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Posted by Paul (in NY) on Thursday, May 01, 2008 at 19:38:05 :
This may be of interest to anyone who has done, or is planning on converting a Power Wagon or M37 to Centered Rear Differential using the WC Centered Rear Axle Housing.
Part of the Bumblebee Diesel conversion is using a centered rear differential (3rd member). After installing the 3rd member in the axle housing, I inserted the axle shafts and easily found the differential splines. The axle shafts pushed in easy like it should until the axle flange came to the studs on the hub. At this point it would no longer push in easily. In fact it almost did not line up with the axle shaft flange holes. The more it went in the tighter it got. This condition existed on both right and left axles. Knowing something was seriously wrong, I pulled the axles and eventually pulled the 3rd member for inspection. Both axles plugged right into the 3rd member sitting on my bench, without issue.
The US Army TM 9-1808-c, " Power Trains and Interchanges; Military and Civilian
Trucks, 1941 to 1971 Dodge", Page 29, paragraph 6.1 (c) goes into great lengths to explain that;
"The axle centerline of a 9.625" third member is 1/16" further away from the mounting surface than is the center line of the 8.750" unit to allow for the larger ring gear. Axle housings made after SN 82037271 were modified by placing a 1/16" thick ring over the studs and welding it on, to space the third member 1/16" out. These housings use only one gasket and have a tag welded on that says (Use One Differential Carrier Gasket), Unmodified housings with a 9.625" third member, Three Gaskets must be used to provide proper spacing. If a truck exhibits rear axle breaking off near the outer end, the reason may be that a 9.625" third member is mounted in a unmodified housing with only one gasket".
I am well aware of this condition as I have done centered rears on my Carryall and M37 using 3 gaskets. I checked to be sure I had 3 gaskets on the Bumblebee Axle housing, there were three gaskets. Strange, I knew I have a mis-alignment and was stressing the axle trying to push it in. Its time for a Pepsie break and figure this out. After a lot of looking and thought, I ended up inspecting the gaskets I installed and comparing them to NOS Military Gaskets.
I opened a box of new NOS Military Differential Gaskets and measured the thickness on quite a few gaskets. They all were .036" thick. Working backwards,
The steel ring used on modified axle housings is 1/16" or .0625". Add the thickness of 1 gasket, .036" gives a total space out from the face of the base axle housing of .0985". I am using a unmodified axle housing so 3 gaskets are required. The gaskets are .036" x 3 = .108". Thats only a difference of .0095" from the modified housings with a welded ring and 1 gasket, which the Military considered acceptable.
I then cleaned the 3 gaskets that I had installed on the axle housing and measured them. Immediately I noticed that the gaskets were DIFFERENT ! Instead of being smooth like the NOS Military gaskets I always use, these had a pebble texture on both faces and they felt THINNER. I measured them and found they were .026" thick. I had a few more new ones and they too measured .026" thick. So knowing that the modified housings with the welded spacer and one gasket measured
.0985" space out from the face of the axle housing I calculated the following;
These new gaskets are .026" thick x 3 used = .078" total space out from the face of the axle housing. Using the known space out on a modified housing of .0985" and the difference with the thin gaskets of .078", I had a misalignment of .0205". I then installed the 3 Military NOS Gaskets .036", the 3rd member and installed the axles. The axles plugged in normally as expected.
I had bought these thin gaskets on Ebay a while ago. They were loose, not boxed....maybe they are reproductions ? They fit perfectly on all studs, the die cutting of the gasket was perfect. They were the right size gaskets, but the wrong thickness. Be careful what you buy.
So if your doing a centered rear conversion, be sure NOT TO USE a differential gasket other that NOS Military Gaskets that measure .036" thick. The NOS Military Gaskets come individually boxed in the typical brown waxed envelopes. If you dont use the .036" thick gaskets, you too may be pulling the differential to change gaskets. Pulling a 3rd member is not my idea of a fun job !
If you have done a center rear conversion and found the axles difficult to insert, you too may have the wrong gaskets. You need to change them to relieve stress on the axles. I now have ordered a bunch of Military NOS Gaskets for stock and tossed out the .026" thick gaskets. I suggest never buying loose Differential Gaskets unless you can measure them to assure the .036" thickness.
Paul
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