The MGA With An Attitude
T-NUTS For Seat Mounting in FLOORBOARDS - FR-111B

T-nuts for seat mounting, viewed from bottom of floorboard.
Rusty one is original. Silver one is replacement from Clarke Spares.

Some original MGA (perhaps most of them) used T-nuts in the floor to secure the seat slide rails with 1/4-inch bolts. My MGA uses wood screws, two or three in each end of each lower rail installed from the top. The Service Parts list calls for HPZ0405 Hex head Pointed tip Zinc plated screws, 1/4-28-UNF x 1/2-inch long, 8 pieces (and spring washers). This implies that the T-nuts are 1/4-28-UNF thread, but they are not listed separately in the SPL. Apparently they were originally pre-installed in replacement floorboards, so not listed or sold separately.

Original T-nuts (the rusty one in photo above) were rather small in diameter, may have had three tiny spikes in the flange to bite into the wood to prevent rotation, and three small tacks to hold them in place when the bolt is removed. Current replacement T-nuts may be larger in diameter with a flat on one side of the flange ("D" shape flange) and still three holes for tacks. The one in picture above came from Clarke Spares And Restorations.
I believe the depression in the wood (above) is not originally machined, but it is caused by tightening the bolt, and settling over some decades of use. It is common for the flange to have three little spikes to dig into the wood to prevent turning while tightening the bolt. If they have longer tabs they don't need the tacks. McMaster-Carr has them with 1/4-20-UNC threads (not fine threads), long prongs, no tacks, as cheap as $0.09 each in packs of 100 (picture at right).

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