I wonder if this happens as a cascade effect or all at once failure? I've known about this weakness for some time but have never seen it in person or pictures. Thanks for sharing.
Bert
1952 M37 W/W Rebuild @ 59% complete
Engine rebuild @ 95% complete
1985 M1009, 1990 M101A2, 2008 M116A3 Pioneer tool trailer
MVPA # 24265
NRA Life Member
NRA Cert. Personal Protection Pistol Instructor
NRA Cert. RSO
Class III RSO/KCR
The common cause of this failure is simple. The snap rings are available in 4 different thicknesses. You need to install the thickest ring that will work in your application. Most people pay that no attention using whatever ring is handy. If the ring used is not thick enough; the assembly has a slight amount of end play. Any end play at all allows the assembly to move forward every time you shift gears, eventually either the ring is forced off the shaft or the spline grooves in the shaft become fatigued and break off. If the transmission is built correctly to the best it can be; they actually give little trouble.
I'm not as familiar with the 88950 trans, but some of the small bits for the later NP420 (snap rings, etc.) are the same as used in the NP435, and small parts kits for that trans are readily available from places like Novak Conversions and quad4x4. (Edit: after cross referencing the part nos., it looks like none of the snap rings or shims are common between the NP420 and 88950.)
The mainshaft in the NP420 is vulnerable to the same sort of failure. Of the three NP420 transmissions that I disassembled in the last year, two of them had broken mainshafts similar to the photo in the first post above.
As Charles mentioned above, too much freeplay between the snap ring thickness and the width of those grooves is not good. Too little freeplay isn't good either, i.e. don't try to wedge a too-thick snap ring into place.
“When a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, IT IS THEIR RIGHT, IT IS THEIR DUTY, TO THROW OFF SUCH GOVERNMENT...” -Declaration of Independence, 1776
Elwood,
Thanks for trying to help. The snap ring and splines in our transmission looked OK, but the ring did have some fretting and I don't plan on attempting to reuse it.
Hopefully I can find something that will work. If I can find one that's too thick I'll have it ground to size.
I guess that ring/spline combo takes a hit every missed shift when the dogs bounce across each others faces. Especially in the 2-3 shift.
When I rebuilt Chris's transmission he contacted Charles and got a selection of snap rings from him in varying thicknesses.
Charles has them made for him. I put my calipers on them and put them in order of thickness. The first one I tried was the thinnest and it was too loose. The next thicker one fit in the grove very nice.
Home
