Howdy,

I am chasing a drivetrain vibe and would like to check my math on my modified crossmember. does anyone have an lt1 t56 out of a car they can measure for me? i need the most accurate measurement someone can give me from the centerline of the output shaft to the trans mount surface on the tailhousing(as square as possible too.) i am trying to determine how much i should have moved the crossmember perch down to swap from a 4l60e to a t56. i am trying to rule out the driveline angle as a possible cause of vibration since i had none with the 4l60e, so i want to try to get the center line of the t56 output shaft as close as possible to where it was on the 4l60e. unfortunately my trans is in my car and i would have have to take the whole thing apart just to get a measurement and on top of that it wouldn't be as easy since it is tucked up under the car, etc. Any help it greatly appreciated!

Mike

 

Sorry dont have a measurement but a suggestion have you checked your pinion angle, easy way to gain some insight.

 

Instead of working with parts you're no longer using, get an angle finder, and compare pinion to output angles. This does not require disassembly of the car and will tell you whether you need to shim the mount or modify the crossmember.

 

i actually did use an angle finder to set the trans up as it is now. when i measured it the angles came out less than a degree off, so that is why i moved on to the driveshaft being the problem and had a new alloy one made with new ujoints. i also changed the drums to see if they were off balance and i switched the wheels around. the only things i can come up with besides driveline angle are some sort of imbalance in the rear axle itself, the 5/6 gear in the trans slipping on the shaft(this part i supposed to be a press fit and instead it slipped right on) or just a really bad chassis harmonic since i have poly body mounts, but the vibe sounds too much like it is related to the driveline. since i don't recall having this vibe when i had the 4l60e in it, i figured a good and simple test is to try to get the output shaft right back where it was with the 4l60e. i am not expecting any magical results, just want to satisfy the curiosity, then i am going to have to try some other cheap ideas before i pull the trans back out again, pull the tailhousing off and start messing with internals again.

In other words, tried it, just want to take a shot in the dark for the hell of it.

Mike

 

Wouldn't be the first 27 spline mainshaft that I saw that was bent. Most of the time if we see it has more than .010 runout is could cause a vibration.

 

that's something i wish i had measured when i had the trans apart. i purchased it after someone else had rebuilt it, turned out that whoever built it left out a spacer on the back end so it had to come back apart to add it in, and i bought it after that. after a few weeks the reverse blocker went so i changed that, then, after a few months i took it all the way apart to reseal it with rtv since someone used crappy gasket maker and, from jacking it up and down, a leak formed near the front adapter plate, at which point i also added bronze fork pads on a steel fork and billet keys. i think someone who should not have been messing with this trans had it before me. looking back i should have just sent it out to be checked and rebuilt. i may still do that. by the way, the vibe comes it at about 75 to 80 mph, and from what i have seen it only happens it 5th and 6th but i don't really want to run it at 80 in 4th, so i will have to test that out again to see. the 5/6 gear slid right on and off and i read somewhere that that could be my problem. lesson learned: just buy a new one and get it over with.

 

What was the angle difference w/ the 4L60-E? Did you use the front case mount location or the tailhousing mount?

There's a good thread on ls1tech about the 5-6 fix recommended by some (repair by oversizing the non-spline section of the mainshaft) - you should read it if you haven't.

Have you ruled out out-of-round tires by running them up to speed while on jackstands?

 

Having the 5th 6th driven gear assembly slide off is not a problem. It does become a problem when the splines wear out. I did a Cadillac CTSV T-56 the other week it had 200k highway miles. It had really bad wear on the shaft and the inside spline of the 5th driven gear which drives the 6th driven gear needless to say it vibrated and made noise in 5th. It needed a shaft and a 5th 6th driven gear assembly.

 

http://www.ls1tech.com/forums/manual...vibration.html

(if your problem exists in 4th and 6th, this is not the problem with your car.)

 

To be honest with you i didn't remember to measure the 4l60e before i took it out since i was in a hurry, but we won't get into that. by the time i realized i forgot, i had the flexplate off and the trans out of the garage, but i settled for the fact that measuring the angles of the trans and rear was just as good since that is how you are supposed to set these situations up anyway. my own fault.

the 5/6 gear problem is what i have been leaning towards for about a year now. there was a thread on the problem a year or two back that i had read, but because i had just got this whole setup back together again i was in no hurry to pull it back apart as i am sure you can imagine. i bought this from a guy who rebuilt it after his friend tried to rebuild it and messed up, who then sold it because a pressure plate bolt came loose and he thought the trans was beat. his friend bought it from a guy from the southern east coast who pulled it out of a car and sold it. someone at some point tried to drill a pressure plate bolt out (i'll leave that one to your imagination) but instead, the bit took a turn due to the hardened bolt and drilled out a hole in the flywheel which no one caught until i was about to put it all together and realized that the bolt wobbled when you screwed it in. i took it apart twice, once to replace the reverse blocker ring and once all the way to reseal all surfaces, at which point i realized that someone left the shifter detent spring out but put the ball in, which explained why the shifter was so sloppy. In other words, i am just trying to convince myself that trying something easy would at least save me the heartache of pulling it apart only to find the 5/6 wasn't the problem.

I have somewhat ruled out the out of round tires because it really only comes in when the trans in under load at about 80 and is worse in 6th than 5th. this leads me to believe that due to the angle of the gears, the load in 5th and 6th forces a load on the 5/6 gear parallel to the main shaft and, since it is not press fit, it goes into a "bounce. the problem with diagnosing this is that there is so much road noise, exhaust noise, and just the fact that i have poly body mounts so there is a lot going on it terms of sound and vibe that it can be difficult to really know what is a contributing factor and which sound and vibe comes from where.

i need to test this car at 80 in 4th to see if i get any vibe. a year ago i hatched a plan to just yank the tailhousing and use a retaining compound i had looked up. it claimed to restore a press fit to parts that have lost it but are still low clearance. i think it was a loctite product.

Sorry for the long read, i want to thank everyone for chiming in.

Mike

 

you're on the right path. Good luck.