The MGA With An Attitude
SEAT RAIL Mounting Position -- INT-109A

At 03:07 PM 3/19/2009 -0600, Dane Hanson wrote:
"Can you tell me where to position new seat slide rails on my new wood floorboards. I believe the rails need to be parallel to the tunnel and side rails...right?

You can just set the seat down wherever you like and screw it down. The tunnel is not straight forward, but the side frame rails are. The slides should be parallel to the side frame tube, and put the release lever on the outboard side.

The slides have pretty good travel distance if you get them set down midway in the stroke. I can move the seats back until the backrest is against the side curtain bag and nearly under the body tonneau. I can also move forward until my tummy just about touches the steering wheel. My wife had even shorter legs than I, and she could get it far enough forward to reach the pedals okay.

See here:
INT-109 - Seat Mounting
FR-112 - Floorboard Templates
floor.pdf - Full Scale Floor Templates

I think the PDF templates are not exactly right for seat rail bolt location. Whoever drew them up got the holes a little cockeyed so the seat rails would be angled a bit when they should be straight fore and aft.

My seat rails have three wood screws at each end. Short wood screws may work loose after years of use and weathering, but I beefed it up some. I have 1/2" thick floor boards where originals were only 3/8". I also installed 5/8" thick plywood strips under the rails to raise the seats a little, allowing for 3/8" carpet padding and 1/4" carpet thickness. I screwed the spacer strips down first with lots of wood screws. Then I used longer wood screws to anchor the seat rails with the screws having a bite most the way through 1-1/8" of plywood.

The Service Parts List shows one 1/4" bolt at each end of each rail, assuming there would be a 1/4" T-nut installed from underneath. I don't know what time the cars had screwed rails or bolted rails. I find little or no factory documentation for the screwed rails, but I have owned 4 different MGAs ('56, '58, '57, and '58), and I seem to recall they all had wood screw type rails. I actually swapped seats between the '58 and '57 in the late 60's. If you use single 1/4" bolts in each end, I strongly suggest that you put a BIG fender washer and nut on the bottom to spread the load on the plywood. New replacement rails would most likely have single bolt ends, because they screw down to the steel floor in the MGB.

Addendum February 2015:
Seat rail locating diagrem, compliments of Clarke Spares and Restorations. Click for larger image.
seat rail bolt locations in the floor

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