Then there was a big kaboom

onefastmustang · #1 (**permalink**) 10-08-2007, 12:15 PM

So I am cruising up a hill on the highway near my house and suddenly I hear a big bang from under the hood. then a rapping noise and it seems like the engine is only firing on 7 cylinders. Fearing the worst I took the valve covers and oil pan off. I find nothing wrong so far. the rods and pistons look ok from below and the valve train looks ok from above. I removed all of the rockers and there appears to be no damage there. Could I have lost a lifter?? I plan to pull the webers tonight and see what lays beneath but if anyone has any ideas at things to look at I would be much appreciative. I have built a few engines, this is my first one to go kaboom.

Rick Parker · #2 (**permalink**) 10-08-2007, 01:17 PM

Have you pulled the plugs?

onefastmustang · #3 (**permalink**) 10-08-2007, 02:15 PM

Not yet.. Ill do that when I get home from work. anything I need to look for in doing that?

lineslinger · #4 (**permalink**) 10-08-2007, 02:48 PM

Did you actually turn the engine over (by hand) while you had it dismantled? Are you sure all the pistons are still attached to the rods..timing chain and gears still in place?

onefastmustang · #5 (**permalink**) 10-08-2007, 03:04 PM

I haven't.. I did drop the pan and drained it through a filter, no shrapnel so far. Ill try that tonight as well after I pull the plugs.

olddog · #6 (**permalink**) 10-08-2007, 04:00 PM

A little late now, but my first move would have been a compression test. You may still want to put the rockers back on and do that, before you pull it further appart. At least then you will know what cylinder to concentrate on.

onefastmustang · #7 (**permalink**) 10-08-2007, 04:30 PM

Well I pulled the plugs on the side I was hearing the noise and the 3rd plug back had the electrode collapsed and the plug was on the black side.. does it sound like something exploded inside?

lineslinger · #8 (**permalink**) 10-08-2007, 05:17 PM

Not clear what you mean by collapsed.
If the electrode was flattened out the piston may have gotten loose and "crunched" it. Maybe you only lost a rod bearing or a wrist pin went sour, all the connecting rod bolts look good? Check them by hand to make sure nothing stretched or broke.
Perform a slow turnover by hand with all the plugs pulled making sure each piston is following its correct time and path. If that works out it seems a slow disassembly is in order until you can locate the sore spot.
There are way too many variables to do any real analysis without more specs on your engine, give us more details of the motors behavior when it went boom, before and after, did you check oil temp. oil pressure, engine temp? Maybe post some pictures of the plug and any other areas you suspect.

olddog · #9 (**permalink**) 10-08-2007, 05:41 PM

Describe electrode collapsed a little better.
Is there parts, missing from the plug?
Does the plug look like it has been hit or stuck by something?

There is really nothing to explode technically. Look over the carburetors, for a missing throttle plate or choke plate screw. I tiny piece of metal can just about beat the inside of a cylinder head and piston to death. If a valve head popped off I would expect more damage. It may just be a plug failure, but you have to make certain that ALL of the pieces are out of there!

Do you have a friend that has access to a fiber optic camera? Sure would be nice to see what the inside looks like. If you pull the intake you can look in the intake valve and see a little bit, but not enough to be certain.

PS
Sorry about the redundancy I didn't see Lineslinger's post.
I typed this and clicked save. Took a ride and came back, and it still wasn't saved. Clicked save again.

Rick Parker · #10 (**permalink**) 10-08-2007, 05:44 PM

If the ground electrode has been impacted and is crushed against the center electrode, then you probably have a broken piston. Sometimes the tops separate right above the ringlands (this was an issue with some of the OEM stroker pistons) it has been documented, and discussed here previously......sorry.

onefastmustang · #11 (**permalink**) 10-08-2007, 07:10 PM

Quote:

Originally Posted by Rick Parker

If the ground electrode has been impacted and is crushed against the center electrode, then you probably have a broken piston. Sometimes the tops separate right above the ringlands (this was an issue with some of the OEM stroker pistons) it has been documented, and discussed here previously......sorry.

You are correct. That's exactly what happened.. Maybe I need to change piston manufacturers. I guess the block comes out this weekend and I rebuild it. I had speedpro hyp pistons in there. I guess another brand/type is advisable at this point..

onefastmustang · #12 (**permalink**) 10-08-2007, 05:44 PM

Here is what I pulled out of there for a plug.. I am pulling the heads tonight to see what lurks inside..

olddog · #13 (**permalink**) 10-08-2007, 05:57 PM

Since the oil pan is off and the plug told you what cylinder to look at, do as Lineslinger said "rotate engine slow by hand with plugs out. Watch the suspect cylinder's piston. At top dead center (or bottom dead center) rotate the crank back and forth. See how far you have to move the crank before the piston starts to move. Compare to some other pistons. If there is any play in a rod or wrist pin it will likely show up with this test. If your eyes are going (like mine), put a dial indicator on the piston.

lineslinger · #14 (**permalink**) 10-08-2007, 06:40 PM

Something (piston) had to of hit that electrode to bend it that far. There may be a floating part trapped in the cylinder as old dog said, more than likely a valve cap or a piece of of a valve cap, maybe a stray piece of metal/fastener got sucked down into the cylinder, but pretty unlikely.
Are you using the older style sodium valves? They like to break.
Its a PIA but at least you shut down early before more damage could happen.
Hmmm...interesting looking hair to the left of the plug, you been fooling around with the Mrs. on that table top?

Rick Parker · #15 (**permalink**) 10-08-2007, 07:21 PM

There was an identical situation several weeks ago. Almost the same picture.
Only forged pistons belong in these motors, we generally "Ask too much of them."
Were you able to locate the missing piece? If you are lucky it got pumped out the exhaust without causing any other damage.

onefastmustang · #16 (**permalink**) 10-08-2007, 07:30 PM

I have not found the missing piece.. I suspect your right on the ejection of the part. I guess I need to find some new pistons and replace all 8. I've looked over the block and heads and they appear to have no damage to them..

Rick Parker · #17 (**permalink**) 10-08-2007, 07:30 PM

I once "Closed up the gap" in a NGK plug when I miised a shift on a 1800 cc Datsun and buzzed it close to 9000 RPM with stock rods. One rod momentarily stretched and allowed the already close dome to impact the electrode. I thought I had bent a valve until I saw the plug.

onefastmustang · #18 (**permalink**) 10-08-2007, 07:40 PM

Yikes.. Well I guess I need to talk to my machinist buddy tommorow and see what he can recommend. It is a 392 stroker kit but its in a 1969 351 block which has a bit shorter deck height. because of that I had to have the pistons cut a little bit. probably should have gone with a different head gasket instead.

Any recommendations most accepted....

olddog · #19 (**permalink**) 10-08-2007, 08:05 PM

When you pull the pistons, look at the land area of the pistons above the top ring. If the pistons do not have enough clearance, when they heat up they can grow to a diameter larger than the bore, resulting in a break just like yours. A lean condition will heat a piston more and make it tighter. There is always some cylinder running leaner than the rest. If there is a lot of wear on the lands, that was very likely your problem.

I would measure the piston clearances in this engine. Then compare that to what these type pistons should have. Also if you have the build sheet, compare the clearance now to the clearance when it was assembled. It only takes a 0.001" change to make all the differance in the world.

Buy the way, although forged pistons can take the most punishment and are best for maximum performance, they also require the most clearance. If you have quiet exhaust, you can hear them slapping when cold. Warm up is critical.

olddog · #20 (**permalink**) 10-08-2007, 08:17 PM

Oh buy the way, pull the valves on that cylinder and give them and the seat a good look. I had a throttle plate screw go through an engine and the intake valve closed on the screw, cutting the seat and valve. It only looked like a scratch, but it was deep. The valve couldn't seal on the power stroke and it kept lighting the fuel in the manifold under load.

So learn from my misfortune, check them before you put it back together.