SM420 -> H41 Question (1 Viewer)

[CruisinGA]

Looking for input from the peanut gallery-
I installed an SM420 when I was 17 and 100% focused on beating myself up on whatever rocks I could find. Fast forward 17 years and I have two small kids and so I am doing a lot more bouncing around town but not ready to totally trade in my wheeling card (still run 38's). The effectively 3spd SM420 isn't great around town and my 3spd transfer is destroyed. I have a good tcase core and rebuild kit but considering not just replacing the tcase it but upgrading to H41 and split case for better road manners and split case durability/quiet. Trying to make sure it will actually drive better and not totally kill offroadability.

H41 1st gear- 4.9 1st * low range = 9.3
SM420 1st-7.0 2nd- 3.6 1st * low range = 16.1

H41 is a nice split of the SM420 1st and 2nd. The final low range is no where near as low, but honestly I didn't use the 1st-lo that often. It might be nice in Moab, but east coast most difficult obstacles required more wheel spin than 16.1 was going to get me.

Anyone made this change?

 

[bsilva132]

I was in the same boat a few years ago as well. It was cheaper for me to pick up a FJ60 rear end with an ARB and sell my FJ40 rear end with a Detroit. I am only on 35's so the 420 isn't terrible (not great either) but it works fine and the t-case is in good condition. Still an overbuilt kid hauler at this point but oh well.

 

[CruisinGA]

Fortunately I put in an ARB in the rear back when I was 16 because this 40 was also my daily driver.
It’s really the need to replace tcase that is making me want to get an upgrade out of the work rather than back to how it was before.
I hope someone has gone from sm420 to H41 and will comment!

 

[matzell]

I have not done that swap but do have a H41/Orion in a 40. To do it over I would find a early H42 split case(short version from a 60) and install a 4:1 gear set. Factory 4 spd is great for driving around town and the low range gear set would help you off road. The H41 is a little low 1st for street use but I feel like I am riding the clutch if I start in 2nd. I have a H55F in one as well but I feel the H42 is the best all around shifting and ratio for street use, although the H55F is nice for overdrive. It has about the same low 1st gear as the H41.

 

[CruisinGA]

I have not done that swap but do have a H41/Orion in a 40. To do it over I would find a early H42 split case(short version from a 60) and install a 4:1 gear set. Factory 4 spd is great for driving around town and the low range gear set would help you off road. The H41 is a little low 1st for street use but I feel like I am riding the clutch if I start in 2nd. I have a H55F in one as well but I feel the H42 is the best all around shifting and ratio for street use.

How big are your tires?

 

[matzell]

H41 has 33's with 4:10 diffs, 2F
H55F has 33's with 4:10 diffs, 350

I had a 350/H42/stock 4spd case/4:10's and 37's. that was nice too, you could wind the gears all the way out!

 

[1911]

I am running an H41/Orion with 3.70 diffs and 33" tires, and a 2F. 1st gear is low but not obnoxious for street use; I use it all the time unless I am pointed down-hill at a complete stop, then I start out in 2nd. Pretty wide spread between 1st, 2nd, and 3rd, you just wind it up to 3,000 rpm to shift. But on the top end in 4th gear, I am only pulling 2500 rpm to go 65 mph. Low range gears are nice for off-road.

 

[CruisinGA]

I am running an H41/Orion with 3.70 diffs and 33" tires, and a 2F. 1st gear is low but not obnoxious for street use; I use it all the time unless I am pointed down-hill at a complete stop, then I start out in 2nd. Pretty wide spread between 1st, 2nd, and 3rd, you just wind it up to 3,000 rpm to shift. But on the top end in 4th gear, I am only pulling 2500 rpm to go 65 mph. Low range gears are nice for off-road.

Thanks @1911
My 38" tires and 4.11 gear ratio results means the H41 first gear would be ~22% "faster" than in your case. If 1st gear is usable on street for you then it seems like it would be for me too.

 

[CruisinGA]

I have not done that swap but do have a H41/Orion in a 40. To do it over I would find a early H42 split case(short version from a 60) and install a 4:1 gear set. Factory 4 spd is great for driving around town and the low range gear set would help you off road. The H41 is a little low 1st for street use but I feel like I am riding the clutch if I start in 2nd. I have a H55F in one as well but I feel the H42 is the best all around shifting and ratio for street use, although the H55F is nice for overdrive. It has about the same low 1st gear as the H41.

Interesting thought. The H42 first gear is same ratio as Sm420 2nd gear, and with a 4:1 gears in the tcase, I'll end up with a crawl ratio that is almost as low but much better splits (the giant jump from SM420 1st to 2nd was always a pain crawling. Hard to keep momentum.

If I'm comfortable with never having a gear lower than 3.55 on the road then this might be a better route than H41 for road manners and nicer gear splits in low range.

 

[MoCoNative]

I'm running an H41 in a 73 with the 1F and 235/85R16 tires (32"). I really like the lower gearing for crawling around. The 3 speed just plain sucked, now I have much better control and power from the wimpier 1F. The lower 1st and bigger gaps in the gearing is annoying for street driving though. It works fine, I almost always start in 1st, unless I'm rolling a bit, then 2nd will work fine. The thing I was most disappointed in, is the use of 3rd on the highway for long grades. That tenth lower has me totally rapped out at 55MPH. Compared to my stock FJ60, 3rd is actually usable when needing to downshift and still stay with traffic on a mountain pass here in CO. Going up to the larger and heavier tire size gets my F really wheezy on grades though. The H42 is also better gapped and far more comfortable around town. If I had to do it over, I think I would have forgone the expensive H41 and just put in a H42, but I do like the lower gearing off road. I suppose I need to upgrade to a late 2F, it would probably be able to keep me moving in 4th on the highway better, the FJ60 seems to do better in that respect.

My 73 still has the stock 3 speed T-case, if you do the alteration in the low gearing you have the best of all worlds as your chart suggests.

I like to go here and play with gearing and tire sizes to see how things will really look. Gear Ratio Calculator - http://www.grimmjeeper.com/gears.html

The H41 in their table is the early super super low version (5.3 1st), use the H55 pull down to get the common H41 gearing, or you can enter your own.

 

[BC40]

with 38s and 4.11 you'd probably be good with the h41 1st gear for street driving especially in hilly areas. My fj40 has 33s and 4.11 and I usually start in 2nd gear with h41. but with my bj60 with h55 33s and 3.70 I start in 1st nearly always so as not to abuse the clutch too much. the gear splits are much more road friendly in an h42 for sure. you'd know best if you never use 1st gear in your sm420 on the road. if thats the case then cost of h42 with split case with 4:1 gears isnt much different than cost of an h41, and you can get the 4:1 gear set with underdrive high range (10% or so IIRC). of course then you still need to sort out something for a parking brake.

 

[4Cruisers]

Your parking brake is on the back of the transfer case, a US market split case won't have that. Solutions include an '81 and later FJ40 rear axle (hard to find) or a non-US split case with drum parking brake (pricey but available if you're patient. At one time @orangefj45 had the parts to convert a US split case to the non-US version, he may still have some. BTW, the overall length of the early H41/H42 with one-piece case is about 1/4" longer than the late H41/H42 with split case.

 

[orangefj45]

Yes we stlll have a few of the OEM e-brake setups for split cases in stock. We also stock the rebuild kits, lower gear sets and much more ......

Georg

Valley Hybrids

 

[CruisinGA]

@orangefj45 Thanks for mentioning it. Called your shop today to get a quote for H42 w/ split case w/ 3:1 gears and parking brake on the split case.

 

[Vandal FJ]

I will throw this in the pot to stew for a while, but my friend has an NV4500 in his and it's a nice compromise with a granny gear 1st and a 5 speed?

 

[CruisinGA]

That's fair @Vandal FJ , just not sure NV4500 makes sense with F engine. Seems like NV4500 are normally seen with v8 conversion since that also offers opportunity to move engine forward and keep rear driveshaft length copacetic

 

[Vandal FJ]

That's fair @Vandal FJ , just not sure NV4500 makes sense with F engine. Seems like NV4500 are normally seen with v8 conversion since that also offers opportunity to move engine forward and keep rear driveshaft length copacetic

You are COMPLETELY correct... duh... I need to read more carefully. LOL

 

[Living in the Past]

@orangefj45 Thanks for mentioning it. Called your shop today to get a quote for H42 w/ split case w/ 3:1 gears and parking brake on the split case.

Something to keep in mind is around 10/82 the split transfer case had a provision for the H55F fifth gear oiler. Didn’t need to be mated to the H55F five speed to have this. If you ever think you may want a H55F would plan ahead. But with the H55F better to have the hand brake on the rear axle otherwise will have torque tube behind the transfer case issues. The four speed with squeeze in but not the five speed with a handbrake on the transfer case.

 

[CruisinGA]

Thanks @Living in the Past, I have some rubber overdrive with 38’s and doubt I’ll ever go smaller than 37 but that is something I would have otherwise wanted to know!