Got an 88 Camaro. Swapped the 305 for a built 355 4 years ago and used the stock serpentine setup from my 305 (with ac and smog delete). Ran perfectly fine. This last winter I put together a 383 (using same block) and stuck with the same serp setup. The ONLY difference is I had to use a (new) 400 balancer.

Now, no matter what I do, the belt will NOT stop squealing and trying to hop off the back of the crank pully (about 1 rib tries to pop off for a sec and causes the tensioner to make a loud bang). However, this is only when it is cold. As soon as the temp comes up, it's perfectly quiet from then on out.

I have replaced the alternator, the ac delete pully, the entire tensioner w\ pully, and the water pump. I've replaced the belt multiple times trying different sizes. A new belt completely solves the problem for about 1 or 2 days then it's back just as bad as ever.

One weird thing I noticed is that my belt got stretched. At one point, after replacing the tensioner, I bought a new belt exactly the same size as the one I've had on it for a month. It WOULD NOT EVEN FIT ON. Couldn't get it to go on with the new tensioner maxed out. Held up the old belt to the new one and the old one was obviously bigger even though it was the exact same part number. My theory is that the old tensioner moved farther than the new one, allowing me to install the belt a month ago, and in that time, the belt stretched a bunch. However I've been running a belt that size with the old tensioner for years with no issue. The belt size is kind of a gamble as I removed the smog pump and just ran a smaller belt instead of buying the smog delete pully. Though I did that 6 years ago and it has never been an issue before.

My next idea is that maybe a 400 balancer is slightly thicker than a 350 balancer causing the crank pully to stick out a bit farther? That would explain the belt trying to jump slightly off the back of the crank pully.

Any ideas?

 

First verify the pulley alignment using a straight edge if you can get to everything. I cant say for sure about the balancer dimensions but I have my doubts. Other than the AC and smog delete, is the belt routing original and correct? What belt routing are you using? Also to clarify, you said when the engine is warm, the squealing stops AND the belt no longer wants to jump off OR just the squealing stops?

 

Get the straight edge out like PaulyC said.
I am assuming that the lower pulley is sticking out too far.
Is the 400 balancer pressed on all the way?
If so, and the balancer dimensions are NOT thicker (should not be), then I have experienced the difference to be due to a thicker crank timing gear. You replaced your timing set with your new 383, correct? Ideally, the timing gears should be in straight edge alignment. No one ever checks for this. I had this problem with my 334. I machined the back side of the lower gear to get the timing gears aligned. Then the balancer and lower pulley also were in alignment with the other pulleys.

Is that lower crank gear on backwards? If not, and you don't want to mess with the lower timing gear, then you can have the amount that it is out too far machined off the end of the snout of the balancer. Either that, or you will have to space out all of your serpentine brackets to match the lower crank pulley.

 

Agree. You've got a pulley out of alignment.