They need to be held in the piston, or they will walk out the end. The four most common methods of retaining them are:
1. Snap rings, or "stock circlips." These are kind of like a ring with the ends twisted in. To remove them, just sqeeze the ends togather with a pair of needle-nose pliers, swear and reach for an old screwdriver or such... OK, they aren't that bad - if one side is difficult to remove, the other is probably easy. They usually keep the pins in place too, but if the forces are great, the pin can push against the ends, which pulls the clip out of it's groove and hey, presto! Engine build time. This actually didn't happen all that often on stock engines that were properly looked after (though my friendly Volkswagen mechanic was familiar enough and the dealership he worked for used circlips.)
2. Circlips, these are pressed from sheet metal, and have holes for circlip pliers to get purchase in and remove (pictured.) These can be a little harder to remove, especially if they are installed backwards - "Hang on!" I hear you say; "backwards?" Yes, there is a right and a wrong way to install these. If you look at the profile, you will notice there is a bit of a chamfer from when they were pressed. The side that was on the bottom is a little wider and has sharper edges - this is the side we insert our circlip pliers from and will be on the outside of the piston when installed. You will note from the picture above, I have put one punched side up on the left and one "flat side" up on the right - the example on the right is how it will look to you from the outside of the piston. This is not only so you will be able to install them without finding why they are called dammit clips and also to remove them one day, this will give the greatest strength or retention. If they are installed the other way, extreme pressure could cause them to go concave enough for the chamfer to ride over the edge of the possibly worn groove and come loose. If they are installed as recommended, this same pressure will tend to push the ring outward, digging into the piston groove any more and they will not come loose in any normal engine operation. If they come loose after this, you probably have something seriously wrong with your engine - a very bad machining job on the case, or a dodgy crank.
3. Teflon buttons - the great high tech answer to you problem, without having to worry about details like installing things correctly! If you have a set, bin them. There is always going to be some kind of side pressure on the pin, which will be pushing at least one of these against the cylinder wall - the other will probably work it's way out there, too. These will then be constantly wiping oil from a patch of the cylinder wall, meaning a section of the piston and rings will not be properly lubricated and the cylinder will receive a little less cooling at this point. The buttons themselves are know for scoring the cylinder wall, though whether this is the actual button, or the above lack of oil, or grit and carbon building up on the button no-one can be sure. There may be a place for these in race engines that are rebuilt weekly, but leave them out of any engine you want to last - I have also heard of them disintegrating in turbo applications. The circlips work, they can be used in slipper skirt pistons, they don't interfere with pin or piston oiling and they are cheap. If you have already had Teflon buttons fitted, your pistons may be damaged beyond fitment of circlips, but you probably need new pistons and cylinders anyway.
4. Spirol-lok or similarly named products are actually like a helically-cut ring which is fed into the groove, then the pressure from the pin pushes them into a firmer position much like the circlip. I have not used these - circlips are cheap, they work and are easy to remove - but I see no reason why they would not be effective. My only concern with these would be removing them after the engine has seen a decent amount of work...
Sounds like good logic to me .Thanks
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