OKA 159 (public)

I thought I'd repost my XT resto-destructo-adventure-mod thread for unrestricted viewing, so here's a quick precis of the life of 159 since coming to be with its third owner.

Purchased in Perth in July 2023, from 2nd owners. It had 205k on it when I took ownership, and has a beautifully fitted out pop-top over factory bus ROP (?) bus body. Not sure if the pop-top is OKA circa 1994, or a very clean body builder addition. More or less factory except for the Swift™ turbo/exhaust/airbox/intercooler upgrades.

During the return journey to Hobart, the first Swift XT turbo took a dirt nap about 500km out of Perth. Changed that out with the much-appreciated assistance of Mort himself. Next hiccup was a little bogging on the beach at Kingston SE (whatever SE stands for) on the SA/Vic border. Let the 285/85/19.5 down from 85 to 30 PSI and dug it out with a plastic breakfast bowl. Finally, while driving through Melbourne on my way to Geelong to catch the last available spirit of Tas crossing for 69+ days, the passenger rear 32 spline axle shaft took a dirt nap on the side of a metro highway. Bashed the tailshaft and grotty yoke bolts out and made it to the ferry with 15 minutes to spare.

Fast forward to September, and I have 35 spline rear and front stub axles to go in, dual ARB air lockers, and rebuild kits for the Dana 60 and Dana 70U. The long-term mechanical plan is also a new outer knuckles/kingpin rebuild and new steering links to replace the sad factory links and tie rods.

I have a 3.9L ISB Cummins 4BT and Allison 1000 from an NT in the process of being crated up to slip into 159, knowing full well that diesel swaps in Tasmania are either going to be on a long waitlist or difficult to organise with limited workshops able or willing to undertake it. As part of that, I have refined my options for mating the engine and trans to the XT and NP205 to an underdrive that may or may not work with the current 23 spline input and future 29 spline inputs, then turn the NP205 into a 4-speed transfer case. It will likely be a mix of North West Fab and Behemoth Drivetrain hardware so I can maintain the Speedo sensor and an upgraded transfer case park brake. Then triple cable shifters to replace the less-than-ideal single transfer linkage. The beauty of rock crawlers in the states all frothing over Dana 60/70 is we get really well-engineered parts that are rated for monstrous abuse - which almost certainly translates well to OKA levels of abuse.

At some point, bar work to protect the pop top and eventual solar farm will go on, as well as scrub bars to form a cab roof rack and offer some protection for the mirrors and cab. Need to sort out new bus body stairs, and install the 2x200L and 1x130L rotomould tanks for new fresh water and diesel cartage. The interior camper arrangement is good, but the bed/couch will be raised to return the tailgate to full storage and provide space for an internal composting toilet and possibly external pullout options for cooking/wet storage.

So far I have fabricated new double 7x5" headlight buckets that fit in the original headlight bracket, replacing the decaying stock 7x5" bucket and whatever it was the halogen flood would dangle from. Lots of little mods and quality-of-life upgrades waiting to go in, but first, go from front-wheel drive to four-wheel drive again! Reversing and low mount front trail camera for navigating features without a spotter, planning to upgrade to LT dash on XT and fit a Garmin RV Vieo head unit that does all the normal audio/gps functions, but can control electronics - if that's possible with the 12/24/48v house setup I have rattling around my head. Generally replacing all factory-era components with bits that have modern part numbers - a simple and easy task really, for the well-intentioned and in some ways poorly executed parts bin special that we all know and love. Wipers, indicators, bulbs, housings, antennas, motors, blah blah blah. Eventually hose work and wiring, bits and bobs. The un-initiated to the love-hate affair that is OKA (or equivalent) ownership might say 159 (or equivalent) is a lemon. I strongly rebuke those comments, it's a 30-year-old 1-of-maybe-300-still-operating wonder bus. The work we put in now guarantees another 30 years of service with irresistible OKA charm.

Big ticket items will also include a mid-mount winch with front and rear guide tubes so I can save weight and position the winch in a high and dry place, close to the power supply. For the inevitable questioning, yes it is possible, no I don't know how I'll run it just yet but I consider it to be a lot easier to run a single winch in single-line pull forward/aft and then double-line pull in the opposite direction - than to run 2x13-15-20,000lb winches from the front and rear. I would love a PTO but for now, I will experiment with my Runva13XP which has the berries to pull an OKA if it's set up properly.

I have made up new Britax heated split panel mirrors with 12v indicators as well! It was a mission to get mirrors that weren't 24v and were worth buying without being wildly oversized. The 200x380mm set I went with needed a mounting adapter, and I ordered mirror heaters separately for when I go above the snow line/generally living in Tasmania in Winter (climate change will it ever gets cold again).



Compared 159 to a buddy's BRAND NEW 76 series cruiser. I know which one I prefer.



As with all OKA it's a work in progress, and there is some minor rust in the usual places. So far the only rust of note is the RHS that supports the front underside section of the cab. Will probably remove most if not all the bus body glass and replace it with GRP panels and lighter, low-profile windows where appropriate. Priority is security, safety and thermal performance.

More to follow

Excellent Ethan, thanks for the comprehensive run-down on your plans. It doesn't seem to matter what mods one does to an Oka, they still delight in every way. We have 059, 100 Oka's older than yours! It has taken us on fabulous trips for the last 12 years, and is always, always, a delight to drive.
Happy days, Paul and Michaela

So in my quest for Mo Powa Babeh (IYKYK), I picked up an NT OKA 3.9L ISB170 common rail 4BT and its factory-fitted Allison and transfer case. Not too worried about the HD150 and divorced transfer case, because having crunched the numbers for the final drive and wheel speed with the Allison (and even with the Spicer on 37s) I have decided to give a third and even fourth option a whirl.



Not that I don't trust my NP205, but based on the following chart I threw together:



As you can see, 1.96:1 gives an acceptable crawl ratio of 51:1 with the Spicer, but an unacceptable 29:1 with the Allison 1st gear. Without getting political, the Spicer deserves respect but is past its supported service life so it's not an option long term - plus I already have an Allison with the 4BT.

If the LoMax goes in before the engine and transmission swap (still working engineering angles, plus finding a workshop in Hobart that can do the swap) then I will have 78:1 crawl with 3:1 low gear. This is getting better! The decision point comes at what angle to set the driveline for the NT spec 4BT. Having spoken to Dean, NT is set at 2 degrees and the XT is set at 7 degrees. Dana says 7 degrees is the "safe limit" for their axles at flat level ground, obviously a large safety margin for axle flex and the weather, etc. Online consensus is 30 degrees is the amount of driveline angle you can usually achieve before something goes bang, not accounting for running out of splines in your driveshaft slip yoke.

The jury is still out on the driveline angle, however, driveline length is at least 6" longer from the front of 4BT to Allison tail housing output, my new bits are still being crated up so I can't confirm this for myself but what I'm working off for now. I haven't gotten into minimum tail shaft length yet, but that is a factor to the following:

1. 4BT and Allison together, nothing unusual about that
2. Allison 2WD tail housing (Part #29537009) swapped to 4WD GMC 6 Bolt tail housing (Part #29544804) I don't anticipate a significant difference in length between the tail housings, they are designed around the same Allison 29 Spline output shaft to my knowledge.
3a. The least attractive option is Advance Adapters Allison 1000 (GM) 29 spl. To GM NP205 which adds 2.875" to the driveline; OR
3b. North West Fabrication Blackbox NP205 Titan underdrive which adds 5.35" to the driveline but allows 4-speed transfer case shifting, which is very attractive for many reasons; OR
3c. There are a few other underdrive options which are various lengths that may be called upon if for whatever reason the Blackbox won't work. Behemoth Drivetrain Offroad Design Atlas II
4. Flanges on axles and transfer case outputs for strength and convenience, driveshafts to suit angles and lengths, wedge axles etc. if required.

Now down to the sexy stuff, by fitting an underdrive at the same time as the 4BT+Allison, I get my final drive from 45:1 which is almost acceptable, all the way down to 123:1 which is very respectable. That takes my theoretical stall speed in 1st Low/1st Low-Low from 6kph all the way down to 1.4kph with the Allison. This is achieved by having a 2.72:1 gear set and 1:1 gearset in the box, which coupled with cable shifting gives you 3 levers to control front and rear axles separately (2wd Low for front and rear), and the underdrive/transfer case for high/high 1:1, high/low 3:1, low/high2.72:1, and low/low 8.16:1.

That might not mean much to some, but it's the difference between scrambling for the anchors and sipping pina coladas, and I much prefer pina coladas. The caveat to all this is always going to be what I can get engineered, because I would hate to run afoul of the constabulary, or worse the NHVR.

Also in very sexy news, you can pretty easily turn a 5-speed Allison 1000 into a 6-speed by swapping out the valve body and running an upgraded TCM. Total package seems to be the $4000USD and core charge (spenno ) but requires no drilling. The only difference between the two is that allegedly computing power prior to 05/06 was not sufficient to process the clutch packs required to achieve 6th! Don't know if I'll do it, but a very interesting tidbit for Allison owners that wish they had a SECOND overdrive ratio.

Just be aware that there are two versions of the 6 speed Allison, they have different ratio's on 5th and 6th and some have different ratios on the first four. "normal" is around .64 and .74, "taller" is .61 and .71
You will find that with the .71/.61 combo that with 37's and a laden Oka that it is bordering on being too tall as revs at 100kph are below maximum torque on a 6bt so a 4bt with less power will be worse.

Thanks, I will definitely go far too deep into researching that particular avenue at some point. It's a lot of money for something that's not particularly vital to my plans. It is nice to have a transmission that is in MUCH wider circulation now than the Spicer, even if it's not well known in Australia.

The 4BT I have picked up is the 3.9L common rail ISB170, my understanding is it was only produced for a few years between 4BTA and 4.5L ISB. The NT it came out of was apparently a little sportier, but the stock tune on an ISB170 is funnily enough 170HP@2500rpm, and 560+nm@1500rpm. I'm assuming that's at the flywheel, but still, I've hypothetically heard it would push the NT with a trailer north of 110kph without much of a worry except in hilly country. Good thing there are no hills in Tasmania. I don't know if the PO of this engine is active on the forum or the 17 different Facebook groups, but I look forward to one day comparing new with old.

A little more restrictive than going flat out 6BT due to less popularity, but plenty of folks report getting 400-600rwhp on admittedly much lighter USA mud slingers running 40-45" + tires and reduction gearing and all sorts of shenanigans - all on Dana 60/70 axles! I know the load of a 5-5.5t truck asking for 110 on the highway, compared to a 1.5-3t cut down (here-unthinkable but in the US totally road legal) comp truck is different... but I like to think if you can get 600rwhp for 2t vehicles, it's not too much to ask for 150-200rwhp for 5t.

The engine and transmission were too good to pass up, even if they don't go in for a few seasons, trips, or even years. It means if I ever Chornobyl my Perkins or Spicer (touch wood) I have a solution that I can slowly acquire the multitude of adapters and spare parts for.

Long term, I want the power to go through things if I need it, but the crawl ratio to eliminate speed when I want to. That means all these underdrives and gear set changes, possible compound turbo setups, maybe a weight loss program, and just enjoying an extremely unique truck... I'm sure we can all appreciate just how cool pootling around in an OKA is. There aren't many in Tasmania, so the few of us there are here always score bonus points.

On that note, I imagine the Perkins would respond well to a compound turbo setup. The mechanics behind a small higher pressure and large lower pressure turbo apply to all turbo diesel engines. The exact boost pressure and how you shoehorn in the honking great big hair dryer would be tricky without a lot of trial and error due to what I imagine is a limited supply of:

A. accessible diesel mechanical tuners in Australia, let alone Tasmania
B. tuners familiar with Perkins Phasers. Not many juiced-up tractors or tug boats getting around, I imagine. I would love to be proven wrong on both.

Maybe I'm going in circles, and all the forum topics that would prove me so terribly wrong were unfortunately lost between the various forum mergings, but I'll take one for the team and report my results.

So today is the day we found out where the axle shaft decided to snap, and just how much fun 159's rear diff career was to remove!



As you can see, it was a simple operation involving a few light hand tools. Now that the LSD is out, we can turn our attention to why the carrier was binding and losing backlash in one spot, whether it was spun clockwise or counterclockwise. My man is pretty sure there is no damage to the pumpkin, though we haven't ruled out the tube bending after 30 years of mixed-use. There is no obvious reason for why the axle shaft failed, but onwards and upwards.



I would like to not have to buy a new 4.88 ring and pinion, but if that's what it takes then it is what it is. We'll see after a quick spin on a lathe, and inspection for wear due to diff glitter rolling around in the oil for the 300-400km of driving I had to do before I could pull the shafts and isolate the carrier.



Today was like Christmas, a huge mountain parcels including carpentry clamps for the habitat fit out and future projects, a very nice micro circuit breaker/micro relay Bussman power distro and a UJ puller.

So an update on 159!

I put down an order for the JB Conversions 3:1 LoMax NP205. The NWF Blackbox would be amazing with the Perkins/Spicer/3:1 but more than a little overkill, so it can wait for the Cummins + Allison swap

As part of the restoration of 159’s rear end, it turns out the carrier caps are spun, and had a questionable repair over 25 years ago. The 4.88 ring and pinion was smashed, so I’m taking this opportunity to regear to 4.56 for the 37s that are on the way, and the eventual 3.9L ISB170 and Allison 1000 5 speed conversion. That meant a bit of fancy machining and engineering to return crush to the caps, otherwise the Dana 70U would be in the bin. The questionable repair and resulting wear definitely helps to explain why it was so difficult to remove the carrier, and might lend itself to how the back end had a catastrophic failure - but it's beyond nailing down a single factor that caused the axle shaft to fail.

The carrier cap solution was easy enough, and now after 10 in-and-outs to fit and set the ARB airlocker, it has become clear the 25+ years of spinning carrier also damaged the inner face of the tubes. So that's up to my man Neil and his engineering connections to clean up before the rear can be finally recommissioned. Interesting tidbit, ARB wants the carrier shims to go outboard of the bearings etc. The original LSD packed the shims inboard of the bearings, and not sure about other variations of diff centre. Neil is going to get the shim pack set, and then machine a single shim to avoid the shim pack all going kaput if the carrier needs to come out ever again/during operating.

Hopefully, he still likes me after all this, and can polish off the front diff air locker and regear before I head for the mainland in mid November! If all else fails, I will sadly have to scrub the 4x4 plans, and be a menace to society a-la all other 2WD motorhome steering wheel turners up and down the East Coast

I decided to put together an Instagram for 159 so that I can document my processes, travels and boo-boos on @okaxt159. Yes, I am going to post in the FB groups and this forum, but it might just help another future OKA owner one day. Even had little vinyl stickers made up, so when the inevitable "What's that" conversation we all love comes up, it's a little easier to escape if required. While also on the search for the little touches, I found really lovely Perkins Diesel logos - but they were bloody expensive! Good thing Chummins Turbo Diesel decals and 3D emblems are a lot more common/affordable!

I'd love the chance to wheel with any other OKA owners on the East Coast, I'm pretty confident offroad in my 2007 Hilux, but this is an entirely different beast. The delays in getting it back on the road have definitely eaten into my time for getting used to how the OKA handles. I also came across a growing community of Canter/Isuzu/Iveco etc truck owners that refer to themselves as Recreational Truckers, which I like the differentiation from RV'ers ahaha.

I don't really have any good photos of the work I have done in between visits to the Diff man, but I have a huge pile of small jobs to complete! I need to get the rego transferred to Tasmania, so people don't think I'm just another bloody tourist with a funny-looking truck, so adding to the long list of bits and bobs and mini-projects, I got some personalised plates made up.. can you guess what they say? Yes, OKA 159, groundbreaking stuff, but we've got a tradition! The pre-Christmas list contains:

4G+5G booster, rather than going with starlink
UHF install, to compliment the temporarily disconnected HF system
Centre console, made up of RAM mount toughbox gear (so I can mount Garmin GPS/av system)
Garmin RV Vieo GPS/av system
Lithium accessory system retrofit (my Hilux is the donor, until I decide on a total system rebuild), and total rewire of accessories to clean up the classic 12v rats nest behind the driver's seat
Total auxiliary lighting overhaul, including outdoor under awning lighting, interior LED lighting, rear and front quarter mini floods
Quad 5x7" headlight upgrade
Rear seating and bed platform raised and upgraded to improve internal storage, and allow for future external access storage
Fit my new 37s!
Fit my Speedhub GPS Speedo + fuel gauge. Hopefully the pollack valve wiring plays well with others, and I can still have dual fuel monitoring via the 4" Speedo and fuel gauge!

More to follow, including photos of the above hopefully sooner rather than later!

In the new year, I will hopefully be swapping the original 1.96:1 NP205 for a 3:1 LoMax. That means I am getting a new spud shaft made up for the Spicer output to 32 spline LoMax input. If anyone has ever considered the 3:1 and Spicer combo, having a run of spud shafts manufactured makes everyone’s life easier and cheaper - so reach out if you’re interested!