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View Poll Results: Is the deep firewall recess really necessary?
I need the standard recess. 23 46.94%
I could get by with half the depth. 11 22.45%
I don't need the recess at all. 15 30.61%
Voters: 49. You may not vote on this poll

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Old 12-20-2022, 10:53 AM
DanH's Avatar
DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 11,873
Default RV-6-7-9 Firewall Recess

Question for 6-7-9 finished/flying owners: Do you really need the deep firewall recess? Could you get by with less depth, or no recess at all, and still accommodate your prop governor or other accessory?

Not suggesting you change it on your flying airplane, merely if you found it necessary.
Dan Horton
Barrett IO-390

Last edited by DanH : 12-20-2022 at 11:00 AM.
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Old 12-20-2022, 11:25 AM
skypilot737 skypilot737 is offline
Join Date: Sep 2020
Location: Vacaville, CA
Posts: 29
Default recess

I have a fixed pitch prop (actually ground adjustable ) so no recess required. It did make a handy place to attach the fuselage rotisserie though during the build.
Harold Martin
Built RV-7A slider N70HM
flying since 2021
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Old 12-20-2022, 11:36 AM
Desert Rat Desert Rat is offline
Join Date: Aug 2018
Location: Wichita KS
Posts: 1,530

I was thinking about this the other day as I was hanging the engine; Since I'm going with a 390 with fwd governor it really isn't necessary, but it sure gives more knuckle room back there to accommodate a straight spin on oil filter.

I don't see much value in getting rid of it, whether the engine configuration requires it or not.

Curious what you have in mind that's causing you to ask the question?
Terry Shortt
AGI, CFI, CFII, MEI, A&P, Janitor
RV7 Empennage & Wing done
Fuselage almost done
Avionics almost done
90% done, 90% to go
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Old 12-20-2022, 11:41 AM
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wirejock wirejock is offline
Join Date: Oct 2005
Location: Estes Park, CO
Posts: 6,866
Default Recess

Mine is an "A" model with the elasomeric gear
Throttle cable goes through the recess. IO-360-M1B. CS prop with rear mounted governer. There is 3/4" between the oil filter and firewall.
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Larry Larson
Estes Park, CO
E-mail: wirejock at yahoo dot com
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Donated 12/01/2023, plus a little extra.
RV-7A #73391, N511RV Registered
I cannot be, nor will I be, held responsible if you try to do the same things I do and it does not work and/or causes you loss, injury, or even death in the process.
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Old 12-20-2022, 02:05 PM
Ralph Inkster Ralph Inkster is offline
Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 2,182

Could manage with a shallower recess, but than I would have to
1- fabricate a replacement cover plate myself
2- modify the firewall inner lower heater diffuser box
Besides, the recess is a good place to set sockets & wrenches when man handling things on the back of the engine.
Now retired - my builder assist shop now closed,

Last edited by Ralph Inkster : 12-20-2022 at 02:08 PM.
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Old 12-20-2022, 02:57 PM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 10,468

It is necessary to accommodate a governor for a constant speed prop if mounted on the rear of the engine. It could be a bit shallower for the governor but that would just make it more challenging doing the control cable hook up, etc. it is not absolutely necessary for clearance on the oil filter but there is just barely enough room to get a standard filter installed or removed with the amount of clearance because of the length of the threaded nipple.
Years ago some people would cut the nut portion off of the back of the filter when they had no recess, so that they didn’t have to struggle with getting it installed and particularly removed because of the mess that can cause. There was probably some variation in the amount of clearance depending on whether you had a Dyna focal or conical mount. Though I don’t know any specifics regarding a difference between the two. If using some of the newer filter options that are much smaller than the standard aircraft engine filter it would not be a factor.
Opinions, information, and comments, are my own unless stated otherwise.
You are personally responsible for determining the suitability of any tips, ideas, etc. obtained from any post I have made in this forum.

Scott McDaniels
Hubbard, Oregon
Formerly of Van's Aircraft Engineering Prototype Shop
FAA/DAR & Pre-purchase inspection Services
A&P, EAA Technical Councelor
RV-6A (aka "Junkyard Special ")
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Old 12-20-2022, 03:44 PM
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DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 11,873

So far it sounds like it is not required, but it is convenient.

Humor me....consider only the minimum required, and assume there will actually be more.
Dan Horton
Barrett IO-390
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Old 12-20-2022, 04:17 PM
agent4573 agent4573 is offline
Join Date: Aug 2019
Location: Mountain view
Posts: 817

PCU5000 governor doesn't require the recess to fit. The straight oil filter adapter requires just a small relief in order to have room to get the threads started. If I had a 90 degree filter adapter I wouldn't need a recess at all.
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Old 12-20-2022, 05:06 PM
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azrv6 azrv6 is offline
Join Date: Mar 2006
Posts: 409

Picture of the recess and governor on my RV6 (mid 1990's vintage build). The attach point of the control cable to the governor extends slightly back into the recess. In this picture the oil filter is off. The 48110 is easy to install/remove with the recess. Without the recess, the oil filter nut would be in the way.

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Dave Binkley
1996 RV-6, O-360-A1A, C/S (builder)
1932 Monocoupe 110, Warner 145 (
Sedona, AZ
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Old 12-20-2022, 06:31 PM
Stevea Stevea is offline
Join Date: Jun 2008
Location: San Jose, CA
Posts: 348

Like Scott says, a shallower recess would work, but it makes installing the governor cable attach hardware just a "bit" more challenging. Would also make installing/removing the oil filter tougher too, since the oil filter on my setup has a nipple on it gets "longer" when unscrewing from the filter adapter.

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Steve Allison
RV-6A - sloooowwww build
First flight - 3/8/14
Phase II - 5/19/14
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