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Breaking piston rings on re assembly- a tip


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Just watching the assembly of an engine on u tube and it jogged a memory. I use the type of piston ring compressor that is basically multi sheave thin sheet steel wrapped in a circle with a ratchet device on the outside that is tightened with an L shaped key. It worked well for a while and then I managed to break two successive piston rings whilst using it. I couldn’t understand why. I had tightened the ratchet as much as it would go, even using the small wrench from my socket set.
The answer was, that as an amateur, the tool probably only gets used once a year (if that) and the layers of steel had dried out preventing the tool from fully tightening. A smear of oil between the layers made an enormous difference. I recommend it if you use this type of tool

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I use a tapered piston sleeve which compresses the rings, but does *not* grip the pistons, this makes assembly easy and safe and incredibly fast, the chance of ring breakage is minimal and very little effort is required to push the pistons into the bore (one finger); if a ring snags (which is unlikely), then the resistance is easily felt. The top of the liner is tapered to allow entry with the rigs expanded. The sleeve is made from an old liner of the correct size, but Wiseco produce piston sleeves in a variety of sizes. These are very effective and cost less than a ring set.


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The problems with these are..

1) they grip the piston which means a lot of effort is required to insert the piston to the bore, (I've seen them tapped with the handle of a hammer), if a ring snags you wont feel it and will probably damage it

2) they are spiral wound and can easily displace in use allowing a ring to creep out just before entering the bore.

3) they are a faff to use as they have to be wound/unwound on every use.

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Me too, I had never broken a ring until I had the problem. Light lubrication does make a big difference. It allows proper tightening of the clamp (use your small ratchet to minimise the faff of tightening)and reduces ( but doesn't absolutely eliminate) the sticktion between tool and piston that oily mentions. In its favour,its (6?) times cheaper than the sleeve type and accommodates different size pistons which the sleeve doesn't. It's not a bad tool.....

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I use the expanding type and although it has shortcomings it suits my needs, also it's certainly a big step up on the split Heinz Baked Bean tin and jubilee clips I used 40 years ago on the first Mini engine I rebuilt!

However, if I was assembling as many engines as Oily does and particularly the same type, I'd buy a proper sleeve. It's a no-brainer.


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