From above.
In 1934 Norton made a primary chain case cover that leaked oil. It leaked oil for 34 years, before they fixed it with the 69 Commando.
A friend and I bought two Commandos in 71, and his leaked oil out of the new and improved chain case cover. The O ring was not put in place correctly.
In about 1970 or 71, I was at a dealers meeting with the Triumph reps. Two things the dealers were having problems with, other than too many warranty issues, were: Oil leaking from the bottom plate on the oil in the frame models, and cylinders wearing out too soon, as in 20,000 miles.
The replies. We are having trouble machining the surface area of the frame, where that plate attaches. From now on the plate will be made from aluminium instead of steel. We hope that it will bend a bit to conform to the frame shape.
We are having problems getting a reliable source for our cylinder castings. Some seem to have too much sand?? in the castings, and are wearing quickly. Also it is difficult to drill a perfectly round hole.
Some of us had recently watched a Honda video showing a machine drilling four nice round holes for a CB750.
Triumph had a 3 month warranty, Honda 6 months. We would get about 1 warranty claim for every 10 bikes with Honda. We got about ( thinking ) 3 or 4 claims for every Triumph. Honda always asked for specific details, and suggestions on how to fix the problem. Triumph were not very interested to hear about it. Just as the entire British industry did not want to hear anyone else's opinion.
The better frames found on most British bikes, did not originate from the factories. They came from outside frame makers, who were easily able to show they were superior. Reynolds for the Norton Featherbed frame. The Rickman brothers, and several others. The Rickman frame was so good, the big name brands would not sell them engines.
My 68 Dunstall had a Reynolds frame.
For those so inclined. Go ahead and buy one of those old British things for too much money. Others who do the research, will be riding Guzzis, Yamaha, Honda and the new breed of Triumph.
UK