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Dumb Newb Driveshaft Mistake


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Five speed conversion issues...  I've been making steady progress but just made a booboo and the question is what if anything to do to recover. This is long winded, but bear with me.


So I fitten the 5-speed tranny in and measured the new driveshaft length at 155mm flange to flange and then subtracted 28mm for the guibo thickness. I got the DS front section shortened by 78mm which should have made the length correct (or so I thought). I installed a new center bearing and used just the front section with the guibo mounted flange to flange as a spacer to position the tranny cross member so the center bearing fit where it belonged. It spun nicely and everything seemed good....


But, when I measured the DS length, I didn't reposition the diff into the center of its movement slots (it was nearly all the way back) and it looks like I measured the DS about 1-2mm long... (DOH...)


So when I fully reassembled the DS and fit it in, it was pretty tight and takes modest force (easily pushed into place with one arm) to push it into place (flange to flange) because it is a bit too long. I loosened the tranny bolts and pushed it back further and got a half mm or so more.  Now, if Iinstall the guibo correctly, the DS flange is pushing the guibo forward by about 1 (maybe 1.5) mm from the flanges just touching.


So... the question is how much front to back axial load is acceptable on the system (guibo, U joints, etc) if everything is lined up correctly. It seems like I can either push things together and accept a little forward pressure on the guibo or take the DS and get it shorted enough to bring everything into a non-preloaded state.


What is the consensus? Put it in and see how long the guibo lasts or get it shortened some more (by the extra 1-2 mm and enough more to bring the diff into the center of its adjustment range) and whack myself on the head while chanting "dont f.. it up next time" to myself twenty seven times as I shell out the extra cash. In looking through the archives, it looks like there is clear consensus that any misalignment between the DS and the front section of the DS is evil. It is less clear if a little forward preload on the guibo is equally bad... It does make it harder to check the gaps while rotating the DS to confirm alignment as the gaps aren't there...  


I am leaning to shortening the DS to the right length unless someone can save me from my overthinking things...

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You shouldn't have "catastrophic guibo failure"--it's impending demise will announce itself with definite driveshaft noise...and even if it breaks, the pieces can't come out due to the six bolts holding it in place.  When I replaced the guibo on my '69, it came out in seven pieces but was still moving the car around--just a lot of driveshaft noise. 


But making sure the guibo isn't flexing (angularity between tranny and driveshaft) will kill the guibo in a few thousand miles...ask me how I found out!




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On my 4 speed, the guibo failed on the freeway without any advance noises or other warning and the flailing loose bolt tore our a chunk of the selector shaft housing... Granted it was an eight bolt, but there really wasnt any warning...

Agreed, have had an 8-bolt guibo puke a few times with no warning on track--guess I did not inspect it closely enough or replace it often enough. As above, in one case it took out part of the selector shaft housing--fixed it with JB weld and git many more miles out of it.


I don't like having any preload on my drivetrain, can't imagine it is good for the bearings. But the car sees only track duty, so loads are high.


Are 6-bolt guibos stronger and/or do they have a softer failure mode?


--Fred '69 & '74tii

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