Project [Rotary] FD RX-7: Part 2 – Fuel System (Engine Side)

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Now that the fuel rail is installed with all injectors accounted for something has to be done about the two holes in the center iron where the primary injectors used to reside. As always there are several solutions for this problem. The first solution is to install freeze plugs with some sealant, but that seemed too easy. The second option was to purchase aftermarket o-ringed injector plugs, but where's the fun in that? That leaves option three, design and machine custom injector plugs specific to the 13B primary injector ports and intake tracts. Obviously we are going to choose the method that requires the most engineering and custom machining.
 


After spending some time with a spare center iron, a set of digital calipers, and some bore gauges it was time to fire up the CAD software and create a representative model of the custom injector plugs and retention plate.

The idea behind developing custom primary injector plugs was originally considered for two reasons, to match the intake tract on the 13B center iron and because the plugs currently on the market that are readily available in the USA don't look particularly well thought out or robust.

Other than getting the general dimensions of the plugs correct, a large aspect of the design was proper o-ring specification and placement. A dual o-ring setup was eventually chosen for its sealing ability and because two o-rings will laterally support the plug for its full length. After the design was finalized, a local machine shop that doesn't mind small, one-off jobs was contacted to machine the plugs.
 

Freshly machined primary injector plugs specific to the 13B-REW. Notice the o-ring grooves, the angle on the bottom face, and keying/clocking flat on the top face.
 

Here is what the injector plugs look like installed when looking down the intake tracts of the center iron. Notice how the angle of the base of the injector plugs matches the angle of the intake tract. Also, ignore the ugly gasket material as this was just a test fitment conducted with a spare center iron that we had lying around the shop.
 

Here are the primary injector plugs installed with the fuel rail and intake manifold. Notice how the 1/4″ aluminun retention plate clocks the injector plugs, orienting them correctly in the intake tract.

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