Piston ring low compression?

I have a 98 Chevy Silverado c1500 350 small block, I recently bent a valve and had to replace it so replaced all the valves and head gaskets and intake gaskets, I’m still having a misfire people thought it was a burnt valve on the 6th cylinder and it wasn’t but I’m thinking it is the piston rings on the 6th piston here is all the compression readings Cylinder 1 = 135
Cylinder 3 = 140
Cylinder 5 = 150
Cylinder 7 = 140
Cylinder 2 = 150
Cylinder 4 = 110
Cylinder 6 = 80
Cylinder 8 = 150
I had tryed a can of restore and other stuff like that and nothing still has a misfire. The engine light flashes sometimes. I checked everything else on the truck and is in good shape is there anything else you guys would recommend me to try? I don’t have time nor money to tear down the bottom of the engine let me know what you all think I should try thx.

Were the new valves properly lapped in? If the valve job wasn’t done right they’ll still leak.

Redo the compression test after adding a small amount of oil to cylinders 4 and 6. If they show improvement, worn rings are at least partly to blame.


Another possibility is a head gasket leak between adjacent cylinders 4 and 6, since both those cylinder have low compression.

Cylinder 1 is also on the edge of being unacceptable. So you have 2 definitely bad cylinders and 1 marginal.

I was wondering about lapping too, especially since the compression readings are all over the place.

Yes I did valve lapping

After the valve job you flip the heads upside down with spark plugs installed and do a leak test with kerosene?

Also, did you check the valve guides? Worn guides may let the valves move around and not seat properly.

No I didn’t do that and yes I did check the valve guides and also put new valve guide seals on it but ik it’s not the valves it has to be the piston rings do you think rislone ring seal will work?


With all due respect, how do you know it’s a bottom end problem, versus top end?

I wouldn’t make any such assumptions, until you’ve performed wet and dry compression tests

And a cylinder leakdown test would also be a good idea

The reason I think it’s a bottom end problem is bc like I said I changed all the valves did the valve lapping and changed the head gaskets and intake gaskets and valve cover gaskets I also changed cap and rotor plugs and wires and checked for vacuum leak so I assume it would be a bottom end problem. The truck does have a lot a power if I press on the gas a good bit but other than that you can tell it has a misfire and has less power than it should in general just doesn’t run right

I also put another spider injector in it from another engine I had and still had the misfire

With the engine running at operating temperature remove the oil fill cap and if smoke is coming out you likely have a galled cylinder and/or broken piston and ring.

You really need to follow db4690’s advice and run a wet compression test and/or a leakdown test.

Those reading of 135-150 are worn and cylinders 4 and 6 are really bad. Compression numbers that low could easily explain the misfire.
There’s no sense in spending any money or replacing anything until the cause of the low compression is rooted out.

That vehicle can’t run as good as you think it does. There’s some perception at work there because the engine likely smooths out when it comes off idle.

As to additives, they may help a little if the problem is related to rings stuck in the lands. Additives will do nothing for major wear, scored cylinders, rings which have lost their temper (not the attitude…), or rings which are galded into the ring lands.

How many miles on this engine and how did you bend a valve? Run through some deep water?

Run a dry then wet compression test. If this engine has lots of miles on it, as I suspect it does, those compression readings seem to say the rings are worn out.The wet test will pretty much confirm it.

Seems like you’ve been driving the truck which would tend to seat in the new valves a bit. The compression test should pretty much confirm ring wear. There really is no magic chemical that can fix this. See if you can find a good short block to sway your fresh heads onto to drop into the truck or drive it until you can afford to re-ring it. Plan on new bearings, too.

I seem to recall that small block Chevrolet V-8’s cylinders # 6 and 8 were somewhat prone to failure due to heat for some reason. Detonation damage to pistons, scored cylinders and stuck rings was often in one or both of those cylinders while other cylinders had no significant damage. And while the small block 400 was prone to the head gasket failing between the center cylinders on each bank the 350 block gaskets rarely failed and I don’t recall ever seeing a bent valve on a Chevrolet V-8 except when a spring broke and the valve dropped. And it’s always worthwhile to turn the crankshaft a full 360* while looking at the cylinder walls when the heads are off and then pouring several ounces of oil on each piston to protect the cylinder walls from trash when cleaning the deck and if the oil seeps past the piston too quickly on a cylinder there is a problem with the rings.

A dry/wet compression test on all cylinders before removing any heads would have saved a lot of time and aggravation.

It’s odd that a valve would bend on an engine like this unless it ingested something, suffered severe engine overheating, or a valve tried to seize in a guide. The latter would be highly unusual.

How could he have done a compression check with a bent valve?

Don’t be a smart alec, please . . .

“I recently bent a valve and had to replace it so replaced all the valves”

he’s ALREADY replaced the valves


I know, but he only removed them once, so that is the removal I was referring to.