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Counting HALF SHAFT SPLINES - MGA & MGB - RA-106

At 05:46 AM 8/24/04 -0600, Larry Hoy wrote:
>"I recenty made a diff swap [my MGB] with another lister. He wanted to put a 3.9 in his MGA .... When I got the diff from him it had the fine spline axle gears, .... Were MGAs ever built with the fine spline axles?"

Yup.

All MGA 1500 had 10 spline half shafts.

MGA 1600 (chassis 68851 May 1959) had involute 26 spline until December 1959 (chassis 82748ww 82892dw May 1959)

MGA 1600 (chassis 82749ww 82893dw December 1959) had involute 25 spline, continuing through production of MGA 1600 MK II and including the MGB models.

The Twin Cam cars changed from 10 spline to 26 spline in June 1959 (chassis 2370) to match the rest of the body and chassis changes for the 1600 type model. I don't know if the Twin Cam ever changed to 25 splines (but it's a good bet), even though production continued to May 1960.

CAUTION: Like Elmer Fudd says,be vewry, vewry careful. The 25 spline halfshaft will fit into the 26 spline differential, but will fail under load. Count fine splines at least three times to be sure.


This picture is included just for grins. This is a fractured half shaft, broken at the outer face of the differential gear. This came from fatigue in September 2004 after 341,000 miles of spirited driving. This included several years of serious weekend autocrossing using a competition clutch and sticky race tires, neither of which wants to slip much during a jack rabbit launch. During the same time frame I was replacing a failed universal joint in the propshaft almost once per year, giving some idea of the harness imparted to the drive train. Having gone that last few years without using either the competition clutch or race tires, this finally failed two blocks from home after a 150 mile day on a casual rally.

I tell you, this car and I have a very good cooperative repoir, so it knows exactly when to fail if it has to. Considering the circumstances that lead to this break, I believe I will change the other half shaft as well, in the interest of avoiding another such probelm at a less convenient time or place.

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