Project Kawasaki Z900 Stocker part 2 Ralph has a blast!
Classic Motorcycle Mechanics|September 2021
For the best finish on his Z900’s motor Ralph wants the best, so he visits Stephen Smethurst Casting Renovation to find out how it’s done properly.
RALPH FERRAND

Last time out, you’ll have heard me gloating about being the fortunate recipient of a far from shiny example of Kawasaki’s 1976 flagship Z900A4 in boxes for nowt.

I did promise the generous benefactor that I would do justice to his kind gift by restoring it to the very best level I could afford to. To this end the naked aluminium castings would need to be cleaned to the very best standard available in the UK and that meant visiting Greater Manchester with my trusty Nikon. To the best of my knowledge I am the first ‘gentleman’ of the press (if that’s not an oxymoron?) allowed to cross the threshold and see the secrets of Stephen Smethurst’s business (called Stephen Smethurst Casting Renovation). Stephen took a wee bit of persuading, but I did point out that to copy him, someone would need a very big bank account given the huge financial investments he had to make, and even then they wouldn’t have the years of experience and experimentation Stephen has undergone to perfect his system of making aluminium castings appear as if they just left the machine shop.

Stephen had worked his magic on engines for me in the past, so I knew that the quality of his work surpassed that of any of the competition’s efforts that I have ever seen or heard of. Most folk believe that vapour blasting is all that is required to clean engine cases, but this is far from the truth. Vapour blasting alone makes parts look better than they were, but it is far from a serious restorer’s goal.

There are various types of media blasting for different purposes. If you are going to paint a part you want to get rid of any corrosion such as rust and old paint, and give a good key for the new paint or powder-coating. A ‘key’ effectively increases the surface area for the coating to adhere to by making it rough, the diametric opposite to polishing. With engine casings, you want the surface area to be as smooth as possible without losing the original texture of the casting, so that in the future the surface is the polar opposite to a key deterring the adhesion of detritus and allied shite. If you are blasting big lumpy steel bits for powder coat, you would dry blast with a coarse media such as aluminium oxide.

I have an ‘affordable’ dry blast cabinet in my workshop in which I use glass beads. This gives a nice key for wet paint, but is not suitable as a ‘finish’ due to the ‘open’ surface left.

In the early half of the last century, they sand-blasted using a media high in silica particles which made for a very unhealthy working environment because as the media hit the work piece it broke down, releasing silicon dust into the air which caused silicosis once inhaled by the workers. As a result, the use of silica was banned from blasting in 1950.

During the 1950s Norman Ashworth developed the first wet-blasting system, initially to make the process safer, but it was soon discovered that the wet-blasting system resulted in a far improved surface finish over that of traditional sand-blasting and became more popular. Norman, together with sons Stewart and Colin, formed Abrasive Developments Ltd., and was the world’s first manufacturer of wet-blasting equipment based in their premises in Henley-in-Arden, south of Birmingham. The family continued to register patents and develop Norman’s invention as the years passed. In the restoration world vapour-blasting has become a standard, being used to ‘cut’ for the removal of paint and corrosion and ‘peen’ for aluminium alloy castings.

Many claims are made about the perfect finish of vapour-blasting for engine components. The ‘peen’ action is created by whole round beads of glass hitting the casting, leaving a very smooth finish, the idea being that the smooth surface of each bead acts as a miniature ball peen hammer gently smoothing the surface.

Unfortunately, the reality is that during the blasting process the glass beads often shatter upon hitting their target, changing them from a ‘peen’ media into a ‘cut’ media. The media is recirculated in the system so the media slurry becomes full of more and more ‘cut’ media and less solid balls.

Another downside is that tiny shards of the glass get embedded in the alloy surface and whilst a powerful jet wash will help remove some of the embedded glass slithers, the high-pressure water is never 100% successful. When the apparently clean engine is reassembled with lots of heat and various gears, cranks, etc., churning up the oil creating all manner of harmonics at various frequencies and effectively causing forms of cavitation, the previously embedded chips of glass lose their previously limpet-like grasp on the inside of the engine casing and go for a swim in the oil circulatory system, effectively forming a not-very-viscous grinding paste able to give accelerated wear to all manner of parts previously machined to close tolerances.

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