Hey so this is my first post on here so please be gentle, i have a 91 with an eBay eBay turbo on the stock bottom end. It has always had low compression of 200,200,150,100 front to back and recently started egregiously burning oil so I pulled the stock motor to do a postmortem inspection while rebuilding the new block and found pistons 1, 3 and 4 had catastrophic ringland failure. The pistons show no signs of detonation either and bearings look fine so I’m at a loss for the cause. I do suspect it’s been overheated in the past because the previous owner did do a head gasket. I’ve never heard of ringland issues on these engines and as far as I know it’s the stock bottom end from 1991. Does anyone have experience with this kind of failure and what know what might have caused it? Thx.

 

It could be ring butting or you just exceeded the cast piston strength. The rig lands have a tremendous amount of force on them, it’s not intuitive. IMO the pistons are more of a weak point than the rods. The rods are at least forged.

 

Those rods are forged?

 

what...

rods are not forged and are generally the known weak point in the BP engine...

pistons will withstand far more than the rods generally.


And to the OP: detonation can take out ringlands like that with no evidence on the piston crown. Subaru turbo motors are notorious for it.

 

Not to cause further thread creep, but I'm fairly positive the factory rods are forged, albeit weak. You can tell by the larger amount of flashing which is common with forged components (the rough spot in the middle of the rod) while cast parts will typically be much smaller. This is from the excess material from the rods needs to disperse under the forging process (see attached picture) while cast parts only show a small line where the molds meet. Forged doesn't necessarily mean aftermarket or strong, just stronger than the cast equivalent due to better grain structure and the lack of potential voids.


Regardless, the rods under normal conditions are going to be weaker than the pistons, but under detonation, the piston could be weaker.
Lastly, OP what were your AFRs, timing, type of gas used, and boost level? Excess heat from lean mixtures could also cause ring butting

 

Thank you Watterson. You saved me the explanation. The large forging line is the proof of the manufacturing process.

The load from speed, inertia is always greater than the compressive load. Many of the failures for this application that I have seen are bent in the direction of rotation which is a telltale of an over speed condition. These rods are good to 200-250hp as long as you don't miss a shift. They probably are good to more as long the max rpm is less than 7krpm. Piston weight is also a factor. Steel has an infinite fatigue life and aluminum does not. The cast pistons might be stronger for fewer cycles but for a longer service scenario they are weaker than the rods.

 

Almost always pre-ignition starts from detonation of the end gases around the perimeter of the bore/piston. This will set up a frequency that will degrade the boundary layer and overheat the peripheral areas of the piston. The piston has carbon on the sides that would have burned off. These failures are from overloading and high cycle fatigue.