So you have a brand new piston in front of you. How do you check the ring-gap? We show you how in this short lesson.
New Pistons and Cylinders
We have used a variety of pistons on these air-cooled engines. Today we have an aftermarket set of pistons to check. The factory piston and cylinder is a Mahle brand, and they are a great choice for the engine, but sometimes price and availability are a factor. When checking the ring-gap on Mahle sets, we have never had any issues and they have been perfect.
This set of pistons and cylinders are aftermarket, with a JE piston, and JE rings, with an AA barrel. Normally new sets come with pistons, rings, cylinders, wrist-pins, and wrist-pin clips. If you are using old pistons and cylinders, if they measured up, then you would have kept the parts together as a set.
New pistons and cylinders need to be cleaned and dried before measuring. Because they have not run together yet, you do not need to keep them as sets.
To see how we checked our old pistons and cylinders, check out our articles.
Checking the ring-gap
Setting up the rings in the barrel is how you measure the ring-gap. To set the rings up correctly, they need to be level in the barrel. To do this we use our depth gauge again. If you had a piston with an even lip you can use it to press down your rings.
Once the ring is in the barrel you can get out your feeler gauge set. Each engine has its own ring-gap dimensions, so check the model your working on. For this two-liter air-cooled Porsche engine, the ring-gap is 0.15mm to 0.45mm
Start with the smallest feeler gauge, .015, and push it in and feel the gap. Work your way up until you can feel the drag on your feeler gauge. So in this case we have approximately a .325 ring-gap, well within our usable range.
What to do if the ring-gap is too big or too small
If your ring-gap is too big, stop! Something is wrong. Recheck the piston is the size you odered, check your rings, check your cylinders. Nothing can be done if the ring-gap is too big, you will have to order a new set of rings.
If your ring-gap is too small, there are ways to set the correct gaps. You can use a ring-gap filing machine to size the gap. The edge of the ring needs to stay square to the ring face, so using a hand tool is not recommended. Be careful as you would have to replace a ring if you go too big!
Putting Piston Rings onto the Pistons
We start with the oil ring assembly. The waffled ring acts as a pump, pushing sump oil back through the holes in the piston to return to the engine. On the top and bottom of this pump ring is an oil scraper ring. These rings do not have an upside, but we want to stagger the ring-gaps.
After the 3 piece oil assembly, we like to install the 2nd ring or compression ring. This ring has an upside and a downside. The ring may have the words, up, or a dot, to indicate the top. If there are no marks, then the profile will show you which way to set the ring. If you look closely at the second ring, you will see a step down on the ring. The step down will face down on this second ring
The top ring has a rectangular profile. They may not be marked up or down, but you can take a look. Install the top ring into the last ring-land. We will set the ring-gap position as we install the pistons into the cylinders, so for now, the rings should move freely.
Up Next in the Air-Cooled Engine Build
We have checked our ring-gap, now it is time to marry the pistons and cylinders to the engine. There are different ways to do this job. We are happy with the way we do it, it may be the method you use, or it may be new to you!
After the cylinders are on it is time to move onto the heads.