PainfullySlow

I am in the middle of tearing down the original motor in the car and came across this:



From what Alan tells me (and linked) the valve covers were in place when the engine was originally painted so there should not be overspray on the valve train. This makes me think that the engine has been gone into.

I finally got the heads off and discovered this:

A 0.017 steel shim head gasket





Flat top pistons that look way too clean for 58k miles





This bore and piston were not touched since disassembly. This is how clean they are. I am also concerned about that little digger in the top of the piston. I checked the head and there is no evidence at all of valve to piston contact.





Overall it just looks way too neat and clean in there to be original. I do not have a proper mic to check the piston bores for size.





Any ideas?? Due to the overspray on the valve train I am certain that someone had been inside this motor at some point. The question is: what did they do?

Dynra Rockets

What year?

Here is a pic of my original 69 "3892673" ZQ3 flattop (-6cc) 10.25:1 pistons @ 55k miles. Yours look like the -17cc dished from the later low CR L48. The machined top of the dinged piston makes me think it is aftermarket. If factory there is a part number cast into the inside of the skirt (as posted below)

PainfullySlow

Sorry, 1972 base 200hp 350.

Mine do look remarkably like yours on the surface anyway. Hmm.

I haven't gotten under the pan yet to check the skirts or conrods. I was really not expecting the OEM pistons to be flat tops with a .017 gasket.

Ibanez540r

Your pistons are not flat tops. They are def. dished similar to if not OE.

The gasket is not stock, nor is the overspray. But, it's not unlikely that sometime over its 46 year life it got fresh head gaskets and some spray paint.

Dynra Rockets

1977 L48 "3169382" piston as posted by REELAV8R in 2012.

https://www.corvetteforum.com/forums...mber-size.html

calwldlife

pistons do look too clean.
no hint of carbon deposits.
wow.
air/fuel must be perfect.
or previous owner did a head job and cleaned them?
and then didn't drive it.

why is it apart?
any running trouble?

keithl1967

Did you have a head gasket leak? I've seen very clean pistons before where cooant was getting into the cimbustion chamber (you'd enotice the white smoke out the exhaust...

Essentially the leak was small enough not to cause catastrophic damage (at least we caught it before anything catastrophic happened), but essentially "steam-cleaned" the pistons in the cylinders that had the leak...

PainfullySlow

Haha, too true. I have been looking at flat top pistons as replacements for the past few hours and I had a brain fart where it got stuck in my head.

It appears that the pistons are definitely stock as I finally got into the bottom end of the engine.



Thanks for that. I tried searching google for a picture before I posted here but apparently my google-fu is weak today.

Heads are similarly clean. It looks like someone definitely was into the engine since those are not OEM gaskets.

It is apart because I am sending the block out for machine work. It ran when I got the car but barely. It had been sitting in storage for many years so I am certain that the carb was the culprit.

No head gasket leak that I could detect. All of the seals on the cylinder bores seemed intact. It would have to have been leaking on all cylinders equally which is very unlikely.


I am taking a guess that based on the slight ding in the #8 piston that something must have happened internally causing one of the previous owners to go into the motor. They replaced the stock head gaskets with aftermarket performance ones however they left the OEM heads alone.

It seems weird that someone would go through all the trouble of putting thinner head gaskets on but keep the stock 1.92 heads. As far as I can tell, everything else in the engine is original. Weird.

Dynra Rockets

What is the part number cast in the piston? Can't make it out.

Fredtoo

I have seen the same thing.

GOSFAST

It's most likely a #3991476, a "standard" 350" SB piston used between 1971 & 1984 on OEM production-line builds!

That dish should be .067" deep??

Thanks, Gary in N.Y.

P.S. That little "nick" on the top is from a valve "kissing' the piston somewhere along the line!!

TimAT

GM used steel shim head gaskets for years. Both OEM and dealer repairs.

sunflower 1972

For your reference. Here's a photo of mine when I replaced my heads last year at about 91,000 miles.

Duane4238

WOW! After looking at the cleanliness of your heads and pistons, I'd have to say the previous owner used "Z-Max" engine oil additive. Soaks into metal parts and keeps them perfectly clean.
Duane

PainfullySlow

That was my thought as well however there is absolutely no evidence of it on the #8 valves. This is why I am thinking that there was originally some issue the caused the valve to hit the piston and the previous owner replaced the failed part, decided the piston was 'good enough' and left it alone other than some cleaning which would explain the thinner gasket

So I am learning but from what I have read, they did NOT come from the factory this way? This would support the above theory.

TimAT

GM built engines for years-- both BB and SB with steel shim gaskets. The composite gaskets were used on Aluminum heads. We used to spray them with Hi-Temp aluminum paint for a sealer. That's what GM built them with at the engine plants.

Dynra Rockets

I believe the L98 aluminum heads used MLS gaskets from the factory. At least the 1990/91 spec gasket I purchased/used was.