Coarse vs Fine-Spline for 35" Tires (1 Viewer)

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JGM

JGM

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Aug 30, 2005
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I know this has been covered, but I haven't been able to get a specific answer.

I am working through my 1971 FJ40 project. I am getting into suspension and axles. Because of its year, I believe I have the coarse-spline axles. I have also seen many people "upgrade" to the fine-spline axles.

I want to run 35's and would like to know if there is a specific reason I should want fine spline axles over the coarse that I have.

For moderate rock-crawling (lockers in both ends) and some daily driving, what risks do I run with 35's and coarse spline axles?

Thanks!
 
your signature says front disks, what kind of setup is it? bolt on from man-a-fre? 60/mini knuckles? depending on what it is you could already have fine spline.

stock was not disk brakes
 
Regardless of the disk brake swap (and unless someone did something stupid), you already have/had fine-spline (30 splines) axle shafts. i.e. the end of the axle shaft that goes into the differential. Coarse-spline (10 spline) axles disappeared (at least in the US) in 1968-ish.

Depending on how your front brake swap was done, you *may* still have coarse-spline birfields - that's why thorvald asked about that. For running 35s I would definitely check that and make sure you're running, at the very least, fine-spline birfs if not upgrading to some longfields or something. All depends on your budget and driving style.

Also, you most likely still have coarse-spline pinions in the differentials, but these are less likely to cause problems than the birfields.

HTH,

Eric
 
The disk brakes on the front are the MAF "bolt-on" style, and I did not do that work, so I am not sure.

I gather from reading between the line that the fine splines are stronger? And from research, I know that there are long-fields available for the fines, but not the coarse.

Bottom line for me is am I okay for 35's and some rockcrawling, or should I go to fine spline...

OR..

Do I go to a dana 44 front. The reason I am thinking through that is because I run a D44 on my 4Runner, and I like having some commonality for obvious reasons.
 
JGM said:
The disk brakes on the front are the MAF "bolt-on" style, and I did not do that work, so I am not sure.
Click to expand...

I'm not an expert on those, but I think that the bolt-on kits do not replace the birfields, so you probably still have coarse-spline birfs.

JGM said:
I gather from reading between the line that the fine splines are stronger? And from research, I know that there are long-fields available for the fines, but not the coarse.
Click to expand...

Fine-spline are stronger. There's really no reason for aftermarket coarse-spline birfs to exist since it's so easy to just swap in the fine-splines.

JGM said:
Bottom line for me is am I okay for 35's and some rockcrawling, or should I go to fine spline...

OR..

Do I go to a dana 44 front. The reason I am thinking through that is because I run a D44 on my 4Runner, and I like having some commonality for obvious reasons.
Click to expand...

Again, it depends on your driving style, budget, level of willingness to deal with trail breakage, etc. You could wheel them the way they are, but understand that you run a big risk of busting one on the trail. Spare coarse-spline birfs are cheap, so carry a couple extras and learn how to swap them. Then when you're tired of swapping them, pony up for some stronger ones and the associated parts to fit them. I've never really messed with the MAF kits, so I don't know how much of the kit can be retained while upgrading the knuckle/hub/lockouts.

I would not do the D44. I understand your commonality argument, but you've already got commonality between the cruiser's front and rear axles, and there's no way I'd give that up to have some commonality with another vehicle - especially one that will likely never be wheeling the same trail at the same time as the cruiser.

Do you already have the 44 in your garage? If so, sell it to a jeep guy who will actually benefit from the increased strength and put the money toward a mini-truck axle and some longfields.

The 44 is not any stronger than a cruiser axle, and a cruiser axle with longfields is miles ahead.
 
Well - I do have chomolly shafts and u-joints on my 44, so it is stout (and expensive as hell). Even so....I just went and did some measuring and it looks like I would have to be creative to get a 44 on the cruiser frame.

If the longfields are as strong, or stronger, then I would rather do that. Besides, the cruiser axle looks cooler anyway. To do that (assuming I don't have fines already, which needs to be checked), what parts do I need? Fine birfs, inner shafts, and R&P?

Thanks, folks
 
Last edited:
E-rock -

Thanks for those replies. Good points all.
 
JGM said:
If the longfields are as strong, or stronger, then I would rather do that. Besides, the cruiser axle looks cooler anyway. To do that (assuming I don't have fines already, which needs to be checked), what parts do I need? Fine birfs, inner shafts, and R&P?
Click to expand...

You keep your differential intact. In fact, you don't even have to remove it. The inner shafts stay, but you'll have to separate them from the birfields and I'd recommend a martack (Click "Technical Links" at the top of the page) while they're out.

Like I said, I'm not sure about those MAF kits, so what you'd need to swap depends on what they include. My first thought is that it'd be easier (and probably cheaper) to pull that whole setup and sell it to someone who doesn't want to run big tires but needs disk brakes. Then do a mini-truck disk conversion on your axle housing (Technical Links). I'm pretty sure the bigger birfields won't fit in your current knuckle/spindle/hub/lockout combo, so you have to upgrade to bigger ones, but they essentially bolt on to your existing housing.

The swap is almost entirely bolt-on and very well-documented.

HTH,
Eric
 
I think that is the way to go. There are plenty of fans of MAF stuff here in the Phx area (I am not so much of one....). I could off-load this stuff.

Eric gave me a useful link to the minitruck swap, one of many apparently. Seems like the cost-driver will be the longfield stuff, but, if your are going to do it.....

Gracias
 
i run 34" and coarse birf's i have 6 spares in the truck at all times and a shot side shaft. Thiy work fine as long as i dont keep the front locker on for to long, i just use it when i need it and then right back off.
 
Check out the website for longfields, there is a chart on there that shows the breaking strength for the 30 spline cromoly birf, its suposed to be as stout as a Dana 60 and Bobby garanties if u break one he will replace it. I talked to him and he was very helpful. That is what i am putting in my rig and i plan on running 37" rubber on it.
 
I have been doing more research and I am finding that 30-splines came in cruisers 68 and newer.

Without opening up my front end, does anyone have reason to believe I DON'T have fine splines now (1971)?

Thanks


BTW - I will be going to newfields / longfields
 
i dont think i would wheel 35" rubber on course spline birfs...if you swap into 27 (fine) spline birfs i dont think you would need the long fields....the fine spline birfs are a bit beefer in the bell houseing and you would need to grind your knuckel out a little but....its worth it i think, and probably much much cheaper than putten out the $650 on longfields, dont get me wrong bobby long has a great product im sure but if it comes down to dollors and cents i would just go with a 27 spline...you will have to change more than just the birf though...i think there is a difference in the spindle length...im not sure, maybe somone with more experience will chip in some info.

and the D44 axle....i wouldnt even consider putting it in place of a LC axle, im sure people will argue this but i think the LC axles are almost as strong as a D60...

note i took fine spline axles and knuckles out of a 60 series so if you took them out of a 40 there could be a difference, i dont know
 
27" tires, coarse spline rear axle in the Napa parking lot :D

DSC00035_sm.JPG
 
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