[Sirrotica]

Going back to the engine mount redesign that happened in the 1970 model year with the addition of the 455 engine. All the stress is concentrated on the #3 bulkhead with the earlier 2 bolt design engine mounts, this can be exasperated by using solid mounts.

The Pontiac engineers did a complete engine redesign for the 455 engines to use the #2 and #3 main bulkheads. I would have bet the block failure happened in the block first, then due to the crank being put into a misalignment, snapping the fragile casting. The bearings still being in usable condition would bear out the theory of the center main was losing support due to the block fracture. Slowing down would be due to increased friction due to the support being lost in the middle of the crankshaft.

Evidently The Pontiac engineers were concerned that the torque of the longest stroke engine to this time was putting the #3 bulkhead in too much tension. I've studied more than a few of these #3 main block failures, many times the common denominator is using the old style 2 bolt motor mount locations in a drag car. Using a transmission brake will further stress the area of the #3 bulkhead to a point where it fractures due to the tension it is placed into. The engine mount redesign would lessen that tension load on #3 bulkhead to half of the early 2 bolt mount location, spreading the load equally between #2 and #3 bulkheads.

BTW to the OP, I feel your pain, been there done that too many times on a Pontiac large journal engine in my own race cars.

Likely a forged crank would have lasted a little bit longer, but the OP states the car was slowing down in the previous run was due to the crank binding in the main bores. Slowing down .4 was the warning sign

That's my theory, feel free to test it.

[73 TRANSAM]

Sorry to see this to anyone's pride and joy. I would not trust a cast crank that is an aftermarket one. The stock 455 is pretty strong. I would use a Scat or Molnar forged crank for the next build. It's not that much difference in price when u compare the aftermath. Goodluck.

[Tim Corcoran]

The engine was supported by a front plate and a mid plate. Next build will be a 400 stroker with a 4.25 forged Scat crank.

[Dragncar]

Originally Posted by Tim Corcoran

The two runs prior to the run when the engine broke the times were down .3 to .4 where it should have been running and the oil pressure seemed lazy to come up and was lower than normal at hot idle. This is the reason I think the crack down the #3 main was already there for a while and just getting worse but I am no authority just what I think. What is interesting is the bearing on the rods looked like new and the mains all looked pretty nice too. This was a low RPM motor I shifted at 5600 and went through the traps at around 5800. This engine started out as a low 12 second street car in 1986 and evolved into a low 10 second race car. Freshened up a few times over the years this was the third crank third set of heads and third cam but the same pistons and rods from 1986. I don't feel too bad this engine served me well thousands of street miles and hundreds of 1/4 mile and 1/8th mile runs over the years.

Sux Tim, at least you got you moneys worth out of it. My old 455 started out life as a 12 second car. Then got it down in the 10s. And I freshened it 3 times like yours only used the same crank.
It slowed down the last time I took it out 2-3 tenths and I put it back on the trailer. Have not torn it down yet but did remove one pair of rocker arms and a lifter that had double the lash after going years without changing.
Questions, how much power do you think you were making ? How big was the cam and what heads were on it ? Compression ratio ?
Good luck on your new engine. Expensive hobby we are in.

[Tim Corcoran]

Dragncar

The first two cranks were replaced during freshen up due to the machinist found very small crack in the rod journal during dye penetrant inspection. Round Port SD Performance CNC Ported E-Heads 330 CFM on the intake don't remember the exhaust numbers, this was before the E-heads had the heart shape fast burn combustion chambers. Compression was 12.3 race weight 3550 LB. Ultradyne Solid roller just under .6 lift 260/265 dur @ .050.

[Tim Corcoran]

Just in case some may not be aware, the 400 block is stronger than a 455 because the distance between the main journal and cam journal is .250 more in a 400. If you're trying to decide to build a race motor with a 455 or a 400 block I recommend going with a 400 the bore size difference is only .03 and with the stroker cranks available now days it should be a no brainer.

[Jack Gifford]

Originally Posted by Tim Corcoran

... the distance between the main journal and cam journal is .250 more in a 400...

Wouldn't it be .125" more?

[Jay S]

I think it would have still broke if it was a 3” block. But who knows. Sounds like it had a pretty good service life.

More than likely you are correct that when the oil pressure loss was showing up the block was cracking. My guess is though the crank was broke before that. You were running a broken crank and had no idea it was broke, and they don’t neccessarily give you any warning signs either. It was nothing you did. I have been amazed how engines can function sometimes with cranks that are broke, but the broken crank isn’t found until it is torn apart with no known issues. Steve C had one like that, and it was a 3” block with 4 bolt mains. We have had 2 engines that we found broken cranks that were found on a routine inspections. One came out in 3 pieces!

My brother and I would swap back and forth every other weekend driving our bracket car. At the end of the track the car got kind of out of shape in the quarter finals the week before with my brother driving. I get in it the next weekend and drove it down to the starting line. It was at Topeka, which is (WAS..they are closing this year!). is a bit of drive from the trailer to the starting line. Everything seems fine. I went into the water box and the engine was vibrating so bad I could hardly see out the window. I drove it back to the trailer and we went home. It pulled a rod completely off the crank and chopped in into pieces on my brothers last pass the weekend before! He had no idea. The car slowed a couple 10ths on his last pass, but it went side ways a bit at the stripes and he had to peddle it. It probably happened when he was going thru the traps!

[mgarblik]

It is possible the crankshaft was at least cracked and bleeding off oil pressure before the final catastrophic failure. We will never really know for sure. I have had 2 engines with broken crankshafts that still ran! Made all kinds of noise but were still running when they showed up at the shop. 1 was an Isuzu Trooper. Knocking like hell but running. The 2 V-belts were the only thing holding the crank in the block. Took the belts off and the crank moved forward and rearward an inch or more. Second was a Small Block Chevy in a truck. Broken in two behind the #4 main bearing. Still running badly and knocking to beat hell. We were able to save that block. The longer you live, the more crazy stuff you see.

[PunchT37]

Originally Posted by mgarblik

It is possible the crankshaft was at least cracked and bleeding off oil pressure before the final catastrophic failure. We will never really know for sure. I have had 2 engines with broken crankshafts that still ran! Made all kinds of noise but were still running when they showed up at the shop. 1 was an Isuzu Trooper. Knocking like hell but running. The 2 V-belts were the only thing holding the crank in the block. Took the belts off and the crank moved forward and rearward an inch or more. Second was a Small Block Chevy in a truck. Broken in two behind the #4 main bearing. Still running badly and knocking to beat hell. We were able to save that block. The longer you live, the more crazy stuff you see.

Yep. Crazy sh!t.


I had a ticket for an "oil leak" on a Dodge truck with the 5.2 mag engine. Drove it in the stall. Ran pretty good. Good oil pressure but, noticed a 1 cyl dead miss.


Picked it up on the rack to find the leak. I did. A hole in the side of the block large enough to put a tennis ball through. Yep, found the leak. The big end of the rod was still on the crank saving the oil pressure. Piston was sitting at the top of the bore. Still running.

[Tim Corcoran]

Originally Posted by Jack Gifford

Wouldn't it be .125" more?

Yes, you are correct .125, never was any good with math

[steve25]

Was that block ever raced on the side motor mounts?
The start of the failure could have began then.

[Tim Corcoran]

Originally Posted by steve25

Was that block ever raced on the side motor mounts?
The start of the failure could have began then.

The car has been an evolution of performance levels from a low 12 second street car to a low 10 second drag car. I don't remember at what performance level I went to front/mid plate setup. If I would have upgraded to a forged crank this probably would have never happened. I always seem to learn the hard way.

[Jim Zeek]

i built a 67 400 out to a 502. filled block, 4.250 bore, 4.500 crank, aluminum rods. it was a butler rotating assembly

[Half-Inch Stud]

Pinging under load could be something to consider avoiding in the next build.

[Tim Corcoran]

Originally Posted by Half-Inch Stud

Pinging under load could be something to consider avoiding in the next build.

What makes you think my engine was pinging underload? 12.3:1 compression and 110 octane race gas. Timing was not aggressive I doubt I had any pinging under load.

[Half-Inch Stud]

Originally Posted by Tim Corcoran

What makes you think my engine was pinging underload? 12.3:1 compression and 110 octane race gas. Timing was not aggressive I doubt I had any pinging under load.

Yea, not you, I probably meant the other Thread with 91 octane at 11:1

In the 80's-early 90's i too had 110 LL feeding a mill; a 472 with flattops and RAIII iron heads for clean power to WOT.

[hurryinhoosier62]

Originally Posted by Tim Corcoran

Dragncar

The first two cranks were replaced during freshen up due to the machinist found very small crack in the rod journal during dye penetrant inspection. Round Port SD Performance CNC Ported E-Heads 330 CFM on the intake don't remember the exhaust numbers, this was before the E-heads had the heart shape fast burn combustion chambers. Compression was 12.3 race weight 3550 LB. Ultradyne Solid roller just under .6 lift 260/265 dur @ .050.

FYI, Tim….dye penetrant/ developer inspections are not terribly accurate as to the amount of cracking involved. Magnetic particle ( Magnaflux), eddy current, ultrasonic and x-ray inspections are far more accurate as to the depth and extent of flaws.

[Tim Corcoran]

HF Eddy current is only applicable to aluminum it is not used on iron, steel or other metals. Ultrasonic is usually used on composite materials to determine if there is delamination or voids. Radiography (x-ray) is very expensive, requires very expensive equipment and would cost a lot of money to have an engineer write a test plan to inspect an automotive crankshaft it, would cost more than the price of my new engine will. Dye penetrant will detect surface discontinuities and cracks not normally visible to the human eye. Mag particle (magnaflux) can detect surface and sub surface discontinuities, there are two basic types wet and dry. Most machine shops use the dry method using a dry magnetic dyed powder and applying a current through a set of prods to the test article. The dry mag inspection is only as accurate as the operator is trained and is not the most accurate method and can miss a lot or give inaccurate results. Wet mag particle is much more accurate but requires more specialized equipment and training and these inspections should only be performed by a level II certified technician. The type of dye penetrant that would be used in a machine shop is simple but is very accurate and does not require a lot or training or skill just follow the procedure that comes with the kit. I have been trained and certified a level II in magnetic particle, Radiography, dye penetrant and HF Eddy Current. I would leave it up to a level III to determine if dry mag or dye penetrant was a better test method to inspect for crack in a crankshaft.

[Tim Corcoran]

Originally Posted by Half-Inch Stud

Yea, not you, I probably meant the other Thread with 91 octane at 11:1

In the 80's-early 90's i too had 110 LL feeding a mill; a 472 with flattops and RAIII iron heads for clean power to WOT.

If you referring to AV Gas I think that is 100LL not 110