Triumph TR4A-250 Chassis to Body Mountings
CHASSIS ROT & FATIGUE
The IRS chassis was a compromise which enabled Triumph to go independent without spending too much. Regrettably this left a few development needs for customers to discover. Designs exist for beefing up lower fulcrum brackets and differential mountings which were not made public as a massive recall would have been financially devastating for the cash strapped company in the mid 1970s.
Originally, these chassis were given only a coat of chassis black enamel. Modern paint or galvanising procedures would have controlled corrosion but fatigue problems were inevitable, so should be faced at the first opportunity. The differential mountings can be tackled from underneath although a popular route in the past was through the rear floor area, which may explain some unusual welding visible when the carpet is lifted. The pins could be re-welded and hopefully reinforced as well but the only proper solution is with the body off the chassis and to replace the pins and associated fixings completely with heavier gauge, reinforced units, which fully box the pins. The trailing arm chassis legs collect water and fatigue too, so should never be repaired: replacement is essential. This requires removal of the upper and lower cruciform (or breast) plates to permit full seam welding.
The corrosion at the rear end of the chassis can be rampant, hence the need for ill. No. 21 (RR1324). Fatigue to the differential bridges also quite frequently necessitates complete replacement. At least the replacement bridges carry all the necessary strengthening modifications. The side braces, ill. No. 6, collect water and rot. After replacement, don’t forget to drill water outlet holes at their bases. Cars can suffer from fatigue to the gearbox mounting brackets and should be replaced as required. A little extra reinforcement here does not go amiss. Don’t for get that the 3/8" washers under the four fixing bolts should be hardened, not mild steel which soon wear and allow gearbox movement.
The front suspension chassis brackets, ill. No. 3 fatigue and are easily damaged in accidents or even kerbing. These must be carefully inspected and replaced if showing any untoward signs. Either way, fitment of the reinforcement plate (Ill no. 36 or 37) is recommended and adhere to the 25-ft/lbs. torque clamping the lower fulcrum brackets to the chassis. Finally remember, your chassis is very old and may have had an unhappy previous life. Accident stress might not make itself known for thousands of miles and may be hidden by paint or underseal.
The packing plates 619395 and 619396 are for use when a greater thickness of body to chassis packing is required than the stated quantity of rubber/canvas pads. The metal plates should be as a supplement to the rubber/canvas pads. The use of additional rubber/canvas pads should be avoided and the more solid packing basis utilised.
It is rare for a body to fit a chassis with identical numbers of packing pads side to side and end to end. So how do you know which parts are right and wrong? Life is very easy if the chassis is bare. All that you need is a flat floor, a piece off string, a ruler and a tape measure. The diagonal dimensions are in the workshop manual (545277). The rest is a simple matter of measurement from the floor to selected points on the chassis.
If the body and chassis are joined, the method used doesn’t change too much. You still need a reliably flat floor. Raise the car from the floor and measure up to the selected chassis points. More care is needed because of larger measurement distances.
Essentially, if the front end is set up parallel to the floor, the rear should be the same. The front needs to be reasonably correct to ensure steering accuracy. Probably as much as 1/2” of misalignment at the rear can be absorbed by careful packing - this depends on individual acceptance. If there is a problem and it’s understood, the body can be built around it. It is suggested that misalignment at the front of more than 1/2” should be corrected professionally especially if other repairs or modifications are to be carried out.
Once the chassis is prepared to the tolerance you accept, it does provide a perfect jig for body preparation. It is obviously a good idea in this case to omit the paint finish until the body is finished. The chassis also provides an excellent transport jig for the bodyshell when it goes for painting, so if it is used for this ensure the clamping bolts move freely in their threads. Pack these threads with greased screws when the chassis goes for painting (or galvanising).