Engine`s gone

[Homeoboxter]

Originally Posted by JFP in PA

OK, lets start with the top ring. Based upon accepted standards for ring end gaps, based upon bore diameters: Most piston ring manufacturers recommend a minimum end gap of .004 inches times the bore diameter for the top piston compression ring. So for a 4 inch bore, the standard end gap would be .016 inches. As you have an 85.5 mm, or 3.366 inches. Using the suggested rule of thumb, which would work out at 0.004 X 3.366 = 0.013 inches, or 0.33 mm. So that ring gap is way too wide.

Mahle, the makers of these pistons have published ring end gap guidelines for the 2.5 and 2.7L M96 engines:

Mahle
Top ring-0,15...0,35mm....006"-.014"

2nd ring-0,60-0,80mm....024"-.032"

oil control-0,50-0,75mm...020"-.030"

Again, your gaps appear excessive. Overly large ring end gaps can cause a myriad of issues, from compression loss to oil consumption, and most importantly, excessive piston rock in the bore, which can result in ring land (the piston metal that separates the rings from each other) failures.

When a piece of ring land material breaks loose, it can get wedged between the piston and cylinder wall, stressing both. I would not be surprised to find that if you pulled the pistons out, you would find multiple ring land failures. I also noticed from your assembly photos that there were significant gaps between the rings and the cylinder walls, indicating oval cylinder liners, which would also exacerbate that piston rocking problem even more.


Lawyers often like to say that, "the devil is in the details". That is also very true for engine assembly.

Thanks for looking into this! We should have had this conversation 2 years ago.. As I can recall I could not find specifications for the ring end gaps so I just went with the new rings assuming they should be ok. Based on the values you posted from Mahle the old worn rings were within or just out of spec, the new ones had similar or slightly smaller end gaps. The picture shows an old side rail from the oil ring, it came out stuck and probably deformed from the ring groove where it was stuck in gunk. With the new side rail there was no gap between the rail and the cylinder wall. Also, the car was running strong, no smoke, no excessive oil consumption. I didn`t find metallic particles inside the oil filter at the last oil change.

I`ll see what`s inside, it will be a great learning experience.

[Gilles]

Hello Homeoboxter, bust just ran into this engine (not affiliated), but it seems that could be a great deal for the price, if it works out you could have with a nice powerplant for your Boxter :-)

https://rennlist.com/forums/market/1313957

[Homeoboxter]

Originally Posted by Gilles

Hello Homeoboxter, bust just ran into this engine (not affiliated), but it seems that could be a great deal for the price, if it works out you could have with a nice powerplant for your Boxter :-)

https://rennlist.com/forums/market/1313957

Thanks! Wow, indeed sounds like a great deal. However, I would have to replace or reprogram the DME for this and modify a number of things, and probably it wouldn`t pass the smog test easily. Not sure if it`s worth the hassle... It would be great though

2,500 mile 2006 Cayman S engine with low leak down test numbers.

Why would one do a leak down test on a 2500 mile engine? It`s basically brand new. And why would one swap an engine after 2500 miles? Just wondering..

[husker boxster]

Originally Posted by Gilles

Hello Homeoboxter, bust just ran into this engine (not affiliated), but it seems that could be a great deal for the price, if it works out you could have with a nice powerplant for your Boxter :-)

https://rennlist.com/forums/market/1313957

That engine was one of my options when deciding what to do with my bore scored CSS. He's dropped the price substantially since I was looking 1.5 yrs ago.

[Homeoboxter]

Hey All,

Last year I ended up selling the Blue boxster as a roller to a friend nearby who needed a donor car for his race car. After he took off everything he needed he gave the engine back to me so I had a chance to take a better look inside. I took a couple pictures before I scrapped most of the engine, here they are, just fyi.

Head off:



This is where the thick white smoke was coming from:



The other 5 cylinders looked totally fine.



The top of piston #1 broke off:




I don`t know how could I miss this completely during the rebuild, but one bank had been resleeved.



Here`s what the other bank looks like from the bottom:



I`ve read somewhere that some early bore scored blocks were sent back to Porsche in warranty and got resleeved. Later these engines had a tendency to fail. Perhaps mine was one of them?

When this engine blew up, it was a hot day and I was stuck in traffic, and the coolant temperature was higher than ever before. I had been going to clean the radiators and check the fans because I suspected they are not blowing at the highest speed. I never got around doing it. Nevertheless, now that I think about it, probably the engine got overheated. Another suspect is the aftermarket rings which perhaps didn`t like the Lokasil bore too much. Anyway, my theory now is that somehow the piston got seized for a moment and pulled the sleeve down, then in the next cycle the rings got stuck in the shoulder of the sleeve and that broke off the top of the piston. Also possible that that the rings broke first, hard to tell at this point. Just thought I share.

[husker boxster]

Classic example of trying to put a square peg in a round hole.