The MGA With An Attitude
CHANGE OF THREADS in Gearbox Housing - GT-123

On 28 August 2009, Art Pearse in Ontario, Canada, wrote:
"Can someone confirm the threads for the clutch slave bolts and the starter bolts are 3/8 UNC?

Good question! I run into this a lot with early and mid 1500 production gearboxes with sliding spline output shafts. Early production started out with fine threads all over, then changed to coarse threads where screws go into the aluminum castings. Clutch slave and starter mount bolts are either 3/8-24-UNF or 3/8-16-UNC, so the answer is either yes or no depending on the production date.

  • Front cover, side cover, and rear housing bolts changed at Gearbox (G)A916 [very early production].
  • Shift extension housing bolts changed at Gearbox (G)A9864 [I think ~September 1956].
  • Mainshaft, rear seal and propshaft changed from early to mid style sliding splines at Chassis (C)20753 [September 1956].
  • Clutch slave cylinder mounting bolts changed at Chassis (C)21513 [~October 1956, 760 cars after first change of mainshaft and propshaft].
  • Starter motor mounting bolts changed at Chassis (C)117331 (by the books). [This has to be a book error, no MGA chassis number that high. From field experience I know this happened in early 1500 production, about the same time as clutch slave bolts, around (C)21513].
  • Mainshaft, rear seal and propshaft changed to rear ball bearing and bolt flange, also to high starter position, finishing Engine (E)GB51767, starting Engine (E)GD101 [(C)61504 January 1959, 1500 high starter engine].
  • Any car may have had a change of gearbox, and some gearboxes may have a change of one or the other parts of the housing. (Been there, done that, all options). The only way to know for sure if you have fine or coarse threads is to look at the parts.

    I am loath to report that I once had a starter motor fall out causing great damage to the engine. Anywhere that bolts in aluminum housings are to be periodically removed and replaced I like to install stainless steel Helicoils for the threads. When doing this it is convenient to change thread pitch, so I make them all UNF fine thread. I do this ALWAYS for the 3/8-inch clutch slave and starter motor mounting threads, and sometimes also for the 5/16-inch shift extension housing mounting threads. That way I can change out a gearbox without having to hunt for different bolts. When I don't always have fine threads for the shift extension I will leave the full set of bolts in the housing while it is in storage so they are always there for next installation.

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