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The engine I'm rebuilding (Skoda Estelle/120, 70's, RWD, rear mounted engine) was manufactured with two different pistons, depending the fabrication years: one piston kind has 4 rings and a cut from the oil ring to the bottom, at one side, and has a flat head. The other piston came with 3 rings and reinforced area below the pin at both pin holes sides, and has a slightly dome head. Both pistons seems to be interchangeable in the same engine, since they have the same overall measurements.

So question is: which design is better?

EDIT: the 5 rings/flat crown piston weights 267 grams, the 4 rings/domed crown piston weights 280 grams.

enter image description here

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There are trade offs, so no clear answer here.

First of all, just to be anal and clear something up ... what you have is actually a 4-ring and a 5-ring piston. The oil control ring at the bottom actually consists of two rings and a spacer.

The main trade off is, the 4-ring piston will have less drag on the cylinder walls, but the 5-ring piston will seal better. A better sealing piston will keep more power in the cylinder, but you'll see additional frictional losses. Most engines I've seen with the 5-ring setup are for diesel engines.

With the piston which is slightly domed, you'll have a slightly higher compression ratio. You'd have to do some measuring to figure out how much the difference would be.

I don't know if you are assuming the pistons are virtually the same or if you have the specs on both. One of the things you need to check is the piston height difference. To do this, place a single wrist pin in the pin bore on both pistons. You'll then be able to measure how much taller one piston is from the other. This will give you a better idea of how much one piston will stick up higher from the other. This may be important when installed, as the taller one may stick out of the bore above the deck surface or interfere with the head (I doubt it, but you should check ir).

One other thing to think about is the weight of the piston. The lighter one may have a slight advantage over the heavier one as it would be able to allow the engine to rev a little higher without fear of something coming apart.

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    Both pistons have the same overall dimensions: same height, diameter and pin location. I will check their weights and check which one is heavier – Aram Alvarez May 3 '17 at 2:05
  • Some engines set the compression ratio by being built with slightly different pistons. A small dome in the top can make a surprising difference to the compression ratio. Even the thickness of the head gasket has an appreciable effect. – John U May 3 '17 at 10:33
  • @AramAlvarez - From the picture I can tell the pistons are very similar in shape and size. The thing is, and I bet you realize this, when talking dimensions with engine parts, we are usually looking at +/- 0.001" making a difference (if you use metric, +/- 0.0254mm difference). This isn't something you can eyeball and make an assumption about ... it's something you actually have to measure with micrometers to ensure accuracy. The only reason I bring this up is, I don't get the warm and fuzzy you are actually measuring these things. The piston height could be off by .01" and you'd never know it – Pᴀᴜʟsᴛᴇʀ2 May 3 '17 at 13:15
  • @Paulster2: correct, but in that engine's lifespan they only made little design changes but cylinder liners, crankshaft, camshaft, head chambers, etc were kept the same. There are about 2 or 3 engine versions but all the parts are interchangeable. – Aram Alvarez May 3 '17 at 14:23
  • @AramAlvarez - No worries. Not really knowing the history of the engine design leaves me a bit anal when it comes to such things. You'll still want to know if any of your parts are within tolerance, especially the clearance between bore and piston. This, too, may affect your decision as to which piston to use. – Pᴀᴜʟsᴛᴇʀ2 May 3 '17 at 14:28
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Use the piston from the "newer" design ie most recent - the engineers most likely made changes to reduce or avoid certain issues.

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Don’t mix different pistons; particularly don’t mix pistons with different weights. The crankshaft is balanced for a given piston weight as well as the pistons being balanced as a set.

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