BBC Scenario

On a BBC I found rocker arm off the exhaust on Number 3, the nut appears to be loose. The intake stud has pulled the threads out of the aluminum head so it was not opening.

Number 2 lifter the tie bar was broken, but the lifters were still straight.

My boost gauge read 46 lbs rather than the normal 31.

Any thoughts on what order things happened and why the high boost?

Should the connecting rod be pulled and checked for straightness?

 

Yes, I would pull it and check it.

EDB

 

for bbc aluminum heads arp makes a special rocker stud that is deeper and catches a few more threads... might be a possible thing to look at?

i'm gonna say that the first thing i always suspect in a situation like this is valve springs. take this next statement with a grain of salt: you can NOT have too much valve spring... but too little is lethal (especially if you are dealing with stainless valves)

you didn't say what rpm this happened at... but i'll just assume we're talking WFO and you tractor pull (right?)

so... would assuming the thing was 'screaming' be a reasonable assumption?

scenario 1-i'm gonna guess that a valve floated and the piston 'assisted it' shutting which lead to your pulled stud and/or your broken tie-bars

scenario 2-the stud pulled 'on it's own' due the the issue listed above and lead to your damage

to answer the chicken vs egg story above you'll just need to see what it tells ya... spring pressures/condition, pushrods, pistons, valves, etc

as for the boost... you went from 31# with 8 exit routes to '31#' with only 7 (soon to be 6) exits... so the boost has to jump. boost psi is always a relative number

as for what goes in the trash... is the bearing mashed? any mark on the piston? if they are PRISTINE you MIGHT be ok... how's that for a bullshit answer?

but if it were mine... lifters/pushrods/rocker/valves from all the affected holes go in the garbage

priorities on making your bbc heads live... valve springs, the right studs, a good (like RM/Brodix/Dart) stud girdle that ties ALL valves together (if you're poor like me you're still running a non-shaft set up), valve springs, no valve seals, open up the guides, really study your geometry, valve springs, and on a raised runner head (and especially if you are using longer valves) the proper guide plate is CRITICAL. did i mention valve springs?

 

Specs

Engine was turning about 6500.
Springs CC 948
320# of seat pressure
that head does have SS valves.
Stud Girdle & ARP studs

it did drop from 8 to 7 cylinders so the increase seems reasonable
I have 3 sets of these heads I have run for 6 years with the same setup. So it is not a new setup.
I can put it a longer stud but it is not going to do any good. The stud goes into the intake port, so the thread length is only 5/8 max.
The situation with the threads might just be fatigue.
Maybe I was just lucky the exhaust came of at the same time!

 

If you lose the exhaust rocker, the intake has a hard time opening that big old valve under extreme pressure. That normaly bend are breaks something on the intake side. Not sure about the #3 lifter. Mabey not strong enough for the extra boost that you gained with only seven holes.

 

I agree with most of what been said above. They are good points.

I have too much experience with BBC valvetrain issues.

After listening to the above... My guess is the exhaust thing happened first. I once had the locking setscrew/tall nut break so it backed off and made for way too much clearence which allowed the pushrod to come out of the rocker arm socket. Once the exhausts fail it has a bad domino effect. After hearing the spring psi is good, stud girdle, stud etc, I'm convinced the intake stud pulled out because the cylinder psi was much higher than normal when the intake opens.
My boost never increased by that much when I would lose a cylinder, which makes me think you had a little bang under the blower I would only ever gain about 4 lbs on a normally 30 psi setup. The tiebar came off because the free wheeling exhaust lifter was able to lift too high.

If these are Dart conventional heads I always put as long of studs or bolts(shaft system) in as I can. But like you said once the stud is into the intake port you can gain no more by length. You can helicoil them, but that's probably not why this happened.

Have you checked your spring psi lately? We've heard what its supposed to be...

 

The extra boost is normal in a situation like this. I would check the rod bearings and rod for sure on that cylinder. It may have just been its time to pull out, a good steel instert will cure this most of the time, we use some that loc and stitch make, with great success. Is it possible that you had a push rod failure and it got under the rocker and jerked the stud out?

 

I agree with Sodak,

I have had them strip when torquing to spec as well as pull out after assembly. To remedy this I weld up the stripped hole and blind tap them all and run a bolt machined to penetrate as close as possible (about 1/32") to the bottom of the hole, as I feel its so critical on the conventional head to get every bit of thread that you can.

Richard G