OVERDRIVE

[Marty, SoCal]

I recently added an ATV overdrive to my '43 GPW, Wow, what a difference! I bought a rebuilt unit from Herm the overdrive guy. Herm is a super guy to deal with.



I decided to make my own shifter setup as I didn't want the shifter to project out of the floor as the ATV shifter would. I modified a second "In-Out" Dana 18 t-case shifter lever and made a 3/4" (19mm) longer t-case shifter pin to mount it between the t-case ears and the transmission. I made my own shifter boot and ring that looks as stock as possible. I made a new transmission floor plate out of aluminum so I wouldn't have to cut the original plate. I heated and rebent the stock Dana 18 shifters about 1" to the right for added clearances between them and the trans shifter. I saved my original GPW shifters to return it to stock later, if I want to.





The overdrive is useable at speeds over 40 on flat terrain, it easily cruises at 60 mph even with my 270 lbs buddy as a passenger! (I'm 200 lbs myself) Second gear OD splits 3rd and second normal gears nicely for fairly steep hills that second would be too low for, yet regular third is too high for. Milder hills require a downshift out of OD into regular third if the speeds are around 50 or so.

Here's a link to my Photobucket Overdrive page:

http://s551.photobucket.com/albums/ii47 ... Overdrive/

[Marty, SoCal]

Some more pics!


ATV Overdrive parts as supplied


Making a new 3/4" longer pin on my 1943 Sears lathe!


Overdrive installed on t-case.


Test fitted on the ground with my spare t-case and transmission case. The blue marks on the shifters are where I heated them to bend them slightly to the right for more clearance. I made my own linkage with 5/16" steel rod, and a 1/4-28 RH threaded heim joint end to connect to the extra shift lever.


Three shifters in a row! Herms' designed shifter link is shown mounted to the OD unit, I didn't use it.


The heim joint connects to a 1/4" hole I drilled in the tip of the third shift lever. The heim joint fits to the 5/16" rod because I welded a 1/4-28 bolt to the end of it.

[OldGPW]

Great Job! I really like the way you were able to do it without hacking the floor up.

Would it be possible to have some detailed instructions on modifying the kit the way you did?

[Marty, SoCal]

Basically, If I had to to it over, I would not have ordered the shifter or boot kits from Herm, they would work great with a CJ or M38 raised floor, but the trans is too close to the floorboards on a MB/GPW for his shifter to fit without adding a giant boot. His shift lever also hits on the side tab of the T-84, which would require you to modify the shifter and/or cut the tab off the case.


Notice how high the ATV shifter bracket would place it over the transmission. It also would place the lever about 1" back of the t-case shifter levers, even with the trans shifter.



To use the ATV shifter would require thinning the shift lever and/or trimming the tab on the trans case

I will say that the ATV stuff is well made, though, as to workmanship and materials, I suspect that the lever is "universal" and the bracket and shift link are specially made to fit the T-84. The shift lever boot also is made to fit CJ's and M38s, the holes are in the wrong spots for a MB/GPW.

I'm working on a M38-A1 and noticed that the "In-Out" t-case shift lever is a good bit longer than the MB/GPW version, it might be a better candidate to make a OD shift lever which would place it up closer to the regular trans shifter.

A lever would not be too hard to fabricate from scratch. The pin on the t-case is 1/2" in diameter and there is about 5/8" space between the t-case shifter mount and the trans case.

The early GPW case, with the right side fill plugs does not have a reinforcing rib, the case would require a retaining bolt to be used with the extended pin. The shifter clears the fill plugs perfectly. This pic also shows that you could use the ATV shift lever to work with the pin mount.


Using a late "H" case required grinding a bit off the rear case side of the shifter to get free movement on the pin. In the pic below, you can just make out the slight grinding that was needed on the left rear of the shifter, near the rib and tab.


I had problems with end play in my "H" case trans causing it to pop out of third, so I reinstalled my mid series GPW case after freshing up the internals. The rib is a bit smaller and the unmodified shifter would probably have cleared the case fine.

[Marty, SoCal]

MAKING A LONGER SHIFTER PIN
For the shifter pin, I used 1/2" hot rolled steel rod I picked up at the local hardware store. I have my grand dads old Sears Dunlap lathe, (circa 1943!) which made making a new, longer pin relatively easy!


My old lathe! The Sears model number is 109-0703


This is the 1/2" rod cut to length on my chop saw. It's approximately 3/4" longer than the stock shifter pin.

NOTE: A lathe makes quick work at cutting a groove in the new pin for the set screw, but you could use a drill press or even a large drill motor held in a vise to spin the pin, along with a file to make the groove. The groove is in the same position relative to the grease fitting as the stock pin.


Measuring for the new groove

I drilled the pin all the way through using the lathe, The drill bit I had was too short to go deep enough, so I had to reverse the pin in the lathe and go in from the other side, too. You could drill it by hand, or use a drill press, depending on your access to shop equipment. The hole is for the grease fitting, and a retaining bolt, if needed. I tapped both ends of the pin for a 1/4"-28 tpi grease fitting or SAE bolt. (6mm x 1mm threads would work just as well, for European or British use if US standard hardware is too hard to find). With an early case, a bolt will retain the shifter, as there is no rib on the case to hold it in place. With a later case, use a matching thread set screw and locktite to plug the hole so the grease won't come out.


Manually tap both ends of the pin for the grease fitting and plug


Machine the taper on the end where the grease fitting goes


Polish the shaft with sand paper


Drill the ports so the grease will reach each shifter


Test fitting the new pin. (Mid series GPW case shown)

Not shown is how I hardened the pin by heating it dull red and quenching it in used motor oil. It came out a nice blue black color.

[OldGPW]

Thank you very much for the extra photos, I need to get an overdrive and these steps should guide me through the installation.

I can see why you set the ATV shifter aside, it doesn't look like it would really fit with the idea of a stock body.

[Ed Menees]

Very nice installation. I also installed an OD a while back. Maybe it will help if you don't have access to a lathe. I love my OD.

http://www.g503.com/forums/viewtopic.php?f=10&t=134640&p=778226&hilit=overdrive#p778226

[OldGPW]

Hello Ed,

Thanks for the link, I am REALLY looking forward to getting an OD sometime. Soon I hope!

Have heard nothing but good about them.


In either yours or Marty's experience, has the "front end" shimmy been a problem when using an OD? Or is it about the same as one without an OD?
I have heard it can be a problem at higher speeds.

[Marty, SoCal]

I don't have any shimmy with my GPW, and I've had it up to 65 mph. The combat wheels have a good dynamic balance, with weights inside and out, I have unlocking front hubs, and a add-on steering damper on the tie rod. It's also important that the knuckles have the proper preload on the kingpin bearings.

[Ed Menees]

I have never had a front end shimmy on my GPW. I've never balanced the tires and they are very smooth at all speeds. No damper or lock out hubs.

[OldGPW]

Hello Marty & Ed,

I have never experienced the shimmy without an overdrive, so I was curious if the OD would affect that.

I suspect that people who experience it probably have worn steering/axle components.

Thanks for the replies.

[Marty, SoCal]

Update: Took the GPW for a long freeway cruise yesterday, it holds 60-65 mph pretty well, at about 3/4 throttle. At 65, the 4 ply Denman 600-16 NDT's seem to start to follow the grooves in the freeway a bit, but no shakes or shimmys! At 65, the WW2 style canvas top seems to flap alot, I probably will remove it for longer drives.

[Marty, SoCal]

Took a long four day camping trip in the GPW to the SoCal Desert Training Center, Palen Pass area to be specific! I went with a bunch of buddies in their modern jeeps, (plus one other G503, a '44 MB that the owner towed out). Total miles was 437, approx. 360 of which was on the freeway. Averaged 62 mph, top speed 66 while on the freeway. (I could have gone faster!) The GPW (which has the 8-1 aluminum head) was able to maintain 50 mph up the 10 freeway through the Banning pass, with a 20 mph headwind. Had to go to second gear, OD though. On the longer, less steep up- hills, I dropped the speed to 55 to 60mph, normal third. I burned about 30 gallons of fuel. Max temp was 190*f on the uphills. I was carrying about 300 lbs of gear, tools and water.

Off road, the OD helped greatly as I was able to split gears between first and second, Sometimes first was too low, but second was too steep, OD, first was perfect! I also used low range third alot, in and out of OD for driving on the smoother sections of the trails. The OD unit seems to give an almost perfect split between the three speeds of the T84.

One problem area I need to work on is the oil transfer between the t-case and the trans, after about a half hour at speed, the trans starts to vent excess oil out of the top cover. The oil was actually seeping through the floor plates! It's coming out of the small vent hole in the top cover. The trans has all the mid series features, with the shielded bearing and slinger. I may have to go to a double rubber sealed #307 bearing to stop the oil transfer. I lost about 3/4 quart over the whole trip, topping the t-case off twice. If I get the oil loss problem under control, I wouldn't be afraid to drive it across the country.

EDIT: Link to my trip pics! : http://s551.photobucket.com/albums/ii47 ... th%202010/

[artificer]

At 65, the WW2 style canvas top seems to flap alot, I probably will remove it for longer drives.

A gentle reminder....
Don't forget that windshield assembly was not designed to take the continual stress of high speed driving especially with the top top flapping & trying to rip the frame off....time after time.
Would be absolutely terrible to end up wearing the screen....at any speed.

[Marty, SoCal]

Yep, the top was folded up under the seat for the trip! I also open up the windshield about 2", it helps cut the buffeting. All my gear was tarped down in the back, too.