Modifying a GT6 Gearbox for a Spitfire

by Joe Curry

Because the GT6 gearbox has a very close ratio gear set, it is excellent for installing in a Spitfire race car. However, there are a few modifications that must be made to adapt the gearbox. The differences are in the bell housing and the input shaft.

1. The input shaft is too long and must be cut down approximately one inch leaving the tip the same size as that of the one from a Spitfire. The best thing to do is take the GT6 input shaft and a shaft from a Spitfire 3-rail transmission to a machinist and tell him, "Shorten this longer shaft to the same length as the shorter one and leave the end the same dimensions as the shorter one."

2. The splines on the GT6 input shaft are larger than the Spitfire splines so a special clutch disk will have to be used for this purpose. Ted Schumacher at TSI Import Automotive can provide this part. Be sure to specify the diameter of the clutch disk with the splines for a GT6.

3. The GT6 bell housing is deeper than the Spitfire one and therefore will not be used. However, the seal collar that presses into the GT6 bell housing must be removed and installed in the Spitfire bell housing. The hole in the Spitfire bell housing will need to be enlarged to accept that collar. But the result is a positive seal instead of the scroll seal that is typical of the MkIV full-symchro gearbox. A very good addition is an aluminum bellhousing from a Herald gearbox. This will save several pounds of weight over the cast iron one used on Spitfire 3-rail gearboxes.

4. The Spitfire throw-out bearing mounting hub will not fit over the GT6 seal Collar, so it will need to be bored out, since the GT6 hub will not work with the Spitfire operating lever. To ensure that it is sized correctly, take both the collar and the hub to a machinist and tell him, "make it fit". While the hub walls appear to be too thin to bore out to the correct size, the GT6 hub has the same outside diameter and therefore the same wall thickness after boring.

NOTE: While it is possible to install the gearset from a late GT6 into a MKIV full synchro 3-rail gearbox, I prefer to use the GT6 box because it has spring loaded thrust washers on the laygear. This minimizes laygear float and takes up any slack in the thrust washers.

The following is a comparison of the gear ratios:

Gear                     Spitfire                   GT6

First                     3.75:1                  2.65:1                        

Second                    2.16:1                  1.78:1

Third                     1.39:1                  1.25:1

Fourth                    1.00:1                  1.00:1