C15a front hub bearing

[Hanno Spoelstra]

Originally Posted by Jordan Baker

When I look over the timken data pages there appears to be no difference between the bearings other then the “d-bore” measurement. That difference is 1.588mm with the 395s being the slightly smaller one. My initial thoughts were that the 399a had a higher load rating or something similar.
I’m curious on the change over in with regards to serial numbers. Was that a significant change in production? It may be interesting to see as various hubs could be mixed and matched on trucks today.

I'm not a bearing specialist, but looking at the basic dimensions:

Timken Part Number 395-S - 394
Dimensions
d - Bore: 66.675 mm | 2.6250 in
D - Cup Outer Diameter: 110.000 mm | 4.3307 in
B - Cone Width: 21.996 mm | 0.8660 in
C - Cup Width: 21.999 mm | 0.8661 in
T - Bearing Width: 21.999 mm | 0.8661 in

Timken Part Number 399A - 394A
Dimensions
d - Bore: 68.263 mm | 2.6875 in
D - Cup Outer Diameter: 110.000 mm | 4.3307 in
B - Cone Width: 21.996 mm | 0.8660 in
C - Cup Width: 18.824 mm | 0.7411 in
T - Bearing Width 21.999 mm | 0.8661 in

I see differences in d Bore of +1.588 mm like you said, and in C Cup Width of -3.175 mm.

Clearly the Chevrolets after s/n 184–23657 had an axle shaft which was 1.588 mm larger in diameter. While the Cup Width may still fit in the hub, fitting the 399A over the earlier axle would result in a sloppy fit and thus an early failure.

HTH,
Hanno


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