Blowby, undersized Piston Rings, and 1/2 second

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maybe your machinist didn't fellow the piston to wall spec for the hyper piston which is alot less then cast or forged. the wall finish does look a bit rough but that maybe just the way it looks in the pic.
 
360Z28 said:
are those scratches in the cyl.wall on the lower left hand pic?
Click to expand...

If you're talking about the third photo down, it looks like a hairline crack in the cylinder wall. I had it happen in about the same place at 150k on a 3.9 V-6 Dakota. The truck was never raced, but did a little towing with dolly beyond the ususal urban pickup stuff. Ruined my day...
 
I remember that is was the tin can i was using to position and the center rings, it left a slight mark, which wiped right off with a little oil. However, the block was bored/honed when i got it, so it is probably not the correct piston to wall clearance, on top of everything else.

I did not know what I did not know.
 
The Machinist Should Have Pistons Before Any Boring Or Honing Is Done. Not All Pistons Are The Same. It Seems That You Have Gotten Quite A Few Replies To Your Concern. They All Seem To Be Good Advice. I Think You Are Being Steered Into The Right Direction. As Far As Getting The Wrong Parts, Trust No One. Always Check & Double Check Everything.
 
jimmyray said:
Interesting. I was "sure" that I put them in according to the directions supplied with the rings, but then...

I plan on pulling the motor in a couple of weeks, and going through it. I recently had Bobby at BJR go through the heads, and have eliminated them as a root cause. I am thus left with these possibilities:

1. Uneven surface of the block deck causing blowby past the head gaskets - Bobby and I did notice there was carbon black between cylinders 4&6 and 3&7, as well as in other places, on the block side of the gasket.

2. Piston slap, out of round cylinders, excessive gap between the cylinders cuasing slight rocking of the piston or poor ring sealing

3. Ring issues - installed or gapped incorrectly, the undersize issue, or perhaps they were collapsed at start up due to a lean condition or running hot.

As you all note, the only way to know for sure is to pull it and disassemble it. However, I am going to try a leakdown test first. Incidentally, I have issues with leaking out of the dipstick at higher RPM from the beginning, it's worse now, though, since I have been inching the shift point up from 5500 towards 6500.

Hey 69Chgr, when you rebuilt, what changes did you make, and what was the result in 1/4 times?
Click to expand...

Hey Jimmy,

I see the dot on the second ring, but I was referring to the top ring. I found the ring manufactures instructions to be a little vague on my set. I'd be more interested in seeing if the chamfer on the top ring is facing up(towards cylinder head) than I would the dots. I learned a hard lesson on mine. I only ran one time, and that was an 11.98 @ 113 before my motor puked so much oil out the front seal and valve covers at Ennis, Texas I had to stop. After fixing my rings, I had Ryan @ Shady Dell perform a Stage 2 port job, mill the heads, regrind the roller and match the intake. I don't know what it will run now as my new combo ate the A998! So I'd say a considerable much more, even still on pump gas. I could have sworn I've seen your car at the Texas Motorplex one year at the Mopar Nationals.
 
Jimmy,
There's HUGE differences in abilities, equipment, and experience out in the world. So dont feel bad. It ran, you had some fun, now it will run better. That's why we do all this crap...lol. I would suggest you keep rading up on things like square decking and why it's better and more expensive, and what a shop need to do it. Same thing with cylinder wall honing and machined finishes. Go into the shop armed with info and if they cant show you the fixtures or answer your questions, find a new shop. I drive over an hour and pass shops friends of mine own to go to a guy that is on top of his game in all three catagories. It's worth it to me to get things the way I want them.
 
I wonder if he re-torqued the heads after the engine was brought to operating temps the first time.
Prolly re-torquing the intake/valve covers/headers/etc woulda been a good idea too.

I cant see the deck being the problem. wouldn't the head gasket blown out long before?

Judging by those pics... I'm convinced the cylinder bores are the culprits.
 
FWIW, I got home last night and checked the box that the rings came out of; I kept the ends of all of the boxes, as well as all of the paperwork for just such a thing. The rings were in fact 0.030 over, and they are supposed to be installed with the dot facing up. So now it must be an uneven deck or poorly clearance pistons. Hope to pull it next week.
 
And the honing job.... Tell who ever is doing the job to make sure they finish is fine enough for moly rings... The one you got there isnt.
 
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