Loki1199

Hi Guys,
got a big complex question. The shop that built my motor screwed up somewhere and something apparently isn't properly balanced. My car is in the shop getting a new flex plate for the third time. The shop guys say "it's fixed. Your thrust bearing is fine, so that's not causing the cracked flex plate. Something internally isn't properly balanced, or you have the wrong balancer and that it causing the problem."
So, my question is - If it's not play in the crank, or a faulty torque converter, what the hell else can break my flex plate every month!?
TIA for any info you can give.

SSC

The flywheel on a 350 should be set to a nutral ballance or if you wanted to be picky you can have the ballance matched with the crank and damper, but I doubt they or you are willing to pull the engine down again and send it out. What 99% of the time cracks the flywheels is user abuse. Ive cracked several flywheel and even had a flex plate dig a couple on inches into the ground. Why did thery break because I broke them Theres a few companies that are producing SFI approved flywheels that are ballanced and there fairly cheap I just cant think of the names at the moment.
Good Luck
SSC


Oh BTW how hard is the tq hitting you when you take off? It can cause a problems if it is screw up.

RB83L69

Does the transmission have a strange whine that changes pitch according to engine speed? Was the block align-bored, or is it a 70s 350 block? If so, you're in for an interesting situation.

Guest

The wrong starter can cause it as well, the starter will smack the flexplate instead of meshing with the teeth, and it will break it in short order. Engine balance wont break the flywheel unless its out so bad that the engine would probably let go first.

Loki1199

Yes the block was align bored and yes the tranny does have a whine that changes pitch.
What does that mean?

Loki1199

Wouldn't the wrong starter cause the teeth on the wheel to crack? This problem is causing the whole damn thing to break, but it hasn't lost any teeth.

Loki1199

Oh and the tq isn't hitting too hard. I haven't had a chance to really even beat on the motor cause I've had so many problems! I've never done neutral drive drops, or anything like that, I don't rev it to excess. It's never even been really raced.

GRENDLE

i broke about 5 on my second gens. main offenders : engine backfiring (usually after you've reinstalled the distributor) , loose flexplate bolts!!!! these need to be torqued (properly) in a star like sequence (like a wheel) and red thread locker is a must!!!! also get a flywheel turning tool (like $15) if you stick a screwdriver or something in there to turn/hold it while your installing the torque converter it will flex the flexplate and it will most likely break again. and lastly (from my exp. ) is a bad trans mount, i went through 4 shifter cables and 2 flexplates before i found out mine was bad (and it looked good). starters will usually break teeth and or lock up on the flexplate. also loose trans to engine bolts will do it too. thats a wierd problem that no one seems to have heard everytime i ask. my TA 's flexplate is cracked in almost every hole on it!! (thats why i got it for $600, it had a bad "rod knock" lol ) get the thicker sfi approved one that should stop it. there's a few on ebay right now for like $30 (better than $90 at the store)

Loki1199

I just picked up my T/A and she's running better, though she has shakes like crazy. The shop says something is out of balance which is causing the vibration, which in turn is wrecking flex plates. They looked at the alignment of the crank in relation to the converter - ok.
Checked the end play on the crank - ok.
Checked the dowel pin locator on the tray (whatever that is) - ok.
Out of balance is the diagnosis.
They installed a thicker flex plate and said that may prevent future cracks. Now I'm worried that the vibration is going to wreck somethig else. Crazy vibration!! Shakes like mad!
Where should I look first?

ATOMonkey

If something is messed up in the balance check that your harmonic balancer is properly installed. If it is good and is spinning nice and smoothly make sure it is not for a 400. Your 350 should be internally balanced and a 400 is externally balanced.
If that is all good then the machine shop that built the engine really screwed it up somehow. Take the car to them and show tell them that they completely blew the balance job.

rezinn

ttt

RB83L69

What that means is that now your crankshaft doesn't line up with the trans input shaft any more.

What you need is some offset dowel pins, to raise the trans a few thousandths of an inch with respect to the engine block. I'd suggest about .015"-.020" of offset.

jcb999

Is it new?

Loki1199

It's the stock balancer for the block... or it should be. I don't think they would have swapped that when they wouldn't need to.

The whining noise that I noticed doesn't happen in every gear... only first. As I understand it, this is just the way first gear sounds. It's not loud or grinding, just a nice spinning noise like it's gaining momentum - I've noticed most cars with TH350's have this sound.

Everything lines up as it is supposed to according to the shop it just came back from.

Do balancer wear or break? All they are is a big spinning counter weight, right?

jcb999

If the rubber ring on the balancer is deformed or out of place, the balancer can slip. I would make sure that it is not defective.

RB83L69

Balancers do wear and break. They do not crack flex plates however.

Don't belive what the shop told you. It defies all logic and common sense. Think about it... align boring consists of cutting some meteal off the main bearing caps, and re-boring the main saddles through the block, a few thousandths of an inch higher than wherever they were before. It's not possible for the crank to be in the same place any more.

Ask the shop how they checked it, and ask them exactly how far they moved the crank up when they align-bored it. If they can't tell you, then they aren't checking, and they don't know or don't care. Think about this while you wait for the opportunity to do this... how would you check it yourself?

I can tell you how it can be checked, having been down this road a couple of times already. Let's see what your shop says they did to "check" it, and what the exact location of the crank centerline is with respect to the bell housing dowel pins.

Loki1199

The issue comes in here - the shop that just did my fly wheel isn't the same one that built my motor. I haven't the faintest clue how they would check the alignment or anything down there, honestly. I do know that these are the guys that swapped my blown 700r4 for the th350, and they are the guys that replaced my fly wheels when they have cracked. I also know that these guys built sick race cars. The shop owner has the fastest Stock motored Nova (A Frame?) in the states and he built it himself.
So, with that said, what else could crack a fly wheel and cause my car to shake if everything is lined up the way it should be respectively? I know the motor was align bored. I Know the thrust bearing is not worn and there is no end play in the crank.
So.... please help. I have this love/hate relationship with my car and I can't afford a new one or a new motor.

RB83L69

The way you check the crank for being lined up to the dowel pins, is to make a plate of of a dimensioned drawing of the engine block (available in the old Chevrolet Power Manual among other places) that bolts to the back of the motor, with enough precision that a difference of as little as .005" can be measured. All you really need are the dowel pin holes and a hole for the crank centerline. A rod the diameter of the pilot hole in the back of the crank and a hole slightly larger than that by a known amount, measured with wire-type feeler gauges, gives the most accurate results. It's possible to sort of measure it with a manual trans bell housing and a dial indicator stuck to the crank, but the tolerances inherent in that arrangement will probably prevent a meaningful measurement.

Misalignment is essentailly the only culprit that explains your problems. I honesly think you need offset dowel pins: your situation is exactly what those are made to take care of.

http://www.moroso.com/catalog/catego...?CatCode=23004

"The simplest explanation that fits all the facts is usually the right one."
— Occam, ancient Greek philosopher; some things still hold true even after thousands of years

Loki1199

Offset dowel pins?
This may be a naive question, but... what are the dowel pins holding to being with? Are they responsible for crank placement? If they are, then a set of offset pins are needed every time a block is align bored?
Thanks for all your help guys. Knowledge is power and all that jazz, and I hate going to a shop and sounding like a schmuck.
I've pulled out a block a couple times and helped build one, but I was with someone who had a lot more expereince.

RB83L69

They are what locates the trans to the block. The bolt holes are huge and have all sorts of tolerance, but the 2 dowel pins fit tight to the bell housing or trans case and positively determine how the trans and block line up to each other. The idea is that it's supposed to be impossible for the trans to not be in a direct line with the crank. If you look at the steps in how a block is made at the factory, the very first machinig they do is to square the bottom of the block and the back of the block, and then drill the 2 large dowel-pin-looking holes in the oil pan flange, and those 2 in the bell housing flange. Then as the block casting procedes through all its finishing steps, all other measurements of it, including the crank centerline, the cam centerline, the lifter bores, the cylinders, the deck (for squareness), etc. etc. are all made by plugging those dowel pin holes into the jigs that hold the block for each of the machining steps.

The offset ones are very often required when a block is align bored, because as detailed above, that procedure moves the crank upward in the block, such that its centerline is no longer in the same as the centerline of the trans input shaft. This forces the torque converter or clutch gear to be at an angle instead of a straight line, and puts all kinds of stress on the torque converter, pump bushing, and flex plate in an auto; or in the clutch gear bearing, the bearing between the shafts, and the pilot bearing in a manual. The offset dowel pins move the trans upward a slight distance to restore the alignment.

GRENDLE

i've seen the bad alignment waste thrust bearings in a hurry, like 30 min. but never seen them crack flexplates, not that i doubt that. but if all else is ok that would be a good place to start. also make sure everything else is good (motor mounts, trans mount and crossmember alignment,bolts etc.) all these are cheap and should probably be replaced anyway. your vibration is an issue though, i've seen bad pulley's do this but not crack flywheels.

Jim85IROC

iTrader: (2)

Call me stupid, but I thought when you had a block align-bored, it along with the bearing caps were overbored, and the appropriate oversized bearings were used so that the crank sayed centered.

SSC

Yep, it should be centered but shiznick happens.

This is an odd problem one that shouldent happen. I would check the engine and tranny mount for that vibration. If the vibration only happens in 1st than we can rule out the drive shaft.
SSC

Loki1199

It even vibrates in neutral. Not the drive shaft's fault

88WS6TA

When a block is align bored the caps and the bottom of the block are shaved to make the hole smaller. Then the holes are bored back to the stock size.