Small Block 400 Chevy

You don't have a broken crankshaft. Whoever told you that does not really understand what a crankshaft is. Harmonic balancer is a possibility. Many things could cause a rough idle that smoothes out at higher rpm's. You don't give enough information to provide anything more insightful than that though.

So - what's the deal with this loud squealing noise? Have you determined that it is not from a belt slipping? If it's not a belt, and it's not something like an alternator bearing (which could also cause vibration problems), then I'd start looking into the possibility of bearing issues. A spun bearing will emit a nasty squeal. It's not uncommon for that squealing noise to go away at rpm.

Normally I would agree with Sheps statements, BUT I have seen two engines with broken cranks that ran. One was snapped in two inside #1 main. It was a salvage engine that a guy had bought to replace his. Near as I could tell it had gotten dropped on it's nose or had come from a wreck and had gotten hit on the front. It came stripped and he didn't look it over very well. The other was an older 350. It broke between #1 and 2 rods. Not sure how that one came about. It went to the scrap yard without being opened.

However I would strongly suspect that it isn't the case here, since neither one of those engines were turning the front accessories. And they didn't run "fine" but they did run.

Take a look at the balancer and make sure it is the one for a 400. It will have a cast in counter weighted area on the back. If you rotate it and it is the same thickness all the way around it is not a 400 dampener. If this a a factory engine that hasn't been apart then it's doubtful this is the problem. Has the transmission ever been out of it?

Loud squealing noise could be a bad/loose belt.

Did you just rebuilt it or do any repairs on it? Last time I had a squeal it was an intake manifold gasket.

You MUST use a sb 400 harmonic balancer AND flywheel or ring gear. But I think if one of these were wrong your vibration would get worse at higher rpm, not better.

Good luck with it!

I've seen one. A Pontiac 421. Broke right at the last rod throw. Here is the story. Guy has this Pontiac towed to my shop and says the transmission is bad. So I pulled it, and looked inside. No problem. So I called him. He said it broke a few months ago and he was starting it once a week to keep the battery up. Said it made a hell of a noise. I started looking and found I could wiggle the flywheel like the bolts on the crank flange were loose. Pulled the pressure plate and checked the bolts. They were tight. So I dropped the pan. Broken crank. First and last for me:)

Al

yeah there is a belt loose from the balancer thats just hanging there on the tip of the crank. i don't think it's the alternator cause it's a new alternator but then again it could be. well i guess the good news is that it's not the crank . if you can you can e-mail me at snipped-for-privacy@leisurebay.com. thank you very much

NO but it isn't broken. If the fan, water pump and alternator are turning it isn't broke.