Yes, I agree the diff' should have a drain plug. And I have an 8-page micro-analysis to prove it.

 

I think you hit the nail on the head. This particular leak seems to be coming from around the nut in the center of the yoke, not from behind the yoke as you would expect from a leaking seal. Thanks for the tip on marking the nut. I will give this a try.

 

Well I haven't done much to the 1981 itself, but I have been working on "the project" by stripping everything I reasonably could from the donor and getting it hauled off. Memo to self: parts trucks are a lot of work!



The other development is a junkyard find that I made last week... a Doug/Nash dual range overdrive unit (DNE2). This will ~not go in this truck. It will go in my diesel 4-door (I will post details in that build thread). But... the Gear Vendor's overdrive that I had planned for the diesel will now go behind the I-6. For a guy who isn't all that keen on auxiliary overdrives, it is ironic that I am ending up with one in each of my trucks...



The one thing that I will say for this drive train is that it is the only way to get a close ratio gear set AND overdrive behind a straight six. I've heard stories about swapping ZF internals but no proof that it has ever been accomplished.

My past I-6 trucks have all had 4.10's and manual 4 speeds with the non-synchronized granny 1st gear. Driven as a three speed, the gear spread I am used to has final drive ratios of roughly 12, 7, 4 to one. This truck has 3.00 gears, so my final ratios will be:

1st- 12.06
2nd- 7.23
3rd- 4.23
4th- 3.00
OD- 2.34

The first three will be about what I'm used to, and from my conversations with David I should really like the two additional ratios up top.

 

Can't wait to see the completed project! You will be impressed with gas mileage [considering being a 4X4] when you cruise at 55-62 mph. Maybe on your way to Skiatook in September!

 

You will love the tall gears. I did in Dad's truck with the ZF5, and can't wait to get the ZF behind the 460 in Big Blue.

 

Not sure what you are speaking of here. The zf came behind the six from the factory on the little bit later trucks. It's the easiest cheapest way to get overdrive, especially if you have access to the donor and it's the same wheelbase.

I am going by memory here, but I want to say the Doug Nash unit is losing favor because you can't get parts for it anymore. The Gear vendors is still supported, they only draw back to them is the expense to purchase it in the first place. And they have a control box I hope you have also.

 

Thanks for the encouragement! I've always gotten around 15 mpg (empty) with my straight six trucks, and they sure start to complain around 55 mph on the highway. I'm hoping I can do better.

I've asked this before with no response, but it does not hurt to ask again... does anyone here know what might make a T19 pop out of gear while engine braking down hill? Mine does it in all 4 gears when holding the truck back, never does it while pulling, and is otherwise the smoothest, quietest truck transmission I've ever driven. I have googled this to death and the only potential culprits I have come up with are either a worn shift lever tip, or fore/aft slop in the output shaft. Mine is excellent on both accounts, and overall in good condition. It's not a big deal, I've lived with it that way and it does not bother me much, but if it were a simple fix now would sure be the time while it's out of the vehicle.

Another question I have is what to do with the fuel tank fume recovery system. How well does this work? Should I leave it in or delete it and put a regular old school vented cap on it? I don't like to smell gasoline any more than the next guy, but it seems like a lot of extra junk and clutter, and I still smell gas. I deleted the mid-ship tank which had been bypassed for many years. I plan to just run off the rear tank or possibly upgrade to a 32 gallon Bronco tank if I want more capacity.

 

Yes, the small block ZF is the easy way to get overdrive. The thing is, it has the wide ratio gears. There was talk of putting the close ratio (diesel) gear set in a small block ZF case, but they have different input shafts and it isn't just a case swap. I've never heard of it actually happening.

True, the DNE2 is not supported. However, all the bearings and seals are standard GM TH400 transmission parts. The electric shift motor and limit switch are Eaton 2-speed differential parts which are still available. Unless you have total case, shaft or gear failure, they are generally very serviceable still. They do not have clutches like the GVOD and are very strong. They can be left in high range in all gears unlike the GVOD which is just what I need for the diesel. I will detail this more when I get time to update that thread. Things are moving and shaking with that build as well.

Edit: yes, I have the complete electronic control components for both overdrives

 

Very few aftermarket carbs have the requisite vent connections to capture the evaporating gas fumes, and since the gas in the carb usually gets a lot hotter than that in the tank, a lot of the evap is from the carb. So, without that connection I haven't seen the need to have the charcoal canister and the necessary plumbing and solenoid valves to make the evap system work.

 

Thanks Gary, I think I will take the system out. My truck has the catalytic converter and EGR deleted, and there is no evap recovery on the carb (which as you pointed out is the main offender). There isn't much point, then, in trying to keep the tank recovery system working.

 

When I de-smogged a few years ago (still with both tanks operating), I removed the charcoal canister, solenoids, lines etc.
I teed the breather tubes from the tanks together & just have a rubber tube with an in-line filter installed at the end, to keep dirt & bugs out. I kept the emissions fuel caps.
Occasionally I smell gas vapor outside the truck when the tanks breathe, but have had no problems at all.
One precaution would be to have the breather tube mounted higher than the top of the tanks.

I capped the float bowl vent, & left the carb to vent internally to the air cleaner.

More recently Chris (ctubutis) described his non-emissions F350 tank venting system to me........from the factory it was venting in the same way, except his breather tubes were secured to a chassis cross member separately (not teed together).

 

Theoretically if you keep the roll-over valve that is in the top of each tank, then a overfilled tank shouldn't leak gas. But if you plan on any deep water encounters, you probably should run the lines together and up high somewhere.

I have worked on a 6x6 army truck a little bit before, they had the pumpkin vents, transfer case and tranny vents, and the fuel tank vents all plumbed together with lines that ran up front and ended up high on the firewall for high water crossing. And I believe that was the normal setup, they had a special high water kit you could add to them.

My 89 diesel does not have a evap system from the factory, they just have a short hose going from the tanks and clipped to the frame with a little plastic thing in the end of the line to keep bugs out.

 

That's what my 86 diesel has, and it is also what they put on non-emissions Canadian trucks with gas engines. The frame clip location should be just fine for me, I don't plan on driving through any deep water.

 

I hear you on the parts truck as I have one I am using for parts too.


I would like to hear how the 300 six with the OD unit works out for you as I would like to go with a GV unit some day behind my 300/T18. I am thinking when I go with the GV unit to change the rear gear to 3.55 or around there as I don't think the 3.00 gear will not keep the six in the power range when in OD.


As for not being able to split the lower gears with the GV unit I have seen it posted you can by post the control unit and split every gear. I will see if I can find the post on it as I think I saved it, I save all I can find on GV unit installs.


As for the tank/carb/evap system. I plan on using most of it. I do have the rollover valves, yes 2 as I have duel tanks, run up to the canister. I also plan on the carb vent running to it and will pull a vacuum on the canister when motor is running.
I do not have any of the solenoids so they will be bypassed.
Dave ----

 

Dave,

I have been discussing this topic with David (1986F150six) extensively, and based on his experience I don’t think I am going too tall. He runs 3.08 gears and the SROD .71 transmission in his truck which puts him at 2.19 final drive ratio. Granted, he runs 215/75 R15 tires (27.7” diameter) which are smaller than what I will run, but my 2.34 will feel very similar. David’s son has a 1984 F150 that came with 2.47 gears and the SROD, which put the final drive ratio at 1.73. It got 26 mpg on the flat, but even the slightest incline called for a downshift. He ended up putting 3.55’s in it which essentially brought it down one full gear to where OD was equivalent to 3rd before the gear swap.

I think 1.73 is over-geared, but low to mid 2’s is perfect for the straight six in a F150. The I-6 has a low rpm torque peak that is almost identical to my diesel, and the more you can drive near the torque peak the better the straight six feels. My 2wd center chunk had 2.75 gears in it, and the little bit that I did drive it felt great with 31” tires. Because of the front wheel bearing issue I never got it above 55mph, so I am sure at highway speeds it could have used a little bit more gear.

My main complaint about the 2.75’s with the T18 would be the start out gearing. 2nd gear starts felt a little hard on the clutch, and for uphill starts I felt I had to drop into granny to get it rolling. I’m not a fan of the non-synchronized 1st for regular driving use. My close ratio (diesel) T19 solves all of that. I loose the crawl gear on the transmission itself, but I have the transfer case low range.

I am aware that you can bypass the GVOD controller and leave it locked in OD in all gears. However, Gear Vendors put the speed control in place to prevent excessive torque force on the conical clutches. The GVOD is a pricey little thing, and I personally don’t think I would use mine that way. As I’ve said a bunch of times, I’m not a big fan of auxiliary overdrives. They are clumsy to use, and they add complexity to the drive train. It’s just that many more bearings and seals and fluid to maintain and parts to break. I would prefer an OD transmission and proper axle gears for the intended use… but the GVOD came with my diesel and the DNE2 dropped in my lap at the junkyard while I was looking for a ZF so I will run with what I have available.