A small follow-up to the exhaust pipe situation for my 1972 GT750 J rebuild. Although I did say in the previous post that I was finished with cutting open any additional pipes, I lied ! Given the cost of having these re-chromed, I decided that the small pipes I had available needed a closer look just to be certain the internals were all sound.
This photo (just click or tap to see a larger version) is actually of a later 1975 through 1977 inner, lower pipe, and again you will note that as with the other large later model pipes, there is an additional lead baffle plate (the one closest to the engine) which is spot welded on both the inner and outer skins, which prevents you from just slicing the pipe end to end and lifting off the outer skin. With this pipe, you can see that all the baffle plates that the removable baffle slide into are broken. They actually have rusted, and then broken away from the rear outer skin that they are spot welded to. The rear cone is spot welded to the outer skin on the inside only.
The interesting thing that Ross and I discovered, and that certainly I was not aware of, is that the small pipes do not have wadding under the metal sound deadening mesh at the front section of the pipe. The large pipes have wadding, but not the small ones. Both the 1972/1973 and the later style inner pipes just have a fine metal mesh, similar to screen door mesh, spot welded to the inside of both the inner and outer skins. I had been told by many people that wadding was installed in all the pipes. Having looked at several examples now, it is clear that this is true only the large upper pipes. That is actually a good thing as it means that concerns about neutralising the acid used by the plating company is less of a concern for the smaller inner pipes, as there is nothing there really to absorb and hold the acid. A good flush with water should be all that is needed.
The other thing I confirmed is that the first baffle plate (the one nearest the engine) does indeed differ between the J, K and L pipes and those from 1975 and later M, A and B models. As seen here, the later style inner/lower pipe has a outlet diameter of 20mm. Inner pipes from the J, K and L models all had 29mm outlet holes. This is detailed on page 2 of the Suzuki Service Bulletin GT-7 which you can read at this link on my web site. This is only worth knowing if you happen to be mixing 1974 L inner pipes with later model inner pipes on a bike. 1974 inner pipes are not stamped “GT750” toward the rear of the pipe, whereas the 1975 and later pipes are all marked, however at a quick glance they do all look the same. That difference in baffle plate hole sizes may make carburettor set-up a bit of a challenge for the centre cylinder if you decide to mix and match. Just another thing to be aware of.
The project continues ! 🙂