The Stovebolt Site Forums The Parking Lot Big Bolts 55 differential lock

I've got a 55 gmc 550 tandem axle,, with an inter axle differential lock on it (vacuum).. It looks like a transfer case ahead of the first axle,, with the rear drive shaft, for the rear axle, that comes "over" the first axle, to hook in on the "transfer case".. Is this common for this era? I've not seen this before on newer big trucks. Are there many problems with this kind? It looks like a heck of an angle on the shaft for the U joints. I know it's for only a second one would engage it (to get out of being stuck), but it looks strange to me. It, I was told, IS original.

I have pictures of the drivetrain in a similar truck I have seen at a few shows now, I'll email them to you when I get a chance.

A setup like that is not found on any new trucks I know of.
Old Dodge military 6x6 had a somewhat similar setup.

Grigg

Chevrolet used a "Power Divider" that was lever shifted from the cab that sounds about the same. It has one input shaft and two outputs (one to each rear). It had 3-speeds, 2 underdrives and direct. They first started offering factory installed Tandems in 1956 on the 2-1/2 ton 8000 & 10000 Series trucks.

Mike B

Your pulling tandem setup was the typical system used in the mid to late 50's. My father had two chevrolet pulling tandems dumps(1959 models) that used this system. The twin driveshafts were prone to vibration problems, broken u joints, yokes, and had to be lubed every 1000 miles. Most truckers simply used a dead axle arrangement on their tandems, much simpler and less maintenance. The 'modern' tandem came out in 1960 with the power divider built into the front rear and just a short driveshaft between the rears. JAMES

Thanks much guys,, I sure like the help I'll tell you.. I've NOT seen anything like this at all, in my time. The transfer case is off the yoke on the first axle, then it comes way up to the driver side, over the axle housing, to grab the rear axle drive shaft. I know u joints need an angle to run right (most the time somewhat upward), this is up and off to the side of the truck, the rear axle drive shaft has to go. Yea, Grigg, if you would please,, I'd sure thank you.. I do have pic's of it if anyone wants to see it.. Their not the best though, its hard to get a good clean pics of it even under the truck. I sure can see how it would vibrate if one did not engage it after being stuck. ARE they gear or chain driven? It is a rather big housing for it.

Pictures have been sent,

Grigg

I've got an Eaton truck axle book here, and what you describe sounds like the M-series axle setup. And yeah, looks like a real joy to keep goin.
It is all gear driven, no chains to worry about.

Thanks freq2002. Come time to tear it apart, it sure is nice to know beforehand what one will run into. AGAIN, I thank everyone here who responded.. Just can't say enough about you's (good of coarse) :-)

Lemme know if you need some details, I can scan the section concerning the M series and shoot it off to ya.

I went ahead and scanned the whole deal, it's the Eaton truck axles service manual an addendum from 1960. Covers single reduction, 2-speed axles, plaentary double reduction, D-series tandem, M-series tandem, axle shift systems, and all the specs. the addendum covers 3-speed tandem axles.
I've offered it up to the old manual project, but if anyone else would like a copy I can email it all at once. It's an Acrobat file, just under 83 MB. I also have the Eaton speed chart and a truck axle spec sheet from '64.
Let me know!

Hi Grigg...

I would like to see your pics. Can you post them or email me??

Thanks so much. [email protected]

Sure,
Here they are for all to see, start with this one, and the following 4, (5 total):
http://good-times.webshots.com/photo/2916646400080251109tDpxcP

Grigg

Thanks for posting the pics Grigg!!

About 7 or 8 years ago, I came across a GMC cab /chassis in a scrapyard in Darlington County, SC. It had the Pontiac V-8 with the exact Powermatic tranny/Power Divider/Tandem axle combo as the 1956 Chevy in Grigg's pictures. I tried like crazy to buy it, but it was already sold for scrap iron. It was a cryin' shame because the live tandems are very few and far between.