Just finished my 96 lt1 rebuilt yesterday and it fired right up! Sounded great. After the oil pressure came up nicely, and the assembly oil burned off, I revved it to about 3000. The timing chain snapped! Anyone have any idea what might have happened? Everything seemed so good. I'm sick over it. No push rods are bent that I can see. How should I proceed?

 

Remove the rocker arms and do a leak down test. Pray that you didn't bend any valves. I bent 3 when that happened to my modified L98.

 

What timing chain did you use?
I'd pull the heads IOT what caused the timing chain to snap and check for damage

 

I've never seen or heard of one snapping on start up. Wow thats bad. Anything with a chain is a Interference fit. That means at 3000 rpm when the chain broke all the valves hit the pistons.
Some high rpm builds have broken chains due to a bad vibration/balancing issues but again never heard of one going south at 3000 rpm new.
Bad chain or did you forget the cam plate? Cam dowel pin sheared off. Lifter maybe jammed. Piston to valve clearance off.
You'll have to completely tear it down again. I would.

http://speedtalk.com/forum/viewtopic.php?f=1&t=28158

 

Last night I finally had an opportunity to work on the engine. It's still in the car. I think I'd rather have all my fingernails pulled instead of pulling this engine again! Anyhow, I cannot find any reason why this chain would snap. Let me give you a history and maybe you all can theorize with me.

I bought the car with a horrible knock. You wouldn't believe how bad this crankshaft was. It had groves and ridges that measured 3/16". I don't know how I even got it on and off the trailer. Three crank bearings had spun... and spun..... and spun..... and spun! Unbelievable how they kept running it.

The gentleman I bought it from bought it at a storage building auction. I believe this car was the original owners "baby". It is perfect inside and out. But I'm speculating that he died, didn't make his rent payment and the storage building people auctioned it off. I'm speculating that based on the condition of this crank, the guy I bought it from started it after at least 2 years of sitting and immediately ran the snot out of it. Never primed it, never even checked the oil to see if it had any in it! And then when the dry "everything" started to knock, he just kept running it! And running it and running it.

When I tore it down, the top of the motor was pristine. I mean it was as clean as the day it left Bowling Green. I mean there wasn't even a ridge in the cylinder walls. The standard pistons slid right out without reaming. So I bought a standard rotating assembly (new crank, new rods, new pistons, new bearings and new rings. I also replaced the oil pump, water pump and optispark. I didn't feel it was necessary to go through the heads-- they were incredibly clean and on inspection, even the seals were like new. The cam bearings had a little wear from debris, but I was trained in college never to replace the cam bearings unless you absolutely had to, because you'll probably do more damage then its worth (old school, I know, but it's served me well over the years). The timing chain was tight and seemed fine, and a new one for this car was almost $600, so I did not replace it. Of course now I wish I had, unless there's something I'm missing.

Let me theorize what happened and you all can shoot holes in my theory and share your own opinions or agree with me. Go easy on me, I'm sick to my stomach over this and don't need to be told I was a fool for using the old chain. I'm admitting that right up front!

1. I wonder if the chain, though tight, had tons of microscopic crank and bearing debris in it. Even though I cleaned it, it had been weakened by heat and friction from the bad treatment the crank had undergone and just said "nope, ain't gonna take it no more!"

2. Or, is it possible that I missed a spun cam bearing and the oil flow was blocked and the cam (which drives the oil pump) couldn't turn when the pressure built up and the pump locked causing the chain to go bye bye. That seems plausible to me, but I would think that the oil pump shaft would have twisted off before the timing chain. (My next step is to spin the camp shaft and see if I get oil pressure and flow).

3. Cam bearings are way sloppier than I thought and things just started jumping around when I revved it to 3000 rpm.

Lastly, here's the good news in the whole thing, none of the push rods are bent and I seriously don't think any of the valves were damaged. I'm not 100% sure, so I'm contemplating replacing the chain and gears and starting it up. I'll know right away if any valves are bent, and I'll pull the heads at that point. What more possible damage could I do if I started it with some bent valves?

 

If you look at the valve train area on the heads, do you see any valve springs that are lower than others?

And I would not start it with bent valves.

 

for god's sake don't start that engine; if you can't do a leakdown test first, at least pull all the spark plugs and do a compression test; no, or bad compression = bent valves; good compression, everything is "probably" okay.

I don't know where you got your pricing, but $600 is excessive; the stock LT1 chain is a link and pin type, not the most durable chain design...but if the sprockets are okay, you can probably save a bunch by replacing just the chain....

LT4 used a (small diameter) roller chain...a definite upgrade ... and of course there is the LT4 "extreme duty" large diameter roller chain; yeah, expect to spend some $$ for that set.

 

What if I put a straight edge across the tops of the stems? Wouldn't that tell me if a valve is open? The crank turns freely, so I assumed there was no valve interference. Am I wrong?

The $600 timing set was a high performance set. When I first ordered the parts from JEGS I thought I was ordering a set. It was $150. When it came it was just the chain and no sprockets, and the chain wouldn't work with the original sprockets. I called them and they said, "Oh, LT1. That's different. That's a $600 set." That's when I decided to use the old chain. I have since found a "stock" set from Autozone for $60.

 

That's what I was thinking. "Put a straight edge across the tops of the stems." Also, the crank turns freely with seeming no interference between the valves and pistons.

 

A leak down test would really help here. If you have good leak down results across the board, then I wouldn't think the valves are bent, and if they are, I'd think it wouldn't be much to really do anything if the valves are still sealing.

 

I think you're on the right track when you suspect the cam bearings. The load on that timing chain must have been huge! With that much debris from the crank, there's bound to be major metallic particles throughout the oil passages and the cam bearings. How did you clean the oil passages? When the camshaft started dancing around, the chain would get hit hard! Hydraulic lifters? Kiss them goodbye!

You apparently have the rockers removed. Blow air into each spark plug hole. All of the valves should be closed, and every cylinder should be sealed. Do you get air flow due to a bent valve thru any cylinder?

After you get it running, change the oil after just a few minutes of operation, to get as much crud out of the engine as possible. Look for debris in that oil.

 

You cleaned the block without removing the cam bearings? I'd bet the cam siezed.
I've never rebuilt a SBC without replacing the cam bearings, not sure what kind of damage you're worried about.

 

The valves will have a tendency to try and straighten out when they slam against the seat and are likely to snap off and fall into the cylinder. You can imagine what happens to the head, piston, and cylinder wall when that happens.

 

You already know that you have some serious problems in that engine. Don't try to shortcut a thorough and absolutely accurate check out of the entire engine. You already tried that, and you destroyed your timing chain, and who knows what else! There are no shortcuts allowed if you want an engine that will run right and be reliable. It's no fun when you can't trust driving your nice Corvette!! Do the job right! You'll be rewarded with the satisfaction of a fun ride thru your favorite canyon road!

 

Cam turns freely and seems well oiled.

 

Ah. I see your point.

 

There's the rub. Removing/replacing the plugs on this car means pulling the engine. It's damn near impossible! It's easier to pull the heads!

 

If this is an indication of your skill level, there's no helping you. Sell the car.

 

I'm thinking you had a "Latent" failure do to [Pin & link] problem with the chain it-self and not a valve train induced chain failure, I've seen Cams brake in two before a chain fails!

Now, Even on an Interference engine I have seen Belts break without incidence so you may have dodged a bullet, but you need to be sure.. to do this you need to inspect all of the valves, and rotate the Cam to be sure there are no Hitch's in the valve train.

Just my two cents..

 

AGREED Changing the plugs on these cars requires a universal an extension and a ratchet lol