Rather than call the story “premature ejectulation or spontaneous eruption,” I toned it down a bit. It was a challenge to put an appropriate handle on this story, due to the unusual nature of what occurred. Read on…
In the process of having the 1963 CB77 parts powdercoated and refinished, a decision was made to have the alloy parts (wheel hubs, brake backing plates and fork crown) all bead-blasted and clear-coated with a polyurethane paint, instead of powdercoating it all with clear.
After a week of waiting, the call came in to pick up the parts, most of which were sandblasted and powdercoated black, except for the painted items, previously mentioned. Each of the 34 parts were refinished and then wrapped with protective foam from the supplier. I didn’t inspect them all on the spot, as their work is generally performed properly and parts given great care.
Stopping to show my friend, Lea, the results, we noticed some globby, dirty-looking paint blobs on the surface of the brake panels. Assuming that it was just an accident or some kind of oversight, the parts were returned to the powdercoating shop for inspection and refinishing.
The manager looked at the parts and felt that the raised features were metallic in nature, as if they were gouged by a wrench or tool, not created by just some debris/dirt that flew up on the surfaces as the paint was drying. Whatever the cause, he apologized for the condition of the finished parts and put them on a priority status for rework. The parts were left on Thursday with hopes of a Friday or Monday completion date. No phone calls by Monday left me wondering why they were not able to just strip the paint sand/bead blast the parts and quickly paint them again.
A call in on Tuesday had me connected to the production manager, who told quite an odd tale about the parts. According to his experience with the parts, they were stripped, sanded and bead-blasted so they smooth again. Laid out on drying racks ready for painting, the surfaces became rough and pitted once again, as if some kind of migration of metal alloy bits were coming back to the surface. They reworked the parts again and the same results reoccurred. Given the continuing history of this mysterious metal migration, the decision was made to just clean them once again, clear-coat them as before and call it good.
When the parts were ready, inspection showed that indeed the original metal areas which had arisen from around the brake anchor point (where a wrench could have damaged the surface) were now depressions after sanding, but new ones, similar to the first had popped up on the opposite side of the anchor point, where there were none before.
If there are any metallurgists in the audience who can shed some light on the possible chemical interactions at play here, please contact me with your wisdom and I’ll be glad to share it with the rest of the readers.
When some welding was done on CB77 engine castings, my local wizard machinist/welder indicated that the aluminum seemed to have some pockets of zinc, magnesium or other reactive compounds imbedded, which would flare off when he was attempting to lay down a nice even bead.
A quick look at alloy inclusions, listed online, showed many casting option ingredients such as: Copper, Silicon with added copper and/or magnesium, Silicon, Magnesium, Zinc and Tin, in various percentages.
It is hard to determine Honda’s preferred casting formulas in their 1960’s cast products, but we do know that they seem to be long-lived and stable in almost all applications that we have seen. Honda did use magnesium castings for their racing machines, which bled over into the magnesium castings seen on 1959 CE71s and the early CB92 Benly Super Sport streetbike and racing models.
Whatever it was, this spontaneous eruption of metallic flakes was a first for the shop managers and for myself. The parts will just have some extra “personality” to the finishes now. I’m glad that this wasn’t a 100 point show bike project, though. Even though people call me the “Honda Guru” of 1960s vintage bikes, there is a LOT that I don’t know and am continuing to learn with each passing day and project which comes through the shop.
Bill “MrHonda” Silver