1970 formula 400 engine

[Formula 400 Guy]

I have been restoring a 1970 Formula 400. I paid a guy $3000 to build the engine for me. When I took it out for the first time, it went about 5 miles and the engine blew up. He took it apart and says he cannot find out what caused it to blow up. The block is cracked, the cam partially came out the back of the block, the distributor is destroyed, The pan has a hole in it, two broken rods, 4 bent valves, several bent push rods, you get the picture. He says the only thing he can figure is that the Flywheel is a neutral balance and it COULD have caused the engine to fly apart. When it was idling, it was smooth, I revved it up and it sounded fine. I am told that ALL Pontiac parts are interchangeable and this could NOT have caused the problem. Thoughts????

 

[melsg5]

1. Neutral balance? Was it the same flywheel the engine came into the shop with? Does he know the concept of internally and externally balanced flywheels applies to Chevrolet not Pontiac?
2. Has this shop rebuilt Pontiacs before and did you have any recommendations from someone else who had their engine done there?
3. What does the harmonic balancer look like and is it new or used? If used did the outer metal ring slip?
4. At what engine speed did it blow up?
5. Was it broken in on a dynamometer?
6. Were new camshaft bearings installed, if so do they look damaged from something other than the cam walking backwards?
7. What viscosity oil was used and any oil additive with zinc?

 

[Formula 400 Guy]

Blown Engine

It was NOT the original flywheel, the original was too worn for the clutch to make, I bought one from a friend in Pennsylvania that does nothing but build Pontiacs. He builds race engines and I was told he was good. I bought a new balancer for the engine. It blew up at about 40 mph, maybe 2000 rpm's. It was NOT dyno'd. It did have new cam bearings installed and I had just put new 20W 40 oil in it.

 

[melsg5]

did you add a zinc additive to the oil?
how do the camshaft journals and lobes look?
it was a new cam and new lifters?
were any engine parts magnafluxed?
does the car have an oil pressure gauge and if so do you recall the pressure reading?
You must realize diagnosing why an engine blew up is hard to do over the internet. Did you ask your friend his opinion of a cause and if so what was it?

______________________________

 

[cammerjeff]

Its odd that the cam walked rearward, I have never heard of that, a stock Pontiac engine has a retainer that bolts on the front surface of the camshaft to keep the cam from walking out the front of the block.

Only thing I can think of is the block may have been cracked between the crank journal and the cam journals. I have seen that on quite a few 68 and later 400's and 455's. Doesn't seem to happen on the earlier 389, 421, or even 428 Blocks.

I would ask the builder his opinion, I agree it is hard to diagnose over the internet, and even harder with out pictures.