a valve and seat arrangement similar to the intake and exhaust valves in your engine. An internal spring in the release is calibrated to close the valve after the initial compression stroke of the piston is completed.

With a cost of around 75 bucks for a set (plus machining, gaskets, and assembly), some serious consideration would be in order when you look at the cost of battery and starter replacement.

While installation is pretty straightforward, it will vary slightly from engine to engine. It would be wise to consult your local shop or dealer before deciding to go ahead.