FAB/Cowl Clearance issue

bullojm1 · #1 11-12-2008, 11:44 AM

I just fitted my FAB to my fuel controller (IO-360, Precision Vertical Induction). How much space is recommended between the side of the FAB and the side of the cowl? I have about 1/2" now and am concerned if that isn't enough to compensate for the engine vibrations. Am I worrying about nothing?

JonJay · #2 11-12-2008, 01:25 PM

Depends on where the clearance is. I have about 1/2" between my mixture control and it does hit on occasion and has caused a small hole in the cowling after about 150 hours. My airbox has 3/4" to 1" clearance and does not hit.

#3 11-12-2008, 05:20 PM

Quote:

Originally Posted by bullojm1

I just fitted my FAB to my fuel controller (IO-360, Precision Vertical Induction). How much space is recommended between the side of the FAB and the side of the cowl? I have about 1/2" now and am concerned if that isn't enough to compensate for the engine vibrations. Am I worrying about nothing?

Search the archives -- there has been a lot written about this, complete with pictures.

The short answer is that you should remake the FAB mounting plate, allowing the FAB to slide as far away from the cowl and towards the center as possible.

I have some pics here: http://home.comcast.net/~n70ge. Others have posted pics in the archived threads mentioned above.

A 1/2" might work, but it's easier to do it right the first time than rework it later. That engine shakes A LOT at startup and shutdown, and the FAB hanging off the bottom of it is about as far away from the center of rotation as you can get.

dhall_polo · #4 11-12-2008, 08:51 PM

Also, rv7, 360, precision vertical fi servo. Doing FAB work now. I'd say I have mine over as much as one can go and still have good edge distance on the bolt holes. I didn't measure yet, but I'm probably around 1/2" as well.

Are you posting good pics anywhere?

And the mixture arm.. I have the reverse (upward) mixture arm that seems more common now, which doesn't require the FAB mod. In full rich position, however, it comes within 1/4-3/8" of the cowl. Some folks have done welding tricks on the mixture arm. I ain't trusting my welding to something that can shut off an engine. I contacted Precision. They'll have other arm options available, I just don't know which one to ask for yet...

From Peter Neilson at Precision Airmotive
"We have arms offset
about 1" or straight and either 2" or 1.625 or 1.38 center to center."

bullojm1 · #5 11-12-2008, 09:07 PM

Thanks for everyones input. I ended up taking a heatgun to the FAB box and reshaping the offending side inboard about 1/2"+. The heatgun really worked nicely to reshape the FAB. Now I need to reshape the top part and its flange -- that's not so fun. I got the rough shape down tonight and will finish it up tomorrow. So I will be sitting happy at 1"+ total clearance.

As far as the question about the mixture arm and the cowl interference goes -- I have the Sam James short cowl and needed to put a "bubble" in the outside to avoid any interference issues.

JonJay · #6 11-13-2008, 04:12 PM

Quote:

Originally Posted by dhall_polo

Also, rv7, 360, precision vertical fi servo. Doing FAB work now. I'd say I have mine over as much as one can go and still have good edge distance on the bolt holes. I didn't measure yet, but I'm probably around 1/2" as well.

Are you posting good pics anywhere?

And the mixture arm.. I have the reverse (upward) mixture arm that seems more common now, which doesn't require the FAB mod. In full rich position, however, it comes within 1/4-3/8" of the cowl. Some folks have done welding tricks on the mixture arm. I ain't trusting my welding to something that can shut off an engine. I contacted Precision. They'll have other arm options available, I just don't know which one to ask for yet...

From Peter Neilson at Precision Airmotive
"We have arms offset
about 1" or straight and either 2" or 1.625 or 1.38 center to center."

I didn't trust my welding either so I had an expert TIG welder do it for me. Precision wanted $100 for their short offset arm. $10 and mine was done, probably better than new. The very end of the mixture arm bolt, the one coming out of the servo, became a point of contact. I will be adding a small bubble for clearance this winter. On my set up, that bolt became the closest point to the cowl and you can not reposition the servo. It took 150 hours to even see where it was occasionaly hitting, I assume on start up or shut down. No harm, no foul, but needs to be fixed.