Hey Guys!

I've finally got to try out my rebuilt T56 from Texas Drivetrain Performance and it's so much more fun compared to the 700r4 it came with. The transmission shifts smooth as can be. Unfortunately, I noticed a vibration. Above 3500RPM or so the engine vibrates. I looked at many T56 swap threads before my dad helped me tackle this project. I have an 89 TPI with a 1 piece rms. I'm running a stock replacement rockauto clutch and flywheel from LUK. The flywheel/clutch is a 93-97 LT1 pull type clutch setup. Is it possible that the "stock replacement" parts aren't balanced properly? I thought 87+ engines were all internally balanced in the front and external in the rear. The new flywheel had the counterweight on it like it should. I had the stock GM LT1 flywheel from when I bought the transmission, but it was really rusty. The GM flywheel had dowel pins in the outer perimeter of the flywheel for balancing. I read a few tech articles from other members on here about using dowel pins to fix vibrations. I ended up pulling the dowel pins out of the old flywheel and found out that they don't fit into my new one. Any thoughts guys? Am I stuck pulling the trans out and getting it all balanced together? The LT1 harmonic balancer should be the same too as far as I'm concerned?

Thanks!

 

Another thing to mention is it's not the driveshaft. I had a driveshaft vibration after my 3.73s and I ended up getting an aluminum driveshaft that fixed that issue. The 3500RPM vibration happens in any gear, including neutral, and with the clutch in.

 

Did you install a transmission pilot bearing? Your engine would not have had one having been an auto trans.

 

Thank you for the response. I did install the pilot bearing included with the kit. It went in easy.

 

a perusal of the LT1 boards will show a number of balance issues with the stock style T56pressure plate. there was even a TSB. the problem has likely gotten worse with cheaper imported parts.

I had similar issues with a stock replacement PP. Got an engine shop to spin it and the flywheel. the flywheel was good, but the PP was woefully out of balance. spent moderate $ on an upgraded clutch/PP and it showed up with no balance holes drilled into it. it was also indistinguishable from the stock piece I took off. a call to their tech support and they could not confirm that it was balanced, only that it 'should' be balanced by their supplier. sent it back.

purchased a LUK clutch/PP assembly from RR. It actually had balance holes drilled into it and remains vibration free.

any machine shop that can balance a crank should be able to test the balance of a flywheel/pp assembly. they should be able to spin it attached to a crank balanced with weights and see if the balance changes. they may not be able to balance it, but I paid like $20 for mine to be checked.

if your PP does not have any balance holes drilled around the circumference, it's likely not been balanced. even if it does (my first one did) it still may be out of balance.

 

I was kinda afraid of it being the pressure plate. Either way, the trans will need to come out. Hopefully sometime this week I'll be able to get it pulled out and have the whole assembly sent to a machine shop. They should be able to balance it if I told them the front of the engine is internally balanced and the rear is external correct? If the flywheel is properly balanced, couldn't they balance the pressure plate to the flywheel's balance? Thanks for your input. Funnily enough, I'm running a LUK clutch/PP assembly from RockAuto as well. Evidently not all are casted the same...

 

I had this same vibration when I did my T56 swap. Ended up being the LT1 flywheel that was out of balance. Took it in to a machine shop and had it balance WITH the pressure plate. Vibration is gone.

 

Considering I have my stock flexplate that came with the engine from the factory, I'll get the flywheel balanced to the flexplate. Then I'll have the pressure plate balanced. I'll keep you guys updated if I can pull the trans this week. Thanks for the input guys, keep it coming!

 

My 3rd gen was also plagued by this when I T56 swapped. Same RPM range and I even started off with a LuK flywheel from RockAuto too. It vibrated out of the box so I took it all apart and had it match balanced to the original flexplate. Machine shop said it was off a good amount, but it still vibrated after balancing. Ended up building a 350 to replace the 305 (it had other issues) and that "balanced" flywheel vibrated with that engine too.

Ordered a used OEM flywheel off eBay and that vibrated too, but it wasn't as bad. In retrospect, this flywheel had been modified though. There was a chunk of counterweight machined off that was not consistent with the other machining from the factory balancing, and that was also where I had to add weight to improve it.

Not wanting to yank everything out again, I used 1/4" round stock from Home Depot and started adding 1.25" lengths of it to those holes in the flywheel until I found a spot that improved it. The trick is to use a punch to make some raised areas on the rods so that they fit snug. And don't drive them all the way in, leave just enough sticking out to be able to grab the end with side cutters to pull them back out. It took a lot of fiddling around but I got it good enough that I could drive it and not feel too bad, but it was definitely still out of balance and I did not like exceeding 3500 RPM. I would only attempt to balance it like this as a last resort or to fine tune a very minor vibration.

Ended up dumping that car but I kept the engine and trans to swap into my other car. I picked up another used OEM flywheel off eBay and reused the clutch disc/pressure plate/TOB and now it's finally smooth. I'm wondering if that first flywheel had a machining error and the pressure plate was not concentric or something. The second flywheel was just bad luck. Third time was a charm though!

 

UPDATE: I sent my original flexplate, flywheel, and pressure plate to the machine shop. It turns out the pressure plate balance was off slightly off. The machinist balanced the flywheel and pressure plate together and marked where they should match up together. I got the transmission in tonight, but I have a few things to wrap up yet. I'll let you guys know the outcome when everything is back together. Thanks for all of your replies guys! It really helped lead me in the right direction.

 

UPDATE 2: Everything is back in the car. I tested it out and the vibration is still there... I'm beginning to think it's the crossmember or something else. I'm a little upset given I had my whole flywheel and clutch balanced and it still vibrates. The time it took me to pull and install the trans is worse than the $42 I spent getting everything balanced.

 

Could the flywheel, pressure plate, or disc be warped? I would've thought the machine shop would've noticed when they balanced it...

 

So I ended up trying the dowel pin balancing from the GM TSB. I tried multiple positions, but found out that holes 4,5,6 to the left of the counterweight with 6 grams was the best. It still vibrates a little bit from 3800-4200, but everything else is significantly better! 4500+ is as smooth as can be. All of the testing was done on jackstands. When it stops raining everyday, I'll take it for a drive and let you guys know the outcome!

 

do you know the TSB number? I've heard of this one, but not read it. Thanks.

 

Here you go sir.

 

UPDATE: I've tested the car out with three 6 gram dowel pins in the flywheel. The vibration is 95% gone now. In conclusion, anyone that has a flywheel/clutch vibration after swapping in a T56 should try to follow the gm technical service bulletin. Throwing 1/4 dowel pins in the flywheel is significantly easier than dropping the transmission, getting it balanced to L98/LT1 specs and still having it vibrate

 

Glad to hear that did it, wish mine had been that "easy" to solve!