924Board.org

[WEASEL149]

Looking into pistons and piston rings over the last number of weeks I found myself asking the question of why so many top compression rings break in these engines.

I don't believe that the majority break from detonation anymore.
The reason I say this is that although all my top rings were shattered, my pistons are literally like brand new. The crowns are perfect and so are the ring lands and grooves. There is no sign of detonation or erosion anywhere and there shouldn't be as I regularly ran my car at 0.8bar WITH 951 intercooler and correct fuelling (98RON).

So reading up on ring material there are basically 3 materials:-

Grey Cast Iron - fine for the majority of everyday cars but try to bend it or subject it to major load and it snaps or shatters. It also doesn't like extreme heat so is not ideal in turbo/super charged engines.

Ductile Iron - approximately double the strength of grey cast iron and you can literally bend and twist rings made from this material. Can be used in forced induction applications.

Steel - stronger than ductile iron and also makes an ideal choice for extreme applications and/or when you want to run thinner rings with less tension.

If you take a cast ring and dangle it from one finger then tap it with a metal object you'll just hear a dull thud.
Take a ductile iron ring the same way and when you tap it, you can hear it ring or resonate.

Using the above info I took a top ring from a spare 86.5mm piston I have and tried the ring test - ring resonated.
I then proceeded to bend the ring and found that I could twist it round and round and bend it back to shape without it breaking.

When I tried the same tests on what was left of my own 87mm top compression rings the ring wouldn't bend AT ALL. It merely snapped into 3 pieces.

Not only that but the 86.5mm ring had a bevel/chamfer on the inside. The 87mm snapped ring was a regular rectangular profile.

This actually brings up more questions than it answers really.
Mahle UK only sell separate ring sets for 86.5mm.
87mm and 87.5mm rings can only be bought now WITH pistons.

So has someone changed the material spec at some point without considering the implications?
Were my 87mm rings aftermarket ones and therefore the incorrect material spec?
Are the rings supplied with oversize Mahle OEM 931 pistons of a different material spec?

Please be aware that this only applies to the top compression ring. All the second rings I've tried have been cast iron because that's all they need to be.

This picture shows the difference - the top ring being the ductile iron one:-



A link for anyone interested:-

http://www.aa1car.com/library/ar293.htm
_________________
1979 UK 932

[gegge]

Have you tried the Alfa Romeo 33 1.7l piston rings? Ok, they are 87mm and need minor adjustments, but the top ring is chromed and secong is moly. Most likely cheaper as well. I belive it was "Racing" that used them in the beginning. He was the first one to use VW flat-tops by the way.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[MikeJinCO]

Many eons ago I worked for a large US piston ring manufacturer called Ramsey which is now part of Sealed Power. Back then(the 70's) all OE rings were ductile iron and are probably the same now. Replacement rings are a totally different story, even the OE replacement parts. The quality is not nearly as high and parts that fit sometimes got substituted for the correct parts. Even the GM replacement parts, particularly oversize, were not nearly as good quality(in their case the roundness and radial pressure distribution, not material) as OE as the replacement parts department is their own profit center and they don't have to meet EPA standards.

Some of the small manufacturers were absolutely dreadful, back then Hasting rings were barely fit to put in a lawn mower. When you get to something like our engines(old and very limited production) unless you go to a specialty manufacturer you are going to get whatever they can scrape together. Obviously the specialty piston manufacturers should do better, but who knows.

Bottom line either go to a specialty manufacturer or at least find some OE replacement parts. A good top ring for a KIA is probably as good as the top ring in Porsche these days.

From a tour I took several years ago the method of manufacture has changed very little in in all these years. I'm sure the radial pressure problem has been greatly reduced with the increased computerized design and pressure measuring capability not available back then.
_________________
Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[MikeJinCO]

Have to modify my statement a bit as my memory must be failing. Hastings had the foundry where we got all of our castings. There were several small manufacturers whose names I don't remember that made the garbage parts.
_________________
Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[WEASEL149]