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Author Topic:   406 SBC - I think it blew up. What do you think?
posted May 24, 2003 09:20 AM
Here's the deal.

I have a Jr Motorsports 406 that I bought last season and raced about 10 nights. It was never overheated or over-reved (it saw 6500, thats it). I was stored all winter and I put new heads on her and stuck it in my race car.

For the past couple weeks, I've been fighting with it because it would run up to 4550 rpm and than run like crap. Below 4500 - fine, above junk. While messing with it yesterday, it stopped turning over while cranking it. The motor won't move and the starter makes horrible noises scrappin the flywheel.

Than I removed all the plugs. All looked perfect except number 6. Number 6 looks like its coated with in layer of aluminum and the electrode is bent over. So I guess the piston has let go. But why did it do this while cranking? The last time it ran, it ran fine (no smoke or evil noises, just the 4500 thing).

I haven't removed the head yet, maybe all questions will be answered than.

posted May 24, 2003 09:33 AM
my guess is that you leaned it out, and it being on # 6 or # 8 is where they do that most of the time. be sure to look at the block in that cylender wall for scaring . i have done this more than once from fuel pump and blown head gasket.when you pull the head ck the gasket real good around that hole and if nothing looks out of place ck your fuel pump presure. as far as the presure any where from 7 to 9 pounds is good without a regulator any thing over 9 pounds will need one.

posted May 24, 2003 11:27 AM
Update 1:

I just pulled the head off and the gasket was blown between 6 and 4. Also, there is a chunk of piston number 6 missing about 3/4" long.

The gasket was not blown into a water jacket. Would the blow between 4 and 6 lean it out? It would suck exhaust out of 4, correct? I guess my question is, could the head gasket have caused all this?

I'm just trying to figure out what's happened so I know what to do next.

posted May 24, 2003 12:40 PM
that's a common failure on a 400 chevy. there isn't much metal between the cylinders and with 2 hot exhaust valves right there together in the middle, they're prone to failure.

posted May 24, 2003 12:40 PM
My guess is you leaned it out strarted burning the piston and in the process got the head gasket from cyl. tempurature...Did it burn the block between cyl.?......Towman

posted May 24, 2003 12:45 PM
yeah, usually the gasket fails first and if you continue to run it, everthing else flies apart.

[This message has been edited by outlawstock17 (edited May 24, 2003).]

posted May 25, 2003 01:38 PM
What side of the piston is burned , intake or exhaust ?? If on the exhaust side , or toward the center of the piston , this could also be a result of major detonation . Also have a look at the head gasket and see if the fire ring has any distortion to it , other than where it blew out . If it is distorted in any way , then you had severe detonation going on . If the gasket is simply blown out , but still in good form otherwise , then you had a severe fuel problem (lean). Have a look at the other spark plug's in your engine , and see if they have a build up of very small aluminum balls on them . If they do , then severe detonation is occurring . Make sure to check that your magnetic pick up wire's are not hooked up backwards , and if your running an MSD or other racing type ignition box , make sure that the mag pick up wires are not running parallel to , or cross over the coil wires going from your box to the coil .

Part of being an engine builder is forensics , so it is very rare that the source of a problem like this can not be found before you try your next engine . If for some reason you can not pinpoint the problem and cause , find a chassis dyno and spend some time finding the problem with your car before you try and race again .


Hendren Racing Engines
Rutherfordton , NC

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