Howdy, i built a 408w for my bronco last winter and from March to October i rebuilt it 3 times. i've never had to work on one engine so much but when its together it runs like a scalded dog! its a 408 kit from FordStrokers with scat h-beam rod and steel crank, probe pistons, crane custom grind roller cam, roller rockers, RHS pro action 215 cast iron heads, timing gears, fluid dampener,underdrive pullies, edelbrock performer intake, full msd ignition, and an edelbrock 750cfm. ok now for the problems
first, the rocker arm studs came loose, not too much damage, had to put new pushrods and lifters in (someone used pipe dope instead of lock tite on the threads)
second, melted a piston ( my bad wrong heat plug in one hole)
third, broke 3!! pistons ( might have been my bad but the ring gap was a little tight from the machine shop i had put the short block together, so i put it together myself this time)
fourth and last, THE CRANK SHAFT BROKE!!!!! not even running it hard, never lost oil pressure. just broke right after the first main and screwed the block up.

Does anyone think the top end would be too much for a regular 351 roller block? my cam specs are : .571 intake .550 exhaust 268/275 duration and the heads are: 215cc intake 66cc exhaust runners. 2.08 in 1.60 ex. 64cc chambers

Thanks for any opinions

 

Do those engines still have 3" crank journals? It takes a lot of stress to break one of those. How fast were you spinning it? Are you certain that the rotating assembly has been properly balanced? Is the harmonic damper working properly? When the pistons broke, did you notice other damage?

 

I'm no expert, but a spark plug of the wrong heat range isn't going to melt a piston. That's going to happen from a lean condition. Broken pistons ? Broken how ? If it was the ring lands, that happens from detonation too. So I'd look to your carb tuning or choice of carb (Ededlbrocks are known to be lesser carbs than a same sized Holley) The choice of a Performer intake doesn't help either, you need an RPM. Broke crank? That can and does happen, but finding a definate cause can be complicated, a sudden shock load will do it. I've seen broken cranks in diesels too.

 

Yes I found out the hard way that a stock plug in place of a race plug is much too hot. Blew a hole right in the middle of #1 within 30 miles of putting the plug in. I also tried the RPM intake with a gasket match port but found much better bottom end results with the performer ported out. It did how ever have a sudden shock when the u joint broke at a truck pull but that was many miles ago. Enough of a jump that it tore the passenger side motor mount. As for the broke pistons one broke through all the lands, one broke the top off and the 3rd was only cracked. All at the top near the intake valve so detonation is not ruled out but like a said the ring gaps were anywhere from .015 to .020. I've seen broken cranks in my old diesel tractors and still ran it the whole summer with nothing more than a little tick haha guess I didn't get as lucky with this crank

 

A spark plug can cause detonation (too hot of a plug), but there is no way it will blow a hole in a piston. To actually melt a hole in a forged piston, you have to be running very lean. Detonation will usually break the skirts or the ring lands, although forgings have more of a tendency to bend rather than break (although I am not familiar with Probe's pistons). Detonation will also snap a crank in 2. How much compression and timing was you running, and what fuel? Running lean will also cause detonation.
This has detonation written all over it...