Sunday, February 8, 2015

DBR1: Scratch-built distributors

Continuing on with the engine in the DBR1, I discovered that the spark plug holes are enormous, and appear to be 0.045" to 0.050", approximately 3/64" -- this would be a huge 1.125" at 1:1 -- and would be a very poor fit for your typical 0.016" spark plug wire from Detail Master.

Furthermore the two distributors (it's a twin plug head) look to be very difficult to stick wires to. So I went for scratch-built distributors using tricks from a fellow modeler on the Model Car List. Apologies for not remembering the name of the poster; I've misplaced the link. If this is your tip, drop me a line and I will give credit by updating this post. [UPDATE: Thanks to Tim Kriner for the method!]

Basically the trick involves three pieces of brass tube sleeved inside one another. Diameters are 5/32", 1/8" and 3/32", all with 0.014" wall thicknesses; the largest works out to an outside diameter for the distributor cap of 3.75" at 1:1 -- about right. The outer sleeve is 3 mm in length; the 1/8" is also 3 mm but is only inserted 1.5 mm into the outer. The 3/32" tube, which must be flush with the 5/32" tube, was cut down to 5 mm which is the total height of the distributor (4.725" at 1:1); it is closer to 9 mm in the photo above.

Do it right and there is a circular annulus of 0.017" radius, between the 5/32" and 3/32" tubes, into which it is easy to insert Detail Master 0.016" wires. The best approach is to put in two adjacent wires as a single long one with a bend in the middle; the bend can get pushed in with a sharp tool and will stay put, hopefully, until you get all the wires in and can dab in some CA to hold it all in place. The scale works well for an 8-cylinder motor; these look a little big for a six, but it will do. Given the raw materials and a decent set of tools, it took me about 90 minutes to make two, including time to screw up a couple of times before getting it right.

I went a step further and inserted some 1/16" rod for the shaft, as drilling 3/32" into the back of the Aston's cylinder head was looking challenging. The photos shows the finished distributors, needing only paint.

Next will be to build up spark plugs. I am considering a range of options including assemblies of styrene rod and tube. 1/32" hexagonal rod, assuming such a thing exists, would be just a bit small to emulate the 13/16" hex of your typical plug ... I'll have to see what is available in styrene.

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