Triumph TR4A-5-250 Rear Axle Unit Assembly - IRS
The original part numbers and ratios of independent rear suspension axle units fitted to Triumph TRs is shown here in the chart. The simple way to check the ratio is by counting the crown wheel and pinion teeth. For example, if the crown wheel has 38 teeth and the pinion 11, the ratio of 3.45:1 is arrived at by dividing the quantity of teeth on the pinion into the quantity of teeth on the crown wheel.
Petrol injected Triumph TR sports cars were the only ones to have the 3.45:1 ratio axle as standard fitment. Carburettored TR250s and TR6s were fitted with the 3.7:1 ratio, along with TR4A (which also had an option for 4.1:1). On the road, the difference between the 3.45 and 3.7 ratio is barely noticeable (unless you are using 60 series low profile tyres on 15" rims) whilst a 4.1 on a 6 cylinder TR feels as though you have run out of gears!
As there were no ratio changes during the production periods of the TR4A, TR5/TR6 PI and the TR250/TR6 Carb. axles they are assumed to be interchangeable between model years. This is true if entire units are replaced. If the internals of a certain model year are used to repair another axle of a different model year, problems may be encountered with non-compatible parts. The major significant change was the replacement of the solid, shim adjusted pinion bearing spacer for a collapsible type, part way through TR6 production. The other items to change internally were the sun and planet gear sets. The early type had plain teeth, the later ones had a groove machined into their tooth edge to aid identification, and so should be fitted in sets of four when interchanged.
We supply from stock, a range of fully reconditioned, ready to fit axle units on a strict exchange basis. These are available (depending upon your requirements and budget) with either a 3.45:1 or 3.7:1 ratio, with optional brand new crown wheel and pinion, and finally with a limited slip differential option (supplied only with a new crown wheel and pinion).
Old exchange units returned to us should be complete and assembled, drained of oil, and be externally clean with no irrepairable damage.
Note that the front mounting plate and the rear aluminium housing (complete with 4 lugs) must be returned. Failure to comply with these requirements will result in retention of part or all of the exchange surcharge.
Availability of the TR4A IRS and early TR250 axles is sometimes limited. It is possible to fit the later TR5/6 axle providing the mounting kit (RR1390) is also purchased.
Alternatively, you can swap over your existing backplate (you will need gasket 134480) to the unit we supply and simply return the backplate with your old unit.