First theory: So supposedly the previous owner of my old Fiero rebuilt this engine... I wonder if he broke off a section of piston ring during reassembly. Then, due to the enormous 1st ring gap (as in 1" long), hot combustion gas flowed into the gap and burned a pocket in the piston.
Second theory: I installed a MegaSquirt engine computer on this engine, and Accel brand (apparently no good) injectors. The engine computer was running a tune fully created by me. So possibly due to a poor state of tune, that particular piston overheated and started to burn on the edge, creating a gap for hot combustion gas to enter. The hot gas would have then burned out a portion of piston ring. I don't know if a piston ring can burn out like that
I don't know if a piston ring can burn out like that
I didn't realize initially that there was a big chunk of ring missing. I don't believe there'd be any chance that a piece of hardened steel ring would burn away. I suspect there were some shenanigans by the previous owner when he "rebuilt" the engine.
It's not obvious to me but I should do some research on what Good and Bad look like ...
Let me add that I was worried the Kerosene leak-down test might have looked bad on cylinder #6 if the piston ring gaps happened to be aligned. When I took the piston out the ring gaps were well away from each other. At this point I feel completely stumped.
Pull the rings off of the piston and check the ring end gap while you’re in there
The gaps look so huge sitting on the table out of the bore. The top ring gap is 0.029" at the top of the cylinder and 0.035" further down, below the ridge. The middle ring gap is 0.025" at the top and 0.040" below the ridge. I didn't measure the gap on the lower (oil?) ring (which was 3 rings itself). Do these gaps seem decent vs specs?
Thank you for posting the specs and figures!!! Reading a bit on the subject I see the gaps can't be too small due to leaving ring expansion room at operating temp. The results of these next measurements seem impossible but I'm posting a picture to show what I did.
Top Clearance: 0.002 - 0.003 (yes, that is 2 thousandths of an inch.) The ring itself is 0.058" thick.
Middle Clearance: < 0.002 (yes, I can't get my thinnest gauge into the space between the middle ring and the groove.) The ring itself is 0.078" thick.
Oil clearance: < 0.002 (yes, same as Middle.) The little sandwich is 0.188".
So are the wrong Rings or wrong Pistons in there? And does this have anything to do with the leak-down failure?
[This message has been edited by Notorio (edited 08-16-2020).]
One would hope that NASA had a Quality Assurance program that would be able to tell that the 'course corrections' they were making were just making the situation worse ...
It seems like things are in spec and check out (It's super weird that everything checks out, yet the fluid leaked so much)...
You should do a hone job on that cylinder before putting it back together if you can to give those rings a fresh surface to seat to, the cylinder wall is looking shiny, maybe it wasn't honed properly by whoever was in there last and it didn't "seat" well?
Watch Uncle Tony here, he's got some good info in his videos... Here's some about honing and inspecting the cylinders...
Watch Uncle Tony here, he's got some good info in his videos... Here's some about honing and inspecting the cylinders...
Thanks for posting. I'll try that ring position test with the flashlight pointing up from the bottom to show out-of-round areas! I don't think I'm up to honing with gasoline ... WD-40 perhaps. Uncle Tony has a lot of good tips there.
Update: I looked for out-of-roundness with the top ring, using the piston to move the ring down about one inch at a time (per Uncle Tony), with a light at the bottom right beneath the crank. I ran the test twice: once with the ring gap clocked at Noon and once at 3PM. Both showed no light leaks around the ring edges. I can definitely see that the area swept by the bottom-most "point" of the piston is highly polished, with no remains of the cross hatch pattern (i.e. corresponds to region between the Red Arrows). This is about the only feature of the Bore that seems suspect to me.
[This message has been edited by Notorio (edited 08-17-2020).]
I don't believe there'd be any chance that a piece of hardened steel ring would burn away. I suspect there were some shenanigans by the previous owner when he "rebuilt" the engine.
