- Part Detail
- CLICK TO VIEW
TRIUMPH PLATES - Browse Original Triumph Catalogue Plates
GEARBOX and OVERDRIVE TR4A
Part No:
510921
NOT AVAILABLE This part is no longer available
Not available new use our gearbox and overdrive exchange/rebuild service detailed under part number G/BOXEX and O/DEXA
This part appears on the following Triumph Catalogue Plates
| Car Type | Plate | Plate Title | Plate No. | Triumph Part Description | No per Unit |
| TR4 | BB | OVERDRIVE: OVERDRIVE UNIT DETAILS | BB86A | OVERDRIVE GEARBOX ASSEMBLY (Less Gear Lever) | 1 |
| TR4 | BB | OVERDRIVE: OVERDRIVE UNIT DETAILS | BB86A | OVERDRIVE GEARBOX ASSEMBLY (Less Gear Lever) | 1 |
| TR4A | P | GEARBOX: CASING, TOP COVER AND MOUNTING. | P1CC | GEARBOX UNIT (OVERDRIVE) (less Gear Lever) | 1 |
Q: Can I use a 20mm long gearbox long spigot input shaft in a racing (200bhp) application, is the front of the shaft insufficiently supported when fitted to 4 cylinder engine? I race a TR3A in Historic events fitted with a "Police" gearbox.
A:
Further background
The input shaft in this gearbox has 22 teeth on the shaft and 35 on the countershaft gear. The spigot is 20mm long, but I think I need to lengthen this to 33mm, unless a gear exists that has the longer spigot.
I have found the problem with using the 20 mm spigot in a racing (200bhp) application, is that I believe the front of the shaft is not adequately supported when fitted to 4 cylinder engine, and the "skipping rope" action causes jumping out of 4th gear, and eventually, damage to the synchromesh dog teeth.
RevingtonTR Answer
This question is fraught with complications. Trying to use metric dimensions when everything involved is imperial will make matters worse.
I imagine Triumph would have used whatever parts were on the shelf to build a police specification gearbox. Somewhere on the box will be a date stamp saying 22 D 67 or something similar. This would help. Probably on the left hand side of the casing near where the clutch shaft emerges, but could this could be in the same position on the right hand side.
All the TR input shafts have 22 teeth and 35 on the counter gear.
There are 3 different helix angles so whatever is fitted would have to be a matched pair of gears.
Only the early specification used the longer spigot so that would appear the way to go, but....there are 2 bore dimensions to match whatever mainshaft is used and the early one uses a 0.875" spigot so is called imperial. The later one uses a so called metric size, but at 0.833" is neither imperial nor metric. It would be possible to use an imperial mainshaft and early gear pair, but only second hand gears are available. Mainshafts are readily available.
The middle gear pairs are imperial but have a short spigot. You would therefore have to strip the box to ascertain whether the mainshaft is imperial or "metric". Just exposing the spigot will be enough to establish this.
The final suggestion is that whatever input shaft is present, you could extend its length by electron beam welding (although tig might do it) and grinding the nose to size, which seems to be 0.495". This fits in a 0.500" bore sintered bronze bush, so isn't exactly a tight fit anyway. It may be possible to find a needle roller to fit the crank 1" bore, but there won't be anything that mates with a .495" spigot.
You could have a longer spigot bush made, to add maybe 10mm to engage more of the input shaft spigot, in silicon bronze. #
With the Toyota 5 speed conversions we do, we supply a hardened steel thin wall tube (approx. 1mm wall) that is longer than the Toyota input shaft spigot and provided a suitable length is used will extend to the full depth of the spigot bush once tapped fully home onto the input shaft spigot. The spigot bush will in turn need to be bored out approx. 2mm to give 0.005" clearance.
It is important to mention here we have never heard of this all being a problem and it is worth pointing out that we produce race engines at 200 BHP and back in 1988 raced against a well-known competitor with a supercharged car showing 250HP at the wheels. That car managed around 10 full seasons before reliability modification were considered. With that in mind it is important to ensure there is not a mismatch in the gearbox somewhere that is causing a problem.
If you need us to supply you with some parts, once you have established what you need we will be happy to supply. We can also rebuild the gearbox for you.
