Jon M

I just bought a 1995 Pontiac Trans Am LT1. 103,000 miles with motor rebuilt at 70,000. Edelbrock headers, no cats and a hypertech tune. A McLeod clutch was installed last year with the flywheel being resurfaced by a regular Chevy dealership. When the car hits 4500k the whole car shakes like its on rumble strips. Does not matter if its in gear, not in gear clutch in or clutch out the car shakes terribly. Starting with spark to try and figure it out but I feel something is crazy off balance. Thoughts?

BALLSS

Many "rebuilt" motors are "internal" balance vs the external (rear) balance of a stock LT1. If the FW was not "neutral", aka zero, balanced than the motor will vibrate like a dog shiating razor blades

what is odd if the dealership just "resurfaced the FW and assuming it was neutral balanced it should be good. With that said dealerships typically (read never) resurface anything. They just bolt in new parts. If they put a stock FW in on a neutral balanced motor it will vibrate significantly. This will destroy bearings and cause RMS leak over time

Given you get this vibration with clutch in or out and if I understand correctly when just revving the motor...sounds like a balance conflict with motor & FW

A machine shop can neutral balance FW for around $40-$50 or you can buy a neutral balance FW but I would have it checked at a machine shop...my exploding wallet billet FW sold as neutral was 26 grams off

If it only happens at speed than your DL angle can be off or your drive shaft needs balancing, or is bent, or u joints are bad

If none of the above than "grounding" can cause vibrations throughout the car (like when exhaust is contacting frame, x member, etc)

Jon M

I spoke to the guy who did the motor and did not know to ask that question. He does do everything stock and two other shops told me he is OCD about his builds and they trust him.

You are correct, simply revving the motor causes this god awful noise and I suspect it is something out of balance and nothing in the DL. My mechanic wants to double check spark first then we are going to pull the transmission to check the FW.

Luckily this car was owned by an OCD clean freak who rarely drove it so I hope there isn’t irreversible damage done to it already. Thank you for your input.

BALLSS

ask the builder if he did a internal or external engine balance. If the FW is a stock one there would be about a 3" cast in raised weight on the back of the FW. Also usually there are a few drill holes partially in the FW in random places. You could just pull the dust cover from bell housing and rotate engine around until you could see if it is there

your note the Dealership resurfaced the FW. You sure they did not replace it?

If they put a stock FW on a internal balanced motor it would vibrate but generally you feel that vibration at any rpm, even idle

you sure its a vibration and not misfire?

Jon M

I will go back and ask him. We are going to dig into it on Sunday. The dealership said specifically resurfaced and the only new part was the clutch.

The best way to describe it is hitting rumble strips at about 50 mph when at 4,500 rpm. It does vibrate a bit at idle but not knowing these cars that well I assumed it was a 24 year old muscle car noise. My 84 Z28 did not vibrate like this at idle but different motor too. The idle vibration feels like a misfire or an air fuel mix issue.

I have done three plugs on the driver’s side so far. Cylinder #4 is weeping a little oil from the plug hole. We are going to work on this Sunday, but I asked here for all those seasoned LT1 owner’s input in case it was something obvious. And I did hours of searching but only found a few threads like this.

BALLSS

A key answer is if the engine was rebuilt as "internal" balance...which is the most common way of rebuilding as the cars FW or flexplate is not used. Just the "rotating assembly" (crank, rods, pistons) is balanced. Those motors require a neutral FW.

If the PO had the stock FW re-installed with a internal balance motor he certainly should have the vibration also. The resurface should not have killed the external balance part of the stock FW

I would confirm with builder if it is internal or external balance before digging in any further.

If this is not a balance issue than ignition (misfire) is something to check

On the "stock clutch replacement"....the PP are balanced and there would be a paint dab on it to line up with the dowel pin on crank. MANY aftermarket clutches are not balanced at all as the Valeo OEM ones are long gone. There have been more than a few threads where people replace the clutch to find there is a vibration issue and it was the China clone PP that was not balanced.

You can tell if it is as the 3 "ears" on the PP face that engages disc will have holes in them of various depths and quantity (pic)

Jon M

I’ll try this reply again. It was the throw out bearing. Once the pressure plate was off the engine ran smooth with the flywheel on. You can see the bearing wobble when you spin it by hand.

SS RRR

iTrader: (2)

I don't see how a TOB could cause an engine to wobble like that. I'd be more apt to believe your PP is the real issue.

Jon M

I keep trying to post pics but it will not let me. We are having the pressure plate and flywheel checked. You can see the bearing is out of balance with the naked eye.

SS RRR

iTrader: (2)

Dunno... I'd be shocked if it were from a TO bearing, especially because it's fixed on the crank centerline and it's light enough where I can't see how the centrifugal force produced could cause a noticeable engine vibration? That's my guess anyway...

Jon M

We shall see! Still waiting on the machine shop to finish up the PP and FW

BALLSS

Joe...the TO bearing is sloppy and does have what can be described as a "asymmetrical" rotating pattern around the trans input shaft

Given you have no vibration without PP I suspect the PP is what the issue is.

I posted a pic of one as an example of where balance holes would be drilled. If your PP is a clone China made one (they are all that now) and does not have any balance holes drilled in it than that is the most likely suspect

If it does and has a "paint dab" on the PP that paint dab is supposed to line up with the dowel pin on crank

You can use a 3rd party service like postimage to upload pics if you can't otherwise load pics

https://postimages.org/

Jon M

https://postimg.cc/gallery/2ulhfk0n4/

I did not see those style weights on my PP. My FW is cast iron, however it has no paint on it.

BALLSS

Don't see a pic of the "back" side (facing engine) of the FW. That is where the cast in weight is on a stock LT1 FW (pic) and also a few drilled areas to balance it to factory (external-rear" balance

The holes around the perimeter of FW are also used where pins of various lengths (different weight) are put in. Not on stock ones but machine shop can use them to insert pins to balance it. You can see them in th epic with yellow PP. That is when I was using a stock FW with the weight ground off and balanced neutral when I was running a single disc clutch. Notice the holes drilled in PP ears...that is also for balance. The clone replacement PP's often do not have these....which is suspect in your case since you say with just FW you don't have the vibration.

Is the weight still cast in the back side of your FW?

Jon M

It is still at the machine shop but I am about 95% sure it did not have that weight in it

SS RRR

iTrader: (2)

The machine shop should test to see if the flywheel has been zero balanced. It's possible the engine was internally balanced and the flywheel was not zero balanced. It's always a good idea to have the PP and flywheel balanced together regardless of internal or external balance.

BALLSS

^^^yep. First they neutral balance the FW then bolt PP to it and zero out the PP

...But if Op's engine was rebuilt as a stock rear/external balance motor than the stock FW, with cast in weight, would be right...AND a PP that is balanced neutral...which is why the original Valeo's had balance holes of various amounts & size (depth) drilled in the PP.

If Op's PP has none of these holes it is likely not balanced correctly and needs to be neutral balanced

Op has to confirm how his engine was rebuilt. Internal or external balance.

Jon M

Engine was rebuilt externally balanced. They found the PP needed a 15 gram weight on it. And the FW does have the drilled holes in the back.

BALLSS

while the FW may or may not have some balance holes partially drilled in it...a stock FW absolutely has a "cast in" weight (see pic I posted, top has about a 2 1/2" cast in weight)

If the PP needed 15 grms….was that to make it "neutral" balanced? (generally they "remove" material to balance, not add weight but whatever method as long as it works)

Jon M

They welded a weight it. I did not see the other side of the FW as I am out of town and its back together. Apparently the vibration was fine but as soon as the transmission was put back in, it still vibrates a bit. He said the pilot bearing was oval.