It's crazy that only those 4 valves on #5 were bent. I wonder how that happened? Only thing I can imagine is with the engine being starved of oil, #5 cylinder got a little too hot, causing the metals to expand just a little too much during a heavy load scenario and the piston kissed the valves at TDC. Interestingly enough, the hole on the piston top may have been cause by pre-ignition which can happen if the temperature is too high.

It sure is nice seeing a rebuilt 6G72 though. I hope you have fun assembling it, I can't wait to do the rebuild when mine finally goes. I'm only at 215K and my oil analysis came back clean.

Also, with everything being clean as a whistle now, I'd invest in an oil catch can setup. Ever since I installed mine in my 98 sport, there's no more oil dripping out of my plenum when I pull it off and the intake manifold is staying clean now. I actually had trouble removing the cloth plenum gasket because the oil hadn't completely saturated it lol.

 

The damage to the valves is so small, that I think it is related to all the aluminum bits flopping around inside the cylinder when the piston blew up and not to the piston impact. Visual examination of the piston did not show any valve marks on it. Oil starvation would manifest much more acutely in the premature wear of the main bearings. Those look nearly pristine and show no oil starvation or overheating at all (I'll get some pics of the old bearings). The piston damage is definitely related to pre-ignition detonation. I was poking around on the interwebs looking at different things and came across this video: Just fast forward to 5:52 That photo looks just like my #5 piston Still tracing down what caused this pre-ignition event. Working theory is - severely fouled injector on #5 cylinder delivering significantly lower amount of fuel, starving one cylinder of fuel and resulting in very lean condition. Can't wait to see results from the injector testing. I'm not ruling out bad batch of fuel ether. I'm going to drain all the fuel out and use it in my other vehicles as a precaution. I'll get 5 gallons of fresh Premium into the tank to get going.
I like your idea of the oil catch can. I've been contemplating that mod for some time. This might be a time to do it. Which one do you have and how did you plumb it? Mind posting the pictures?

 

I just used a generic $25 kit off amazon. As far as routing goes, I'm still trying to figure out a permanent setup. As it stands I have it resting in a vacant spot on the passenger side fender well so there's tubing being routed as neatly as I can to the other side lol. Here is a picture of my dirty engine bay. I haven't had the time to clean it or find a proper mounting solution. So far my ideas are to either do a overflow tank relocation/delete and mount it there, or mount it somewhere all the way on the passenger side.

UPDATE: Picture below was before I redid the catch can routing

 

Alright, seeing that catch can resting there again got me motivated to come up with a solution. And here it is!


 

Hunter.......that block is looking "sweeeeeeet". Great that he assembled it, and found those slightly bent valves.

I watched the fuel mixture ration video....interesting. Do these Monties have a computer set up so it actually does change the fuel/air mixture ratio depending on temperature, rpms, etc.? Dont want them to run any lean for sure. My 94 with the twin cam (6G74 I think) calls for Premium fuel......I wonder if using the higher octane would actually extend the life of these engines, or if regular (non ethanol) is fine.

Bet you cant wait to get the engine together and installed, and then that first sweet test drive.

 

I'm sure these computers do have fuel delivery tables for all sort of parameters depending on load, engine rpm, etc. I can't tell you more, since that usually happens inside the "black box". People who fiddle with actual tuning can access that data, but we mere mortals can't for now. At least until someone figures out how to hack into the software

 

I like how you mounted it after round 2. Nice easy access. I think I'll go with similar set up too. Looking at how much crank case gasses get recirculated into the intake (judging by the amount of residue and soot in the intakes) I'd like to mitigate that on the fresh rebuild.

 

Another present showed up on my doorsteps today. Got my injectors back from service. They are looking AMAZING!!!! They cleaned up really well (internally as well as externally).. The flow after service is exactly the same on all 6 injectors, so I'm really glad I held off on purchasing a set of new ones. So, that is the good thing, the bad (?) thing is that the pre-cleaning condition of the injectors did not solve the engine blowing up mystery. When I sent injectors for service, I labeled them with the cylinder number and requested pre- and post- cleaning evaluation of spray pattern and flow. I explained that I had an engine failure due to suspected lean run, but I did NOT say which cylinder had failed. I figured this way there will be no bias introduced into the initial evaluation. Well... results are in and of all injectors, #5 is the ONLY one which tested exactly at spec flow rate BEFORE cleanig. #2 and #4 were the lowest (6-8% below normal flow rate) but the fella who did the evaluation and cleaning said that the reduction in flow was not sufficient to cause the failure I described. I'm confident now that this rules out "running super lean" as causing the failure. Reading more on the web and seeing some more photos from different failures, I'm more and more convinced that the failure was caused by detonation. I'm starting to suspect a bad batch of fuel I put into the car before the last trip. Just as a precaution, I'm going to drain all of it out and use it elsewhere before firing up the engine once it's installed.

 

Hmm, interesting. My OEM injectors are green. Are these OEM as well?

 

Yes, these are OEM. Original since the day it was made. What year/model is yours? Different engine displacements are likely take different injectors.