Opinions on compression and fixes


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OK,, here's the story, I have a 90, 4.3 which is coming out of a Blazer and going into a 91 Blazer. When it's on the stand I'm planning on changing crank seals, valve cover gaskets and valve stem seals as preventive maintenance. Checked the compression on it today while it's still in the donor chassis and got a little "oops". Five holes run 160-175 lbs., one hole (#5) only showed 140. Wet it and checked it again and it pegged the guage. So,, common wisdom says "bad rings".

Any opinions on how bad 140 really is? And since I'm running low on money (mowing has about stopped due to the drought) I realy hate to dumps tons of money into this right now, although this is the ideal time as far as doing the work goes. What is the viability of cutting the ridge and pulling #5 piston, then honeing and re-ringing just that one hole?

I had planned on putting an Edelbrock cam and manifold on while doing the swap but it doesn't look good to afford it right now either.

I have no idea how oil consumption is as I have not run the donor vehicle. If I knew it was an oil user I'd probably bite the bullet and hone and ring all 6.

Hit me with any suggestions, I'm open to anything right now.

John DiMartino

PlowSite.com Veteran
If it were mine,Id open it now.if you dont,you will regret it later.If you pop the head,you will know the extent of the cylinder wear,and kno w if there are any deep scrapes in the cyl wall ,or real loose piston.I have done one cylinder before in V8's,didnt like to,but usually because of a broken oil control ring,not low compression.The readings your getting are not terrible,but it is about 15% off what the rest are.
Hey Alan:

If you're lucky, you only have some sticking rings in #5. At 15% of the others, it is a little far off (optimum being all within a 10% range from highest to lowest), but it's not a write-off. If the compression was under 100 psi (on any or all cylinders), then we're talking rebuild time.

Put off the upgrades at this time. Adding horsepower will only further aggravate the problem.

I also wouldn't recommend just trying to hone and re-ring one cylinder. Depending on the condition of the cylinder and the piston, you could actually make things worse. Honing the cylinder will increase the clearance between the piston and the bore a couple thousandths of an inch, making it even more difficult for the rings to seat. That is assuming that the bore is even round still; honing an out-of-round cylinder is, IMHO, throwing money out the window.

In the mean time, fill 'er up with 20w50 and a couple of cans of STP oil treatment and save up for next summer's engine rebuild! (Just kidding!)


Chuck Smith

2000 Club Member

This is a tough call. Since it's a 6 cylinder, I'd be more inclined to pull the head, and have a look. Like John said, at least you will see what the story is. At that point, you can decide to procede further, or just put the head back on. Like Raceman and John said, a 15% difference isn't too bad. I know you will be plowing with this Blazer, and I'd hate to have it lose a cylinder in the middle of the winter. Since you have it on the stand, might as well at least look at it now. How many miles are on the motor? I know guys that have replaced the rings on one piston on V8's, but not on any 6's. The one V8, they just changed the rings, didn't hone the cylinder. That was in a Pontiac (402?) small block. The car went through 2 transmissions, and the motor was still running strong. Maybe it was just dumb luck. It was a beater car we had at a shop I worked at.

Anyway, I say pull the head, and see what you find. Then decide if you want to change the rings.