Want to thank all the forum members for the wealth of information provide here. It has enabled this overaged rookie rebuilder to make the progress I have on my 1971 roadster restoration. My question is regarding my ordering a correctly sized driveshaft for my semi-stock rebuilt drivetrain. I have installed a GMPP ZZ383 crate engine coupled to a GM turbo-400 shorttail transmission, using the Energy Suspension poly motor mounts and Prothane poly transmission mount. With these components in place, it is my thought that all dimensions effecting driveshaft length needed for my car, have stayed pretty close to stock. I was advised to check out Fort Wayne Clutch Company for a balanced driveshaft, and they offer a very attractive price for what I need, so I call to order, and this is where it gets interesting and confusing to me. The tech says, pull transmission yoke out 1" from fully seated position, measure center to center on U-joints, end of story, which puts me at 28.50" length. Maybe I should have just ordered it, but looking at the IRS set-up these cars have, I got to wondering if the subtracted 1" the tech called for when measuring, might be for a straight axle type suspension, and not really apply to the Corvette IRS. So I dug through my reference books and found that a stock 68 thru 72 automatic transmission Corvette driveshaft measured 29.50" center to center, which would eliminate a whole bunch of free play on the drive shaft and yokes on my car. So my long winded question amounts to: Do I go loose as a goose at 28.50, or right up tight at a stock driveshaft length from factory of 29.50". My untrained knowledge tells me the suspension set up of the Corvette keeps the angle and length of the driveshaft in these cars at a relative constant, that the halfshafts are where the changes take place. My initial thought is to split the difference on the two lengths and order a 29" center to center, not to loose,not to tight.
Would like facts on this particular question, no opinions, only because I want to get it correct the first time!

 

What trans yoke are you using? Recall that it has to have the removable u-bolts or straps to release the u-joint in the Corvette. (The same requirement exists for the differential yoke.)

Can you post a picture?

 

If the shaft is bottomed out in the trans, any flex in the frame/trans mount/diff mount could cause it to bind and possibly break something. Even with the yoke out 1", there's still plenty of spline for the yoke to grab onto. And the second part is that if the shaft is a full length, you won't be able to remove or install it without moving a major component. Be a bite to have to drop the diff or move the engine/trans forward to get the driveshaft out. Or in.

 

1968 thru 1972 Turbo Hydramatics and 1971 thru 1972 both manual and automatic transmission, all shared the same transmission yoke, which is the
u-clamp/bolt style, 31 spline. This type of driveshaft set-up requires minimal freeplay for assembly.

 

Hi WW,
Too dumb to help but I did enjoy your picture.
I love running gear photos!!!!
Regards,
Alan

 

Both yokes look right to me.

To install the drive shaft, the trans yoke needs to be pushed all the way into the trans to create approximately 1/2 inch of clearance to get the u-joint out of the yoke.

When I get my car up on jack stands to do a rear end change tomorrow, I'll measure its length.

 

Thanks Larry,
If you could measure exact U-joint center to U-joint center, and also give me an idea of the distance from the end of the transmission itself to the front of the transmission yoke where the clip bolts to hold in the U-joint. That measurement will give me a fairly accurate idea of how much free play you have in the line. The picture that I posted of the transmission yoke, shows the distance with the yoke pushed in as far as it will go. If yours measures an inch more, than I would think that that inch is probably free-play.
Willem

 

I did some measuring...

o My driveshaft (1978 with TH350) measures 29.50" u-joint centerline to centerline.

o With the driveshaft installed, the front u-joint centerline is 2 inches aft of the backside of the crossmember.

o The back end of the TH350 is 1 inch forward of the backside of the crossmember.

o The difference in the overall length of the TH350 and TH400 is 11/16"
(http://www.tciauto.com/Products/TechInfo/trans_dims.asp)

Therefore, a TH400 should be 5/16" forward of the backside of the crossmember; your picture shows that the trans is correctly located.

However, your yoke (which you said was fully inserted) appears to place the u-joint centerline about 2 inches aft of the crossmember - this is where mine is with the drive shaft installed, which when I pull the drive shaft, it can move about 3/4 inch farther into the trans. (I haven't pulled it yet, but will soon.) Nonetheless, I'd conclude that your yoke is too long for your Corvette application, or if it isn't, it is hitting something which prevents it from sliding farther into the trans.

Could you pull the yoke and measure the machined length and/or post a picture? Meanwhile, I'll measure how far mine will shift forward until it bottoms.

 

Now that the driveshaft is out, the trans yoke slid forward 1 inch such that the centerline of the u-joint was 1 inch aft of the backside of the crossmember.

 

Hi Larry,
Thanks for helping me out. Pulled the yoke and measured it. It matches correct yoke dimensions and has the correct GM part number on it. I also inserted it into my original 1971 4-speed, and it slid in all the way to the yoke base, so I think that the yoke is OK. I do not know what the Turbo-400 I have came out of. I believe it to be a much later model than an original 1971 and I also believe it was from a truck, due to the oversized pan. It has been overhauled by a local shop, and a shift kit installed that is supposed to duplicate shift points that would be comparable to a Corvette transmission. Do you think that the yoke may be to long for this transmission? I was under the impression that the turbo 400 short shafts were physically all the same.

 

Well after checking out and eliminating the yoke as the problem, turned my attention toward the fact Larry pointed out about yoke not fully seating into transmission. After many searches on turbo 400's and yokes, hit on an article about truck turbo 400's having a different output shaft and yoke configuration. Going to call a couple driveshaft shops tommorrow and see if I can still, somehow utilize my NOS Corvette yoke. I want to stick with the 1350 Spicers for strength. I learn something new everytime I progress to the next step with this project. Isn't learning fun!

 

U-joints...
1350s were used for the half shafts, but 1310s were used for the driveshaft with C2s, or 1330s in later C3 cars. I don't know for certain when, or with what applications, they were changed to 1330s, but there may be issues with trying to use 1350s for the driveshaft - it's overkill. Here are the dimensions of the u-joints:
1350 series
* 3.625 inches wide in both directions
* All 4 bearing caps are 1.187 inch diameter

1330 series
* 3.625 inches wide in both directions
* All 4 bearing caps are 1.0625 inch diameter

1310 series
* 3.219 inches wide in both directions
* All 4 bearing caps are 1.0625 inch diameter

Your yoke looks correct. I'd like to hear what you find with the output shaft.

 

I think you are over-analizing this. Unless you are doing a NCRS resto,You do not need to worry about where the yoke sits in relation to the crossmember.
The yoke is the right one, so use it.
The 1 in. that the driveshaft maker said you need IS NOT slop. As all componets are fairly solid mounts, there is basically no movement during use except for the above stated flex. You WILL need this 1 in. to be able to remove the drive shaft after the car is assembled, also as stated above.
So order your 28.5 or even 28.25 in drive shaft, get it in, so we can see a body on that thing!!!!

 

Hey Larry,
Thanks again for all your help. Turns out that the turbo 400 I have, being a truck transmission, has an output shaft that is set up totally different from a car turbo 400. Unknown to me, this difference in output shafts is what is preventing my yoke from properly seating into the short tail of the transmission. The truck output shafts have a threaded hole at the end to actually bolt the truck yoke into place. The truck output shafts also increase in diameter approx 3" into shorttail for increased strength, and this is what is preventing my stock Corvette yoke from fully seating. What I will do in order to bring my yoke to the correct depth in short tail, is counterbore the yoke the same amount that it is overextended, allowing it to fully seat into short tail and get the correct amount of support from the bushing. With this done, it should accept a standard length drive shaft. And you are correct in pointing out that the Spicers on the driveshaft are 1330's. I am hoping I will not find out any other differences that this turbo 400 may have, other than the new Hughes 2500 converter that is installed!

 

Very interesting, Willem. I learned something too!