How do I measure the port timing on my engine?

Although this information was compiled with the 2 stroke model airplane engine in mind, it will apply to just about any 2 stroke engine.

Measuring the Exhaust Port

Measuring the Intake Port

Measuring the Transfer and Boost Ports

Measuring the Piston/Head Clearance, for Compression

First, you need a degree wheel. If you are CAD fluent, you can make one for yourself. CH Ignitions always provides one when you buy an ignition system from them. Alternatively, you can download a Degree Wheel here. The downloaded degree wheel has the advantage of being 360 degrees. With the CH wheel, it's only 180 so you have to turn it to measure the intake port, after you've measure the exhaust port.

Now, you need to attach the degree wheel to the engine. I take a broken prop, and cut both blades off. This allows you to use it as a spacer, and mount the degree wheel on the crankshaft. Tighten your prop nut tight enough that the wheel won't slip unless you want it to. You also need a reference pointer. I used a large metal paperclip--the kind with handles, sort of like a clothespin--and soldered a stiff wire to it. You can clip the paperclip to a cooling fin, and bend the pointer so it's pointing towards the center of the degree wheel. The whole idea is to be able to accurately and repeatably measure the rotational position of the engine. The more accurate your clip is, the better your numbers will be.

It's time to meaure now! Take the spark plug (or glow plug) out, so the engine turns easier. A flashlight will make measuring slightly easier--if you shine it in the spark plug hole, you'll be able to see the ports better. We'll do the exhaust port first. Turn the engine counterclockwise (the direction it spins the propeller) until the piston top is going down, and just opens up at the top of the exhaust port. Write down or memorize the number on the degree wheel, or turn the degree wheel (but not the engine) so the pointer is at zero. You want this point to be the point at which the gasses will just be able to flow through, with the piston top at the top of the port. Now, turn the engine to move the piston down, and keep turning until the piston has gone back up, and stop just as it reaches the top of the exhaust port. If you set your pointer at zero, you now have the exhaust port timing--usually between 140 and 155 degrees. Your reading may be different; I've had a Husqvarna weed eater as low as 120 degrees, and a 26cc Homelite chainsaw as high as 170. Typically, lower timing numbers are for lower rpm. The higher numbers, like 170 degrees, are typically reserved for use with a tuned pipe, although this particular chainsaw was totally stock.

You can repeat this step for the intake port, if it's a crank timed engine and not reed valved. The difference between the intake port and the exhaust port measuring is that the intake port opens as the piston moves up, and you measure it from there until the piston goes up, comes back down, and then closes the port. Most 2 stroke gas engines have this measurement between 115 and 130 degrees, although some weedies are as low as 110. My G45 measured out at 145. If you are measuring a rotary ported engine, rather than piston ported, you will still measure from the instant the port opens until it's fully closed. The timing will be significantly different, though--usually between 180 and 195 degrees. 

The reason rotary valved engines have longer timing is because they don't open until the piston is moving up at BDC--creating a vacuum in the crankcase--and they close just after the piston reaches TDC. A piston ported engine, on the other hand, must have the port open for a symetrical amount of time before and after BDC. It's a compromise to allow cheaper manufacturing.

Now, do it again for the transfer ports. Usually, there are two mirror image ports, one on each side of the exhaust port. If the engine is a reed valve engine, or if it's crank timed with a rotary valve (like most glow fuel engines), then there may be a third transfer port that's opposite of the exhuast port. This third transfer port is called the boost port, and it's common to have different timing than the other two transfer ports. Common numbers for transfer ports are 105-125 degrees, and I've seen them as low as 100 and as high as 130.

Now you've measured the port timing on your two stroke model airplane engine. Next, you'll want to measure the piston/head clearance. I use a piece of solder stuck in the spark plug hole. Put it in untill it hits the cylinder wall, and turn the engine. The piston will come up and flatten the solder against the cylinder head. If your solder is too small, then fold it over and twist it a couple times. After you've flattened the solder, you can measure it with calipers. You want an absolute minimum of .025" clearance. Less than that, and you'll be flirting with disaster if you get any carbon build up. This information is valuable if you want to bump up the compression of the engine.  

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Revised: October 05, 2001 .

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