the flexplate, in the center of the flexplate with it bolts to the crankshaft there's a hole for a dowell pin to go through the flexplate and into the crankshaft centering and locking the 2 together . . . well my buddies isn't there and the mechanic he took his car to said it's not necessary but he doesn't believe him. he thinks this has to do with the vibration he's gettin if its not centered perfect

 

the bolts and crank will center it...ic is more or less to keep you putting it on one way instead of off a bolt hole....

and ls1 are internally balanced so flexplate shouldent cause a vibration unless it isnt zero balanced

 

i let him know, thanks for the fast response

 

I'm the friend, thanks j.p.
So i just picked my car up from the shop and the vibration is worse and its not going into 4th gear now.... It thinks its in 4th because my converter is locking up @ 42mph like its supposed to in 4th only. Because my tuner turned off lock up in 3rd gear completely. And no codes for a bad silonoid or electric issue so i'm guessing mechanical as in installer error!

This vibration issue is killing me though! I'm to the point where i dont want my car back till this is fixed! Kind of a long read but if anyone has any guesses i would really appreciate any advice
The vibration started after I blew the rear end. I get my car from the rear end shop and theres a bad vibration so i look and see my driveshaft is partially twisted like an aluminum can, okay guess the rear end shop missed that but they did a good job on the gears so whatever... So i replace driveshaft with stock aluminum out of a z28 the wrecking yard told me vibration got much better but is still bad enough to not want to drive on the freeway and bad enough that it feels like its going to come apart by 90-100 so not really enjoying my car at all....
had the new DS balanced it was perfect already so thats not it i'm on a stock torque arm with polly bushing so the pinion angle should be fine.

So the car went into the shop because I got on it in 2nd gear with the tires spinning broke something internal as it went into 3rd. trans let go rpm's shot up.. When they pulled it apart...they found the reverse input drum was ballooning in the middle and some of the tabs were kinda flared out this rubbed and grooved my beast sunshell and it actually had to be replaced...
While I was there one of the mechanics was showing me how theres a dowell pin hole in it and he thought i needed one... I thought maybe if there was any play around the crankshaft and the center of flexplate it could have been throw off center when i blew the rear end. I have a sfi approved flexplate that was in before the rear went, after the rear went i swapped converters to a yank pt3800 and the vibe stayed the same. the trans guy thinks the new driveshaft is too short by an 1" to 1 1/2" there is approx 2" from the tail house to the flange on the slip yoke, is this too much? Or he thinks it could be the harmonic balancer which is stock btw... but i dont even think thats a possibility with how bad the vibe is and how it seems worse in 2nd and 3rd gear and stopps completely in neutral when coasting.... I NEED HELP! or a good ******* shop!

 

I had a vibration and my flex plate was cracked in 3 spots. So you might want to look at it again. Also im on a stock rear and i still have a little vibration from my axles being bent jack the car up and put it in gear and look at the rear tires spin plus this rules out anything in the front of the car. If you still have the vibrations you might want to get you wheels balanced.

 

Yea my stock flexplate was cracked at all 3 holes out to the ring gear also, It was replaced with a Hughes "I think was the brand" sfi unit already long before this vibration began.