The Difference Between Single and Three Rail Transmissions

by several Spitfire Mailing List Members

Jeff McNeal asked:

What are the primary differences between three rail and single rail Spitfire transmissions? Is one more reliable than the other? Do they *feel* different when shifting? Does one type feel tighter or sloppier than the other while shifting? Is one type more *adjustable* than the other (linkage)?

Bill Davies responded:

A well maintained 3-rail has a nicer shift than the single rail (IMHO). The overdrive equipped single rail boxes are worse - I believe the geometry of the shift mechanism was changed to clear the overdrive unit, though I've not actually played with one to be certain. Having said that, the 3-rail seems more prone to wear in the shift mechanism.

and Barry Schwartz added:

Well for what it's worth, the single rail was introduced as a "commonization (is that a word??;-})" with some other models currently in production, also referred to as the Marina box, since it was in production use at that time. It was also an uprated, slightly more robust version of the 3 rail.

The basic box is the same (barring the obvious external differences), sharing most of the major bits, but the mainshaft pilot bearing (the needle bearing between the first motion, or input shaft and the mainshaft) was made larger for increased load handling and longer life. The older units are notorious for wearing these out rather quickly. Also, the shifting is *slightly* more precise because all the selecting is done with a single shaft instead of migrating through three to do the same thing, and it adds the benefit of having the almost the entire shifting mechanism enclosed (and lubricated).

The problem with trying to use a single rail on anything that was not originally designed for it is that single rail actually protrudes into the bellhousing area during second and forth gear selections. Later bellhousings have a "cup" cast into them to accommodate this shaft, where as the older units do not.

I have an older aluminum thick flange bellhousing that I am going to modify by re-machining to add this clearance by boring a hole in the proper location, and press fitting a "cup" to be captured between the bellhousing and the transmission, as in the later models for the shaft to enter.

Graham Stretch added:

I have driven both boxes and the main differences are the throw of the stick. The three rail box has a wide neutral line i.e. the cross bar of the H and a short throw on the legs of the H whereas the single rail is a short throw on the cross bar of the H and a long throw on the legs of the H.

There is also no adjustment on the single rail box, with the exception that if you have slack and you can find a better one you can change the nylon bit in the linkage. The single rail box has been quoted as being the stronger box, and from experience I would tend to agree as I used to get through boxes on my herald at a fair rate though each replacement box was probably just a spit away from failure any how!