Wrench Tips #31: Cast Aluminum in a Can

Wrench Tips #31:

Cast Aluminum in a Can

by Dave Coleman

Strangely enough, this wrench tip is one I learned from owning this bike:

I bought the bike (a '76 CB750) in pretty much the condition you see it. It had been lovingly restored by a bored guy in a barn and aside from a new chain, I never had to touch anything on it. 

As far as I knew, all the engine was just clean, bare cast aluminum, as it was from the factory. Take a look here and you can see why I would think that. 

Then one bitterly cold morning, riding along a desolate stretch of California Highway 58 on the second leg of a 700-mile butt-busting ride to San Luis Obispo and back, I decided it might be neat if I could feel my fingers again. With my right hand on the throttle, I would lean down and grab the fins with my left hand, holding the engine until the heat worked through the glove and started to create some feeling. Then I'd crank the throttle, get up to about 80 or so (CB750s were really fast in their day, but their day was a loooooong time ago) and warm my throttle hand while I coasted down to around 40. After a couple rounds of this, I felt the painful tingle of blood returning to my fingers, and noticed a strange silver sheen on my grips. Sure enough, my gloves were striped with silver paint from the engine's fins. Huh… paint…

It turns out that not only is aluminum engine paint a good way to make a not-so-clean engine look clean, but years of copying this technique has taught me that it also seals what is normally a very porous surface and makes it much easier to keep clean.

Generally, any aluminum engine paint will do the trick of making a cast aluminum part look clean and stay clean, but years of haphazard application of this trick had earned me mixed results. The two halves of my S14 intake manifold, above show how the painted surface (on the left) can look unrealistically shiny sometimes compared to the real deal. Prepping Project Silvia's engine block recently, I figured out why.

Here's the block fresh from the machine shop, its porous cast aluminum looking as clean as it ever will. 

And here's the same block after a fresh coat of Dupli-Color aluminum engine paint. The color match was so perfect, it was actually hard to tell where I had painted it and where I hadn't. The Dupli-color is also really thin, which means when you accidentally lay it on too thick in one spot (easy to do when painting something as complex as an engine block), it simply flows into the cast texture and disappears.

Sadly, my can ran dry after painting one side of the block, so I switched to the same old can of PlastiKote I had used on the intake manifold. Now one side of my block looks like chromed plastic rather than cast aluminum. The difference is dramatic. 

Besides the unnatural shine, the PlastiKote goes on thicker, so heavy spots don't flow into the cast texture and disappear like they did with Dupli-Color. This horrible thick chrome stripe will haunt my dreams.

So that's the wrench tip for today: Dupli-Color Aluminum Engine Paint is the stuff you need.

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