Troublesome 427 Tall deck

[John Deere Boy]

Maybe this is the wrong place (maybe the wrong forum altogether?), and if so please point me where i need to be

This is a 427 bbc tall deck in our 76 C65 grain truck, and it's been nothing but problems for the last ten years or so. It melted three pistons in the course of six years, so we bought a new Dart block and had it built by a local respected race engine builder. It then melted another piston a year later so we suspected a lean condition and bought a new Holley 770 Truck avenger carb two years ago. Now it has two more scarred pistons. I'd like to show you a picture, but this site won't let me post a link to my photobucket picture.

Any ideas? The truck only gets about 300 miles a year, but it weighs 65,000lbs loaded so every mile is 100% load. Its got stock heads, cams, crank, headers, and intake. There are no fuel supply problems to the carb. I do suspect the 770cfm carb is too big, but we had the same exact problems with the original 710 cfm speed governed Holley. The coolant never overheats, but i suspect the combustion temperature is pretty darn hot!

[John Deere Boy]

I'm trying to post a picture. Post #2

[John Deere Boy]

Post #3

[John Deere Boy]

Post #4

[John Deere Boy]

Post #5

[John Deere Boy]

And finally

[305c1500ext]

Maybe too much fuel? Could be bore wash from excess fuel.Is fuel pressure correct?

[John Deere Boy]

Nah i'm pretty sure that's not it..i think i'd be able to smell gas in the oil or notice it flooding. Usually when this happens the pistons melt, so i'm thinking a lean condition, or too much compression. Is this motor supposed to have domed pistons?

[1.721afr]

More pics of the bore/piston damage plz. Yes, melting pistons is normally a mixture/ timing issue.

Higher compression will normally tolerate less timing & require more (& better grade) fuel.

From your pics there may be a crank thrust or torque plate boring issue.

[1badbowtie4x4]

x2 on the timing. it's probably getting really hot on top of the piston.

[Air_jordan99]

From the look of the piston and cylinder I would say you are running way to lean. Tops of the pistons should be a black color not the light grey. The pistons heat up expanding into the cylinder walls causing the scuffing that you see.

Once you rebuild the motor ditch the stock heads for some after market to lower the compression ratio some. We dont run fuel like what was produced in the late 70s. If running any additive such as ethanol (e10, E15 or higher) you will have to increase fuel supply and run a higher grade fuel.

[John Deere Boy]

Thanks for the advice...but i'm still not sure what the right answer is or what to change for this rebuild. The block is now going to be .080 over (ouch) and our machine shop says there are no dished pistons available in that size. Does someone really make aftermarket heads with LOWER compression? Not that i really want to dish out the money for heads. As far as timing goes, our engine builder set the timing initially with a timing light, but it didn't have enough power to suit my grandpa and he re-adjusted it, possibly advancing it too far. The truck basically has no mufflers so it might have been pinging without me being able to hear it.

Here are all the pictures i took. Forgot to whip out the camera after i pulled the pistons.

[John Deere Boy]

One thing's for sure: when it goes back together, i'm putting in an air-fuel mixture guage!

[1badbowtie4x4]

looking at the third pic, it looks to already be bored .060 over.

[b454rat]

Those heads are already low compression. If you put another set of heads on, you might gain compression. Plus the stock heads are designed to make torque/power down low, different heads might make power higher RPMs.

Did you put in a new cam and lifters??

[454cid]

You might read this thread. There's a lot of arguing, but it may give you some pointers since the 366 is similar.

http://www.fullsizechevy.com/forum/s...d.php?t=476198