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I have a 98 GMC Sierra, with a 305 in it. I am considering Re-Ringing the pistons because I have already lapped the valve seats and with the rockers off, the cylinder 4 still comes to 0 PSI and cylinder 1 only has 45 PSI. BTW there isn’t a hole in any of the pistons. I was hoping one of you could tell me how long of a job this will be, (pulling the engine and re-ringing and putting new connecting rod bearing in it). I know everyone will say “just put a 350 in it, but I am content with the 305 and if I complete this it will be completely rebuilt. Thanks to everyone you gives me an accurate response.
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Piston rings are not the only reason to have low compression especially when you said 0PSI it got me concerned, also bad valves or valve seats can cause low compression, if there is not a good seat between the valve and the valve seat air will leak right past the valves and no compression, if you want to make your engine run better i would do more than just new piston rings… i would have some work on the heads done and piston rings as well as having the cylinders honed possibly bored… just doing rings… heck that could be just part of what causes low compression.
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how long it will take, i can’t tell you, depends on your experience working on engines, what kind of tools you have, ect… some guys can build a engine over night they are so good, some guys take several days. If i where to re build a 3800 for my car right now, so much love and care would go into every single peace and part it would probably take days!No, it won’t be “completely rebuilt” simply by installing fresh rings. Throwing new rings into a worn engine won’t fix much for long, if at all. To do the job right, you’ll need to get the cylinders bored oversize, honed, and install oversized pistons and rings. And that’s assuming worn cylinders are your issue.
The first thing to do is to determine why you aren’t making compression. A leak-down test is required. It’ll tell you where the cylinder compression is leaking to. When you figure that out, the question isn’t how long it will take, but how much it will cost.
When I lapped the valves I spent several hours to try and make them as good as I possibly could. I had replaced a bent valve on #4 and when I put it back together it was still at 0 PSI
Evil I pretty much just laid it out, heck the fact you have absolutely Zero compression makes me wonder weather or not your compression tester is working… if you had just shot rings you would at least have some pressure, what you would see with shot rings mostly is low pressure and a variation in pressure between cyinders, and during a wet compression test the difference between wet and dry would be quite notable if it was warn out rings.
Low compression can be caused buy a multitude of things, not just bad piston rings, but also bent or broken valves, warn out valve seats, broken pistons, warn out cylinder walls that are way too out of round for the rings to conform to the wall and prevent blowbuy, blown head gaskets, or even you could be getting bad readings from a compression tester. if the engine genuinly has 0PSI of compression there are some serious issue!
for example, my grandfathers Farmall A tractor with good compression has a pressure of 120PSI, a engine that is designed to be crank started if needed with a 4-1 or 6-1 compression ratio, most modern engines have a 8-1 compression at the least and typically those engines have forced induction, my engine is rated at 9-1 and some modern N/A engines are rated at 10-1 compression, you should be seeing pressures between 140-160PSI if i remember correctly, if you are seeing 0 i sincerly doubt new rings are going to fix the issue.
Thank you for your response I guess I should do another compression test wet and dry. And then go from there.
Honestly, with readings this low, I don’t think a wet/dry compression test is going to tell you much more. I’d go directly to a leakdown test.
0 compression is as bad as it gets. Either your compression gauge is out to lunch, or there’s a serious issue with your engine. I’ve seen engines with massive holes in the piston crowns due to pre ignition or detonation, and the pumping action of the con rods and piston remnants was sufficient to register compression readings on the gauge.
We need more info at this point:
Is this a stock engine, or are you running a higher lift cam or non-original ratio rocker arms? I ask because I’d like to know how you ended up with a bent valve.
Also, are you numbering the cylinders the same way Chev does? #1 will be the front cylinder on the driver’s side, while #4 will be the second cylinder from the front on the passenger side.
if its not the compression testor i say its a high possibility with a engine as old as a 305 that he may have broke a timing chain? or the timing went out of wack and a piston smacked a valve? this would enplane a zero compression, a busted piston or a valve that won’t seal because its broken or bent.
Well, OP said he lapped the valves and replaced at least one, which means the heads were off the engine. You’d think he’d have noticed a holed piston.
As for the timing chain/gears/bent valve issue, that’s why I asked about aftermarket cams and rocker arms. I’m not entirely certain if a stock 305 is an interference engine or not, (some lumpy old-school cast iron V8s were, others weren’t), and the web is vague or contradictory on this matter, depending what you read. Some sites say the 305 isn’t an interference engine unless certain aftermarket parts have been installed.
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