Driveshaft question:

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Bill Dedman

Bill Dedman

bill dedman
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I am replacing my u-joints (7260...small) with the next size up (7290...larger), and am wondering about driveshaft length.

The center section I will be replacing at the time of this swap (a 3.55) has the 7260 yoke, and the front yoke is also a 7260-size.

I have a 7290 front slip-yoke to use, and the 4.10 center section I'll be installing will have the 7290 yoke already on it.

A new driveshaft will, of course, be required.

So, my question is:

Is the center-to-center distance from the front u-joint to the rear u-joint going to change whan I change u-joints?

I think not; the only reason for it to change would be if the distance from the companion flange to the center of the u-joint on the two different rear yokes was not the same... and, I don't believe it is different.

I can't imagine that it would (change), but I thought I'd ask, anyway.

Does anyone have any information to the contrary?

My hope is that I can get a new driveshaft made with the identical u-joint to u-joint dimensions, old-to-new.

It's probably an unnecessary step to take (changing to larger u-joints), but I have to DRIVE home from the drag strip, and as we all know; a chain is only as strong as its weakest link....


Now; has anybody ever sheared off the lugs on a SBP 8.75" rear axle????:angry7:

I hadda ask.... lol!

Bill, in Conway, Arkansas (60 miles from the nearest REAL drag strip.... and IT is only 1,000 feet...)
 
Now; has anybody ever sheared off the lugs on a SBP 8.75" rear axle????:angry7:
Click to expand...

Bill,having a new driveshaft made is a wize choice,if you plan on making some serious power,i had a 3" made with large front yoke and the 1350 joint in the rear with billet pinion yoke,find a company to make you one and they'll tell you how to measure for the new one...as for shearing off wheel studs..i broke all 5 off the driverside 2 seasons ago,,thats when i stepped up to Moser axles with 1/2" studs..don't plan on breaking them..:-D
 
Wow, Stroked 340.... sounds like I may need to spend a little more for my own good, here. I've seen cars shear off the wheel studs on a launch, and, it ain't a pretty sight....:angry7: Don't wanna go down THAT road!!! The puny-looking studs in these SBP axles look mighty weak, to me... but, then again, I have an automatic with a 2500rpm stall converter, and 8" Drag Radials, so I may not be ABLE to hurt 'em.... but it's not worth taking a chance.

I already have a spare set of SBP axles; maybe I can get them drilled for half-inch studs. and a 4.5" bolt circle.

Thanks for responding, Stroked340... I really appreciate the good advice!!!

Bill, in Conway, Arkansas
 
I just went from a 7260 rear to new ham that had 7290, I needed a new shaft anyway so I went for 1350 pinion yoke, 1350 trans yoke and had the shaft made with solid 1350 joints... the old shaft measured the same as the new 1350. so I am sure it will be the same.
 
I was hoping it would be the same; simplifies things a lot. I'll just take my old driveshaft with me to the machine shop and have them duplicate it with the bigger joints.

Thanks a lot for the good info!!! I appreciate it.

Bill, in Conway, Arkansas
 
Bill, I don't have any exact knowledge, but I'll give my thoughts.
I guess you have to know that the shaft you have now is the correct length, before duplicating it. I'm thinking a new one w/ bigger u-joints would have a slightly shorter "tube", because the larger diameter u-joints have to be compensated for.
All that aside, there's a method for measuring for a shaft. It considers sufficient spline engagement in the trans and suspension travel. I think, when we had one made for our stocker back in '94, this is what we did. The key things are that it doesn't go too far into the trans when the suspension is at the point where the pinion yoke is closest to the trans, or have insufficient engagement when the pinion yoke is farthest from the trans. I'm sure you're aware of this, though.
I would prefer to make a new one to my own measured specs, as opposed to duplicating an old one that may or may not be the correct length.
I would check this theory before using it. Maybe the shop that builds the shaft could offer advice?
 
You will not have any length issues when going from 7260 to 7290 joints.
When I had a drive line cut for my car I went to pick&pull and got one out of a big block c body that already had the 7290 and just cut it down and trough new joints in it.
When they asked for the trans to rear measurement they had me measure 'car on ground' from the end of the output shaft to the flat of the rear end yolk, works great.
 
Technically the way to measure is car on ground, put the slip yoke in the trans all the way, and measure center to center on ujoint caps and then subtract and inch. This makes sure the slip yoke has suspension travel and wont bottom out in the trans.
 
Gonna do it the "right" way.... You all seem to agree it needs to be measured with the car on the ground, so, that's what I'll do.

I appreciate very much all the good advice!!!

Merry Christmas to you all...

Bill
 
Bill Dedman said:
Wow, Stroked 340.... sounds like I may need to spend a little more for my own good, here. I've seen cars shear off the wheel studs on a launch, and, it ain't a pretty sight....:angry7: Don't wanna go down THAT road!!! The puny-looking studs in these SBP axles look mighty weak, to me... but, then again, I have an automatic with a 2500rpm stall converter, and 8" Drag Radials, so I may not be ABLE to hurt 'em.... but it's not worth taking a chance.

I already have a spare set of SBP axles; maybe I can get them drilled for half-inch studs. and a 4.5" bolt circle.

Thanks for responding, Stroked340... I really appreciate the good advice!!!

Bill, in Conway, Arkansas
Click to expand...

Bill If it was me i would just buy a new set of axles for the 4.5" BP the center register is smaller on the 4" BP so if you have a hubcentric wheel it's going to be a problem . You'll also need to replace the rear brakes as the axle offset for the available brakes for that bolt pattern is different .

Spend it now or spend more later .
 
I thought about that center-hub problem.

I also thought about the brake drum problem.


I am a cheap so-and-so.... hate spending money on anything that slows the car down (and, brakes will do that), so what I was wondering about was this:

Can I re-drill my existing 4" sbp axles for a 4.5" pattern and if I CAN, why can't I re-drilll the brake drums, too? I can find a way to deal with the wheel-register problem. I have a friend who's a machinist, and could probably have him make up two spacer rings to weld into the center of the steel wheels I use on the back to register the wheels on the smaller axle bosses.

Failing all that, if I have to have special aftermarket axles made anyway, and also have to buy wider brakes with the 4.5" bolt pattern, I probably will just stick a complete B-body housing under it, and be done with it. They are only an inch-and-a-half wider than what I have now, and they are still pretty plentiful in junkyards (cheap.) The width issue is easily dealt with by using wheels with a custom-spec back-spacing.

Decisions, decisions, decisions...

Maybe I'll just replace these wheel studs in the existing "sbp" axles with some larger-diameter, Grade 9 studs, and hope for the best.

I just don't want the thing shearing off those little bolts at an inoppertune moment (is there an "oppertune" one???) LOL!!

Thanks again for the advice; I'll probably end up taking it; just "thinking out loud," here...
Bill
 
I have to agree w/ JohnRR. I appreciate the value of a dollar as much as anyone but, as you said Bill, when one of those wheels goes flying, it's a mess. (And it could happen on the street, who knows who gets hurt then.) Aftermarket axles can be had pretty cheap these days. Myself, I'd save up and get a pair and whatever is needed to go w/ them.
I believe you have 10# of boost and sticky tires? That's gotta be one sleeper 4dr that gets with it!
 
SpeedThrills said:
I have to agree w/ JohnRR. I appreciate the value of a dollar as much as anyone but, as you said Bill, when one of those wheels goes flying, it's a mess. (And it could happen on the street, who knows who gets hurt then.) Aftermarket axles can be had pretty cheap these days. Myself, I'd save up and get a pair and whatever is needed to go w/ them.
I believe you have 10# of boost and sticky tires? That's gotta be one sleeper 4dr that gets with it!
Click to expand...

It's boosted and he is going to run slicks ?

Bill contact dr. diff , he has good axles that'll drop right in for under $400, get the big car brakes off one of those junkyard B body or a C body rear and be done. It'll be cheaper in the long run , you'll twist the splines on the stock axles in short order .

yes you can make a ring for the wheels but if you do that you'll have to blow out the hole in your drum too or you'll never get it off . If you run something like a weld wheel which is lug centric then the center register is not an issue , but twisting the axles will happen eventually ...

pay me now or pay me later ...
 
I had a 4-speed 340, 4.56-geared '64 Valiant with a healthy '71 340 back in 1972.... used to race a lot with some M & H 7"-wide, Stocker slicks, on concrete (I-35), and never ever had a problem, but I guess I was just lucky. I was dropping the hammer at 4,500.... felt GOOD!!!!!!!!!!:profilel:

What's the web address for Dr. Diff, please?
 
Bill another argument for replacing the axles instead of drilling out your original axles and putting in 1/2" studs is there's just barely enough room for another set of bolt holes on the axle flange. My car had stock axles that were re-drilled when I bought it and the new LBP studs are extremely close to the holes where the original studs were. You could probably pay a machine shop to weld up the old holes and machine the surface flat again and drill new holes but that'd probably cost 3/4 as much as a stronger set of aftermarket axles.
 
Fishy,

Thanks for that good info. I hadn't looked at my sbp axles in a while, and didn't realize there was an interference problem with regard to the stud location between 4" and 4.5" circles.

Looks like the only smart thing to do is get new aftermarket axles.... or, put a Powerglide in it.... 1.82 low gear (like, that'll be the day!!!) LOL!:angry7:

Attchment is a photo of my Stockton Wheel-made 7"-wide, 15" sbp street rims that are "reversed." They said they had to do that to get the right offset. Powder-coated, too.... Man, I hate to give 'em up...
:(


Bill, in Conway, Arkansas

100_0109.jpg
 
You can get very resonable priced axles on ebay for the abody housing . It just isn't worth drilling the old axles anymore.
 
I didn't know that. I appreciate the tip!

I'll definitely check that out!

Bill, in Conway, Arkansas
 
Man you must be making some good power to be so concerned about the small bolt pattern . Back in the day there were a lot of cars making good power and i don't remember anyone doing that. A good friend of mine ran a 4 speed and would leave at 6 grand all the time and the studs were not the things that failed when things broke.
 
I hear you, fishman... I am probably over-reacting on this issue.

I have to drive my car to the strip (60 miles away) and if it breaks, I have to rent a trailer to get it home (plus bum a ride home, since the only car available to me is broke.)

Not much fun.... so, I'm trying to make it as bulletproof as I can.

I have a 4.10 rear end for it now (Sure-Grip), two BIG batteries in the trunk (see attachment), Drag Radials, and a supercharged 360 that probably makes around 450 pounds/feet of torque. I have a pinion snubber, and an air bag on the right-hand leaf spring to pre-load the suspension, and a 2800 rpm converter.

This strip operator puts down a LOT of VHT on the starting line, so the bite is really good....

I am having a driveshaft made with 7290 U-Joints, and a heavy-duty slip-yoke in an effort to buletproof the drivetrain on this thing
No fun to walk home... :(

But, you're right; It's VERY unlikely that I'd ever shear off the wheel studs even though they're sbp and 7/16", or whatever.... An automatic in all likelihood would never hurt them.

I just have become the sort of guy who wants suspenders AND a belt, where the drivetrain on this ol' tub is concerned. Better safe than sorry.... lol!

Thanks for your comments; they are well-taken!

Bill

100_2849.jpg
 
Bill Dedman said:
I hear you, fishman... I am probably over-reacting on this issue.


But, you're right; It's VERY unlikely that I'd ever shear off the wheel studs even though they're sbp and 7/16", or whatever.... An automatic in all likelihood would never hurt them.

Bill
Click to expand...

Take it from someone whos been there,if your making any kind of power on a well preped track,don't be suprised if those 7/16" snap right off..better to be over cautious then breaking things,,
 
fishman said:
Man you must be making some good power to be so concerned about the small bolt pattern . Back in the day there were a lot of cars making good power and i don't remember anyone doing that. A good friend of mine ran a 4 speed and would leave at 6 grand all the time and the studs were not the things that failed when things broke.
Click to expand...


I went the cheap route, and walked. I found a RR with a 4.88, swapped the whole thing and just cut the driveshaft down. This was in the early 70's and I've had no trouble out of the new setup.
 
For added insurance, get a steel billet rear end yoke from Mancini, Mopar, etc.
 
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