As most Dart owners know, the clutch is not the easiest thing to set up properly under many circumstances.
This type of clutch is known under a few names (PBtec, Centax, Synchro, Speedgrip, etc.) and was derived from the design PB Racing designed and used on their other cars. It was unique in the large scale car market at this time and I can see it being adopted by all manufacturers when the large scale cars need the advantages of this type of clutch.
Shown left is the standard clutch assembly, showing the Belville Washers (yellow) stacked opposing each other to create a spring. Whilst this is a compact arrangement, the clutch tends to be either on or off, as, although there are five springs, they have little movement and tend to bind on each other.
The Belville springs (# 73/230) can also easily become mis-aligned as the shoulder on the retaining/tensioning nut (# 73/225) does not offer sufficient support to the inside of the springs. There were a few different modifications made to the team car clutches to resolve this and the other problems.
What David did was to replace the standard Belville Washers with a coil spring (Coloured yellow in the diagram on the left), which gives a more progressive action to the clutch. This makes the clutch like the rest of the others on the market and makes tuning of the clutch to track conditions both possible and easy.The hole in the centre of the PTFE clutch shoe (# 73/224) was bored out to allow the coil spring to act against the Clutch Shoe Brace Plate (# 73/223).
The dimension on the diagram on the right depend on the spring used, which on the prototype was 24mm, as a cut down black FG Modellsport suspension spring was used.
David has since had some suitable springs made and many of the PBs in his country (Australia) run with his modification. He has also modified the thrust plate so that a hardened thrust washer takes the force of the clutch bearing thrust race.
Whilst I haven't yet personally performed this modification myself, it is a logical and well reasoned one. I have instead fitted the Radiosistemi SVM Competition Clutch to one of my own cars.
What I may do is obtain a spring and either modify the shoe so that the spring still contacts the shoe or make a new brace plate (# 73/223) out of steel, as there is a possibility that the relatively soft standard aluminium one either deforming or wearing unexpectedly. See below for a possible solution.
I think if the Dart clutch shoe/spring arrangement was modified to be like the PBtec clutch shown on the left, it would provide the best possible solution to the problems it has in its standard form.
Once again, many thanks to David Hyde for taking the time to send us details of this modification.