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Old 01-15-2012, 11:17 AM
Pat Hatch's Avatar
Pat Hatch Pat Hatch is offline
Join Date: Jun 2006
Location: Vero Beach, FL
Posts: 898
Default FAB Brace to Prevent Cracks

Just finished my condition inspection and this was the main finding:

There were 3 hairline cracks in the FAB mounting plate on my carburated -6 which I have highlighted with a Sharpie to see them better. This is after 312 hours in 9? years of flying. I have seen many other RVs with these cracks so this is probably about average. This is probably acceptable wear and tear given the vibration of the significant mass of the FAB, but how to improve?

I did a couple of things that others have done. One, I made a new plate out of .090 instead of the .063 of the original part. Second, I added this brace which I made out of a short piece of 1/4" stainless tube and attached with #6 screws as shown using one of the existing FAB AN3 bolts:

I have no interference with the cowl although it's close and this has been a problem in some cases. I think my cracks are strictly vibration induced. Another solution involves adding about 1/4" spacer between the plate and the carb, which avoids making the cutout in the mounting plate. However, I note that many of the cracks I have observed emanate from the bolt holes, even with plates without the cutouts. In addition, I really didn't think I had enough clearance with the bottom cowl to add the 1/4". So this wasn't really an option for me. I did grind some of the excess casting off of the bottom of the carb that interferes with the mounting plate and this eliminated the majority of the cutout.

Hopefully this will improve on this part's performance over time. We'll see.
Pat Hatch
RV-4 (Gone to RV heaven)
RV-6 (N44PH - Flying)
RV-7 (Sold)
RV-8 (Builder & Avionics Installer) Trim Relay Boards/Flap Actuators (Photography)
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Old 01-15-2012, 09:17 PM
Frank Smidler Frank Smidler is offline
Join Date: Feb 2005
Location: Stoughton, WI
Posts: 492
Default Eliminate the rear notches

Hi Pat,

This problem has been discussed in the past. My Engineering opinion is that the notches that extend out to the side aft of the rear attachment is a major reason for the cracks forming. A new plate should be made without the notches and a .062 aluminum spacer should be used to move it away from the carb. Please see the following posts with the details. I now have over 350 hrs on this set up with not cracks.

Pictures at
Frank Smidler
N96FS, RV-6
Flying 1/11/09
1085 hr
2WI6 Stoughton, WI
Formally of Lafayette, IN
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Old 01-16-2012, 07:41 AM
Pat Hatch's Avatar
Pat Hatch Pat Hatch is offline
Join Date: Jun 2006
Location: Vero Beach, FL
Posts: 898

Hi Frank,

Yes, I think you nailed it. I, too, filed off the two areas as you did which I think really helps. I did not use the spacer because I was concerned about bottom cowl clearance plus I didn't want to upset the alignment of the FAB snout with my cowl inlet. But I think you should use the spacer if you are still building and can align everything accordingly. So I had to notch one side as before, but not nearly as much.

The little 1/4" strut was just a little added insurance. I note that a lot of guys with the Airflow Performance and Precision Airmotive vertical induction have had the cracks and they have no cutouts that I am aware of. Notice, also, that one of my cracks emanates from near the forward left bolt hole and doesn't appear to be related to the cutouts. So, I just thought I would give this solution a try and see if I could dampen some of the vibration which must be significant given the mass of the FAB and filter.
Pat Hatch
RV-4 (Gone to RV heaven)
RV-6 (N44PH - Flying)
RV-7 (Sold)
RV-8 (Builder & Avionics Installer) Trim Relay Boards/Flap Actuators (Photography)
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Old 01-16-2012, 08:32 AM
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Rick_A Rick_A is offline
Join Date: Oct 2005
Location: Jacksonville, FL
Posts: 1,572
Default Doubler

The attached pic shows what I did right from the start. Putting the doubler on top of the stock FAB top plate addressed the issue of the casting at the rear of the carb and added strength to the top plate (which I knew had a history of cracking).

I've don't remember exactly but the doubler is either .063 or .050. Riveted and prosealed in place. Getting the rivet layout right, so as not to interfere with the carb or the airbox mount took a little work.

I've got 350 hrs on this now and no signs of any cracks.

Rick Aronow,
Flying 7A Slider;
Jacksonville, FL
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Old 01-16-2012, 09:21 AM
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dougweil dougweil is offline
Join Date: Jun 2005
Location: Hudson, WI
Posts: 260

Here's my experience in preventing FAB cracks. First remake the mounting plate out of .063 6061 T6 aluminum. It is not nearly as prone to cracking. Secondly, get some 1/8" thick cork gasket at the auto supply store and make a gasket to be mounted between the mounting plate and the carb/fuel servo. This idea comes from our local RV guru Tom Berge. This gasket apparently absorbs a lot of vibration and stopped all cracking on Tom's RV-7A.
Doug Weiler

Hudson, WI
president, Twin Cities RV Builders Group
RV-4, sold
RV-7 completed and flying N722DW, 930 hours
Based Lake Elmo, MN (21D)
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Old 01-16-2012, 10:04 AM
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Ironflight Ironflight is offline
VAF Moderator / Line Boy
Join Date: Jan 2005
Location: Dayton, NV
Posts: 12,829

Nice job Pat - we've got struts just like that on all three of our RV's, and the airbox just doesn't move - really cuts down on vibration problems.
Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
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Old 01-16-2012, 11:31 AM
brad walton brad walton is offline
Join Date: Jul 2007
Location: Cypress, TX
Posts: 533
Default Stainless

I made mine out of stainless steel after reading here that cracking was a problem. I don't recall the thickness, but slightly less than the aluminum original from Van's. In a previous thread, it was theorized that the cracking may be caused by the pulsation as each individual cylinder breaths in air, creating a drumming effect that fatigues the aluminum. Stainless should be less prone to fatigue and less likely to crack.
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Old 01-16-2012, 03:05 PM
sdflier sdflier is offline
Join Date: Aug 2006
Location: Sioux Falls, SD
Posts: 80
Default nuts holding rubber seal

I just made another (3rd one) FAB plate for my -6A. I found that the nuts on the bottom of the fab, holding the rubber inlet baffle, were rubbing on the cowl. My impression is that the front of the FAB had some rubbing pressure and the FAB plate broke as a result. I removed the screws/nuts and replaced these with the blinds rivets used on the upper baffling. we'll see.
Craig Hagen
RV-6A N289CH flying 1200+ hours

2021 dues paid
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Old 01-16-2012, 06:37 PM
John H John H is offline
Join Date: Feb 2007
Location: DeLand, Florida
Posts: 53

Here is my nickle in the pot! In my 25 years of building the biggest issue to cracks is vibration. Bet you figured that out a long time ago My RV9 with 300+ hours has the original FAB plate. However I cheated a bit, I used a Microvibe to balance my original MT prop and again for the Whirlwind. The Microvibe gives you the chance to really get vibrations down to a absolute minimum. A super AP/IA friend told me any time you can get your vibration component to less than .1 IPS (Inches Per Second) you did good. I fussed a bit and got both props to .075 IPS. Any change from that point made things worse.

Bottom line pay attention to prop and engine balance, for you are not balancing just the prop but the engine as well
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Old 01-17-2012, 12:04 AM
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petehowell petehowell is offline
Join Date: Jan 2005
Location: MN
Posts: 2,297
Default O-320 vs O-360?

For the guys that have cracks in the plate, O-320 or O-360?

I've not heard of a 320 with cracks in the baffles or the FAB plate. I was told by someone that the pulses of the 360 are stronger and more likely to cause cracks - but have nothing to back that up.

The 360s go faster and get more chicks, so there might be a trade off.....

Any 320s with baffle or FAB plate cracks? Just curious. Mine looked good at 875 during annual, but I am watching it.

Amateur Plane - RV-9A N789PH - 2450+ Hrs
Amateur Radio - KD0CVN
Doggies Delivered - 25+
St. Paul, MN
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