REFLECTOR: Overhead NACAs

[David Rene Dugas via Reflector]

Top NACA.  Works great.
Rene’

No seal in my gap.  The aluminum angle reinforces the edge and the stringer on the lateral edge aids in replacing the top cowl.
Removable covers over spark plugs.











On Sep 1, 2014, at 10:13 PM, Sid Knox via Reflector <reflector at tvbf.org> wrote:

>> For those with the overhead NACA vents for engine cooling, does the duct connect directly to the plenum or is there a flexible joint for vibration? Thanks,
>> John
> 
> John, I have not seen an answer to your query so I will partially answer ...partial because a photo would make this response more meaningful, and I am not currently able to make a photo but if necessary I can in a few weeks.
> Yes, you need a gap (mine is about 1/4 inch) for two reasons, one for, as you suggested, allowance for the engine to move in its mounts, and two, so you can remove the plenum if required for major engine work.
> 
> On each of the ducts, mine has a "garter" fabricated from about a three inch wide strip of tire inner-tube wrapped around the joint with about a two inch overlap (sealed with contact cement) of the end.  The plenum side of the garter is pop-rivited to the plenum.  The duct side uses a (large) nylon tie-wrap for a seal.  This top NACA mod was done (by the previous builder/owner) about 16 years ago and the rubber looks to be as fresh as new.
> 
> You must have access to the top of both banks of the engine for spark plug removal and you *do not* want to have to remove the plenum every time you examine the plugs!  To properly make the plunum a tight fit around the engine requires fitting and sealing the edges (where the plenum interfacs with the engine) with silicone (or equivalent) and this, if done well, requires much time and effort, so once the plenum is installed, you will not be anxious to remove/re-install it every time you check plugs.  On mine, over each cylinder bank are removable panels to allow spark plug access and access to the injectors.  Also, there is a removable panel over the fuel divider ("spider") to allow access to it.  This word description is where a photo is valuable.
> 
> $0.02
> 
> Sid Knox
> Oklahoma
> 
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