12 bolt Driveshaft Length
 

Almost have the new Moser 12 bolt rear and Global West suspension installed. Aside from a few missing odds and end, it's been going smoothly. I installed a new 3R/1350 conversion joint and went to reinstall the driveshaft this evening. Not so fast. Looks like I need around an inch or so shorter.

I thought that 12 bolts were a direct swap, but I guess Moser changed enough stuff that this one isn't. My hunch is that fat 1350 yolk is a culprit. Guess I'll be ordering a new driveshaft.

https://forums.maxperformanceinc.com...1&d=1714530833

It's most likely that 1350 yoke doing that. Ive noticed some of those being as beefy as they are do stick out a bit further as well.

As far as a 12 bolt being a direct swap, yes. Dad's goat still had it's born with driveshaft when we removed it's original rear and swapped the 12 bolt in, the driveshaft was a slam dunk.
When I later upgraded the 12 bolt to a 1350 yoke I also had a beefy driveshaft made at the same time with 1350 ends and a billet trans yoke.

Would've been nice to get a heads up from Moser when I asked if there was anything to know about going with a 1350. By the time I'm done with this swap, I'll have no excuses not to make a pass at the track.

Honestly, by the looks of that shaft you have now, you NEED to replace it. Those look like OE ends.

And if you are, might as well spring for a new slip yoke too.

Pretty certain it is due to the rear yoke. I bet if you got a standard one it would have fit.

Overkill is usually safer anyway, and you've already done the rear, so a new shaft and you will never have to worry about it.


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Don't reuse the shaft ends and do 1350 ones. Yes, you will eat the money of the conversion joints, but again, in the long run, you will never have to worry about it.


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Slip yokes also make a big difference in length. I had a 48.5" on the Camaro/IA with the SBC and stick. IA and TH400 it was too long to fit.
Mark Williams measurements using their yoke ended up 48.5"!

Great points! Yeah, I had the shop provide the slip yoke when I gave them measurements for a new shaft. When I got it back, realized the yoke was longer, and originally thought my shaft was too short, since more of the polished area stuck out.

I had to unbolt it, and slide it in all the way and see how much stuck out to determine that there was enough engagement.


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When I measured using their locations and came up with 48.5 I was skeptical but is slid right in! Factory trans yoke shaft was too long to fit but the same actual length.

Yes, I’ve been running the original driveshaft. I was going to run a conversion u joint which I figured would be fine for a while. I ended up ordering a new 3.5” steel drive shaft with a new yoke from Inland Empire Driveline. 1350 on both ends.

I measured, re-measured and re-re-measured before calling them and talked through how no measured on the phone with them. I hope the bigger diameter fits with where I have the x-pipe. The stock driveshaft was pretty close.

This piece means I’ve now rebuilt or replaced the entire drivetrain which started with the engine rebuild about a year and a half ago. Funny how things snowball. I have no excuses not to make a few passes at the track now!

You could drop the rear muffler hangers a little if you run out of room. Check it at full droop.

I have a 3" one right now, and when my buddy welded up the exhaust, it was on a 4-post, and he jacked the X pipe up and tacked it. I had to drop the mufflers at the bracket 1 notch. At full droop, it just barely kisses the X, has a faint witness mark on it. Know I will be changing that X out when I swap headers, will probably go to a 3.5" shaft at that time.

There's been a bunch of 'funny' th400 yokes out there, some have a hole in the end, so are not splined all the way deep, etc etc., whole variation of them. But if I tell a shop that's doing a shaft and they supply the yoke, it better work, or they have to flip it. And R&R the one on the shaft.

I just have the shop do my u-joints at that time too.


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I asked for a 3” but they strongly advised against it. Figured if I’m having one made, I might as well get one that’s definitely going to hold up. My exhaust isn’t welded so I bet I could wiggle it around to make some more room if I have to. It’s only 1/4” I’d have to find.

3.5" has a higher critical speed limit than a 3".

Yeah if you haven't welded up the exhaust yet you're golden, just will be slightly lower is all.


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FYI - When calculating critical speed, you should shoot for half the max RPM.


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If you get a chromoly 3" shaft with a forged rear yoke and a chromoly front slip yoke it will be overkill for your car and you'll never have to worry about it.

When I do my exhaust on the 2 post lift and have the rear at full droop, I move the X right up to the driveshaft. Usually not actually touching but it really doesn't matter much if it does because in real world conditions with the car on the ground you'll have clearance. The only time the rear would be at full droop like that is if you're Bo Duking rail road tracks, lol. So you don't really need any "clearance" per say when the rear is at full droop on the lift. Typically the X's are installed fairly close to the yoke area anyway, or within 12" of the front of the driveshaft where driveshaft movement is very minimal.

I have a a 4" aluminum driveshaft in one car (Abody) with a 3" exhaust and X system and it tucks up fairly well and has never touched the driveshaft during any driving conditions.

I'm in the same boat and time, Moser 12 bolt will not accept the stock driveshaft. Maybe 1" too long and would put a huge load in the trans.

Ordered a new aluminum 3.5" from PST
32 spline for TH400
Cast yoke handles up to 1000hp
Around $550 range and 3-5 day build

then needed a bigger DS safety loop because mine was like 4" ID, UMI makes a 5" ID.

$6k rear end, $600 DS with loop, ouch.

It definitely adds up. I didn’t even consider an aluminum one as I assumed I’d have to go up to a 4”. The one I have on the way from Inland is about the same price as yours from PST.

3.5" Aluminum is good for up to 1200hp and I'll weigh it but others said about 13lbs, my steel 3" is way heavier.

My new driveshaft was 57.5" and I'm an auto TH400 trans with Moser 12 bolt.

The new driveshaft fits perfect. I don't think there's any risk of breaking this beast! Plenty of room around the x-pipe. It is heavy though. I kind of wish I asked about aluminum.

https://forums.maxperformanceinc.com...1&d=1715383493

https://forums.maxperformanceinc.com...1&d=1715383508

https://forums.maxperformanceinc.com...1&d=1715383552

https://forums.maxperformanceinc.com...1&d=1715383564

Bravo!

Yeah, not sure how much more an aluminum one would have been, but not a biggie really. Critical speed is a little lower but still better than that old shaft.


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