OK, I've got a 91 Z28 with a 5.7, which I rebuilt a couple years ago and never really got it running that great. Today I found out why: I was running compression and found cylinder #5 was zilch, but came up to 30 psi using oil, so I'm thinking it's got a broken ring.

My question is, what are my options? I really don't fancy rebuilding the bottom end (other cylinders show good compression), is popping the head and dropping the pan (with the motor in place) to replace the ring a feasable option?

If not, can I pull a 350 roller short block from a truck out of the scrap yard to get me by? Not many Irocs with good motors in the yard around here anymore.

Just trying to get this thing running decent without dumping a ton of money in her, don't care if it's the fastest thing out there, so any advice is appreciated.

Thanks!

 

Well i wont hypothesize as to why you have low compression although like you discovered it would seem to have to do with the pistons sealing. Now can you do just that cylinder? Again for starters it could possably be done while the motors still in the car although that would be more difficult than actually pulling the motor. Now if your rebuild it you only rebuild what needs to be done so if everthings good but one piston ring then its just a matter of gaskets a set of rings just the real basics. So it shouldnt be too expencive again assumeing everthing else is ok. Next about getting another 350 from something newer you can do that as well as all the stuff thats on your SBC will bolt on a newer one as there the same block basically. Thats assumeing you dont get a LS or LT series motor. Now the question which is cheaper? Now if you get a junkyard motor you never know what your going to get may get a decent motor might get something that was run into the ground by a 16 year old and not maintained. Your motor on the other hand you know what your dealing with and if it was just rebuilt a year ago assumeing it was done right should be in pretty good shape and may only need the basics. It really depends on how much you can get a motor for, if you can get a good guarente on it, and what your personal preference on it is. Personally ide just go through the block yor have as it was just rebuilt and even if you get a newer motor youll still neet to take things like the heads (vortec heads wont work with the older intakes manifolds) and cam (otherwise you may need to reprogram the chip for it to run right) from your motor to the new motor just so it will run right. So really when you look at it that way rebuiling your block may only put you out a set of rings vs a new motor as eather way youll need a set of gaskets. Beond that though eather way youll probably end up pulling the motor why not pull the one you have now as tear it down see how much it will cost to get it back up to snuff and go from there.

 

Thank you for the response!

I think you are probably right on with repairing the existing motor the more I think about it.

As to just repairing it in place - I'm no stranger to jacking up a motor to clear the pan so I can get the piston/rod loose, and you mention repairing it in the vehicle is doable; what exactly is required? Does the subframe need to be separated from the body in order to gain the clearance? Or can just the motor be elevated enough to do it?

Thanks!

 

Well i would think again doing it in the car would be more trouble than its worth but if you were so inclined i would think that if you just jacked up the motor enough to pull the pan you will have access to the bottom end. No need to remove the sub frame or extreme like that. As long as you have enough room to remove the rod bolts your should be fine as obviously the pistons will only be able to come out through the top end of the motor. Now if you want to pull the crank while your down there you will probably have to jack the motor up a bit further but still possible. The biggest thing ide be concerned about is when replacing the rings you'll have to hone out the cylinders and all the shavings will fall onto the crank so be sure to clean it out throughly.

 

Thanks Rolling Thunder, I appreciate it! I'm popping the head in the net couple days to see exactly what happened. I think I might find a cracked piston since pre-rebuild the vehicle originally came to me with a rod knock, one piston had witness marks from hitting the head but I reused it anyhow. I have a hard time believing the rings blew off, but a cracked piston is definitely a possibility.

 

Yea losing a ring would be kinda tough and youde pretty much have to completely lose it to have no compression maby the cylinderbore is out of tolerance? I mean originally i was thinking maby a compression ring installed upside down or something but even that i dont think would yeild that low compression. It would be easy if it was a broken valve or something but thats probably not the case obviously as oil help build up the compression. A cracked piston is a possability i think would be tough but it will be interesting to see what could cause that catastrophic of a failure.