By Dave Porter, Customer and Technical Support at The Bonneville Shop
Table of Contents
The Triumph 650 and 750 ignitions and primary drive systems evolved over the years, and along with the upgrades, came revisions to the primary chaincase covers. The beautifully cast covers are one of the most prominent and eye-catching components on the unit-construction Triumph twin engine. Unfortunately, they are also vulnerable to damage and corrosion, so original primary covers in near-perfect condition are highly coveted by restorers and are becoming more difficult to source. Fortunately, there are reproductions available, so when a concours restoration calls for a new cover, there are three iterations currently available:
Primary Chaincase Cover: 57-1727
57-1727 was the original primary cover, as fitted to the early unit-construction 650 models from 1963 through 1967. The early road-going 650 machines were equipped with the Lucas 4CA contact breaker points ignition, which was statically-timed, so there was no provision for strobe-timing without a lot of bother. The cover features a large and cosmetically pleasing nameplate over the casting covering the alternator. When upgrading an early unit 650 to electronic ignition, many professional Triumph mechanics keep a beat-up later primary cover with the rotor inspection aperture for temporary use in dialing in the ignition timing.
Primary Chaincase Cover: 57-2439
57-2439 was the part number assigned to the next version of the primary cover casting for the 1968 model year. A revised Lucas 6CA contact breaker points ignition was introduced, allowing easier adjustability of the points and precision timing was now attainable. A removeable inspection cover and timing pointer allowed the tuner to observe the alignment of the marks on the magnetic rotor against the fixed pointer, with the engine running. Although the part number 57-2439 is only shown in the 1968 650 parts catalog, the casting was used through the 1974 model year. The subsequent part number of 70-9245 appears in the 1969-1972 parts books and was specified for the 650 models with the two-row, or “duplex” primary chain and clutch chainwheel. This is the part number most suppliers are selling the 1968-1972 cover under, but often describe the 1973-1975 model years as being compatible.
“With the added displacement of the 1973 750 models, a new upgraded three-row “triplex” primary drive system was implemented for the TR7 and T140 models.”
Despite the added width of the triplex system, the same primary cover casting was used, but some minor revisions to the interior of the cover were necessary to prevent the chain from scrubbing the cast-in bosses for the cover screws. A shorter timing pointer was also used to prevent it from scoring the rotor. The part number associated with the 3-year only cover is 71-3555. This cover appears to be out of production at this time.
Primary Chaincase Cover: 71-7465
Chaincase Cover: 71-7465 was the last part number, appearing in the 1981-1985 parts books, however it was preceded by the more common 71-7003 left-shift primary cover, specified for the 1976-1980 models. Vendors and retail suppliers typically supersede 71-7003 to 71-7465. With a revised crossover shift spindle, spindle bush, and external oil seal, the 1981 models were introduced with this specification. The 71-7465 condition is what is currently available in the marketplace, so be aware that the 1976-1980 models may require some modification to accept the last version of the left-shift primary chaincase cover.
The 1973 750 models were sold in great numbers, and many Triumph enthusiasts believe that they were one of the best motorcycles that Triumph ever produced. Given that they sold so many units and were loved by their owners, there are still many of them being enjoyed today, as well as a few 1974 models too. Why the 71-3555 primary cover is no longer available is a bit mysterious, but alas…there is hope if you are in need of a new cover for your 1973 or 1974 Triumph 750. I recently removed the primary cover from a 1973 TR7RV engine that I purchased some time ago, thinking it was the original cover. After closely inspecting it, I noticed there was no drilling for the level plug at the rear of the cover. It also has the short 57-4643 timing pointer fitted. It has the T2439 designation cast into it, but I can’t tell for sure if it is an original factory cover, or if it is an earlier cover that was retrofitted to the triplex chaincase. In any event, there are numerous witness marks on the interior of the cover that illustrate where some fettling may be required to fit a pre-1973 cover to a right-shift 750 chaincase with the triplex primary drive.
“The 1973 750 models were sold in great numbers, and many Triumph enthusiasts believe that they were one of the best motorcycles that Triumph ever produced.”
The point of this article is to illustrate what will have to be done to fit the reproduction 70-9245 primary cover to a right-shift 1973-1974 TR7 or T140 with the triplex primary drive. I hope you have enjoyed reading this blog.
If your primary or timing cover needs a polish, check out our video with tips to get the job done.