I got a Chevy 12 bolt from a 71 Chevelle and Im going to be pulling out my chevy 8.5 from my 72 lark. I read in here on old posts that I would have to lenghten my drive shaft 1.5 inches. When I measured the 8.5 from the centerline of the yoke (face) to the center of the axle tube I got 12.500 inches and when I measured the 12 bolt the same way I got 11.750. For semi precision I used a 1.500 square tube about 2 1/2 feet long and held it sqaure up aginst the face of the yoke with the u joint removed then used a steel square to measure from the yoke centerline to the center of the tube. My question is should I add 3/4 inch to my driveshaft or should I add the 1 1/2 inches as some of the older members have said or can I use my drive shaft as is o No: ?????????
drive shaft length I would what till the rear is installed and then take your measurements. Also you can use the 8.5 yoke if you really need to on a 12 bolt they sell a conversion seal for the yoke. Jim
I figured that the axle centerline was the same when installed in the car, based on that the distance forward toward the yoke was what I was going on about. That being said based on that constant should be enough to tell me what my driveshaft adjustment should be. Another option I found was at a transmisson specialty shop they had different turbo 400 transmission yoke lengths which might be easier and cheaper to do than the driveshaft. What does lengthening a driveshaft usually cost?
drive shaft cut cut ing a shaft is about 50 bucks lenghten may be more. I would save that one and get a yard stock shaft and have that cut down to fit. 35.00 for the yard shaft I would think. What length do you need I might have a shaft here that would work. AS a matter of fact I have a ...oh shoot you said it was a 400TH right this was a shaft for a 350 350TH and a 12 bolt out of a 4 door...well get me that length from center joint hole to ceter joint whole and I will check. Jim
I did a straight swap in my 69. All I needed to get was the conversion U-Joint from the 70 chevelle rear yoke to the buick driveshaft. Precision makes one. Driveshaft length was not an issue for me. :TU:
1969 conversion On the 1969 you went from a 8.2 to a 12 bolt correct??? Those rears are the same from center line out to the yoke. This guy is doing a 8.5 10 bolt 1971 1972 rear to a 12 bolt ....TOTALLY DIFFERENT BEAST. Ok? Jim
last reply If that sounded snooty ...It was not ment to be. I just don't want the guy to think that the info is wrong. yours was just a different case. Sometimes this work smoothly. Was not trying to be mean if it came out that way. Did not realize the way it sounded till I reread the post. Jim o No: :TU:
"To add 1.5"s" is way too much. Typically when one goes from any 8.2 10bolt, type "O" rear, or a 12 bolt rearend to a same configuration 8.5 rearend, one will need a * 7/8" * shorter driveshaft. I always suggest to double ck with the new rear in the car, as I've seen from 3/4" to 1 1/8" shorter work, depending on which trans yoke was used on front of driveshaft. Conversely, in this case, going to a stock 12 bolt flange 12 bolt, & having the now too short oem 8.5 driveshaft, one is going to either need an *aprox* 7/8" longer used driveshaft, or one will have to have a new one built. Substituting an 8.5 rears 3R pinion flangeon the 12 bolt pinion will move the ujoint C/L forward aprox 3/8", but not the full *7/8"* :3gears: Roger
12 bolt 8.5 rear Well looks like you better bolt that thing in and measure. Sounds like the best low cost way . measure twice ...in this case build once...lol. good luck. Jim
I just crawled under the car agin real quick before work and looked, the 8.5 yoke looked like it roughly measured 3.250 C/L of joint to housing face that is about an inch or a little longer than the 12 bolts current yoke. I thought I had read that I could use the 8.5 yoke on the 12 bolt, didnt someone say there is an adaptor seal I would need, and does anyone know the part number? As usual you guys have been very patient and helpful THANKS :beer
I would keep the 8.5" complete and install the 12 bolt Chevy. Use NEAPCO 3-3130 rear u-joint and you will be ok. As for d-shaft length, just install the existing d-shaft in the car and see where it lines up with the rear yoke. Then you know if you need to shorten or lengthen it.
Do you know of an adaptor seal I will need or can I just use the one in athe regular 12bolt kit. If I need an adaptor do you know what the part number is?
