I am trying to diagnose a vibration my car has had since I have owned it. My car is a 1970 Skylark, with a 455/TH400. While searching the forum I have seen mention of different driveshafts for big block cars and small block cars. What would the difference be, length? Diameter? I am assuming my car still has the original small block driveshaft. I also read that the length for the shaft in my car should be 55.10" Driveshaft length should be measured from the center of the u-joint cap to the center of the u-joint cap, correct? Is the 55.10" figure correct? If so, my driveshaft is too short. Using those reference points it was under 54". Also, between the tailhousing and the yoke there is probably a couple of inches at least. This should be 3/4"-1" I believe. The car only vibrates when it gets up to around 60mph or so. I got under it last night and was able to move the yoke behind the tailhousing side to side just a little. Is there supposed to be any play there? If that bushing is going bad could the driveshaft be causing it? I ask because the transmission was rebuilt by a very reputable shop and I assumed they replaced that bushing if it was needed. From Denny's Driveshaft FAQ: Does the short driveshaft theory look like it could be my problem? So far I have replaced the steering and suspension, had the driveshaft balanced, put a posi in it, stall converter and shift kit and had the transmission rebuilt, tried multiple sets of tires and wheels. Overall the ride has improved somewhat, but that vibration on the highway just takes all the fun out of it. BTW, type slow and use small words, I ain't too bright sometimes.
May very well be the problem. A driveshaft that's too short means the yoke is not properly supported in the transmission's output shaft housing. There's a bearing pressed into the housing that often gets beaten up pretty badly because of this. Replacing the bearing would only be a temporary fix. When the rear suspension is at full rebound (the back of the car is bottomed out on the suspension) the yoke pushes forward into the output housing. You only need enough yoke travel to make sure that there's no output shaft interference at full rebound. Devon
1970 455/400 with 8.2" rear end, shaft should be 55.10". Brown/blue tape. 1971 455/400 with 8.5" rear end, shaft should be 54.241".Yellow/green tape. Not sure what year rear you have or what tail is on the TH400. I think it varied a bit between the A-body cars & the full size models.
heres my 2 bits if its not the yoke: is the vibration in the front? steering wheel shaking? Possibly shocks bad? Worn front end parts? Vibration in rear? U-joints messed up? Drive shaft out of phaze? a bent rear axle? worn control arm bushings? Good luck in narrowing it down, Gord
heres my 2 bits if its not the yoke: is the vibration in the front? I don't think so. steering wheel shaking? no Possibly shocks bad? Worn front end parts? Could be, but I replaced all those when I bought the car, I have only put about 2000-2500 miles on it. Vibration in rear? Feels like it, kind of like a "magic fingers" bed. U-joints messed up? Replaced those Drive shaft out of phaze? possibly, how can I check? a bent rear axle? I had them out when I replaced seals last year, I don't remember if I checked them for straightness or not. Wouldn't this vibrate at lower speeds too? worn control arm bushings? Replaced these too. Good luck in narrowing it down, Gord __________________ Thanks! I suppose it's possible after all this work I got a bad part or didn't install it correctly, it just seems to me if that were the case the symptom would have changed a little. Thanks for the replies, I appreciate it! :TU:
A bad bearing in the differential will give you those symptoms. I assume that if you had the original slip joint shaft it was thrown away and has been repalced.
Brian your narrowing it down, if your yokes on each end of the driveshaft are perfectly inline when you look down the tube, your shaft is phazed. Many people including some mechanics do not understand the importance of driveshaft phazing, or how to phaze a driveshaft, it is important. I had an experience lately with a driveline vibration and sent the shaft out to get it balanced, the shop apparently balanced it and sent it back to me. We installed it and the driveline still vibrated, turns out the driveshaft was bent not out of balance...I guess i forgot to tell them to check that too! As Devon mentioned a driveshaft thats too short may make the driveshaft to whip if the slip yoke doesn't have enough spline contact with the tranny output shaft. use the Kiss system and you'll figure it out..Gord
Your driveshaft does sound like it is too short. How are you measuring how much yoke is sticking out of the trans? You have to jack the car under the rear end housing so the suspension stays "loaded". You are correct, there should be about 3/4" of an inch of the yoke showing.
