Driveshaft lengthening sanity check

[asgeirogm]

Hey everyone,

The 78 Monte Carlo I bought last year previously had a 350 with Th700 transmission, but I bought with a Th350; long story short - the driveshaft I have is too short.

I just wanted to do a sanity check before I fork over the cash to have it lengthened, lest I screw up and have to do it again later.

What I've done to figure out how long it should be is jack the rear axle up by the housing, put the slip yoke all the way in so it bottoms out, prop the driveshaft up with some planks near the pinion yoke so it's close as I can get it to being at the correct angle (i.e. pointing directly at the center of the pinion yoke), then measure from the center of the driveshaft outer u-joint to where the center of the u-joint will sit in the pinion yoke. Then I have the distance of how much longer the driveshaft would need to be if the slip yoke was supposed to bottom out in the transmission. Then, subtract 3/4", and now I have how much longer my driveshaft needs to be while still allowing for travel and installation.

Am I missing something?

I think it's worth mentioning that right now, the car is completely void of interior (i.e. it's lighter than it will be), and the tires are bigger now than they will be (both front and back rub, so everything needs to be a bit smaller). I assume these factors will not have a material affect on the driveshaft length for me, right?

Then there's the matter of the angles; just in case it's relevant, then looking at the following diagram, my measurements with a digital angle finder as are follows:
Angle A = 0.2 degrees
Angle B = 0.7 degrees
Slope A = 0 degrees
Slope C = 0.9 degrees

I'm not sure if I need to try to adjust the pinion angle since a 0.9 degree between the transmission slope and the pinion slope is too great, or if it's close enough; or if Angle A of 0.2 degrees is too small, etc. - I just figured that if I need to make changes, it might be relevant for the driveshaft length (although I doubt it), so I should include the info.

Thanks in advance!

 

[Ugly1]

There is probably a tech page on here for it. But with the yoke all the way in measure center of U joint to center of rear end u joint. It’s been a while since I’ve done mine so calling where you’re getting it made to double check. Angle should not change much if that’s the only change. Depending on the HP you plan on having to may want to tell them and have a HD one made. Ask them questions is best.

 

[WanaBa442]

Didn't see it mentioned but if you're staying with stock suspension and rear axle I'd source a shaft from one of the million or so G bodies that came with the TH200 and 7.5 rear
Bolts right in.

 

[El1987]

I'm no expert in this field, but the one time I did have a driveshaft made we measured while the car was in the air and it didn't work out. The driveshaft place told us we needed to measure while the car was on the ground. Second time it fit perfect.

I only mention this because you said you jacked up the rear end before measuring.

Personally I'd go with WanaBa442 's advice and find one that can bolt right in.

 

[asgeirogm]

Didn't see it mentioned but if you're staying with stock suspension and rear axle I'd source a shaft from one of the million or so G bodies that came with the TH200 and 7.5 rear

I don't know if the suspension is stock or not; the car used to have hydraulics before being put on springs again, I don't know if it's stock setup it went back to or not. Additionally, I suspect the rear has also been fiddled with, as the diff cover is clearly not stock, but I'm not sure if anything has been changed that would matter here though.

In any case, I live in Denmark, so I have very limited access to G Body driveshafts, unfortunately I haven't been able to locate one nearby, so all things considered, having the one I have extended seems like my best option.

I only mention this because you said you jacked up the rear end before measuring.

Yep, that's according to all instructions I've found as it's the same as having the car on the ground; I just needed extra room to be able to crawl under the car for doing the measurements

There is probably a tech page on here for it. But with the yoke all the way in measure center of U joint to center of rear end u joint. It’s been a while since I’ve done mine so calling where you’re getting it made to double check. Angle should not change much if that’s the only change. Depending on the HP you plan on having to may want to tell them and have a HD one made. Ask them questions is best.

I'll make sure to go over my measurements and all that with the guy that'll be doing the work, that's a very good idea I don't think I'll ever be going for any real power on this car, so this one should be enough

 

[melloelky]

eliminate all doubt-ask the person(s) making your shaft what their measurement algorithm is,that way there's no question of fitment or rework$.

 

[Rt Jam]

I don't know if the suspension is stock or not; the car used to have hydraulics before being put on springs again, I don't know if it's stock setup it went back to or not. Additionally, I suspect the rear has also been fiddled with, as the diff cover is clearly not stock, but I'm not sure if anything has been changed that would matter here though.

This is a big part of this equation. Will the car sit at normal ride height? How far does the suspension droop if jacked up.

The pull out of 3/4 is a general measurement. It also helps when installing the driveshaft.

 

[86LK]

Denmark? I'll be visiting Europe in June but don't think I can slip a driveshaft into my luggage and then drop it off for you

 

[565bbchevy]