Keeping your fuel cap O-rings in great shape is good fuel cap leak prevention. Just look at what happened to this really nice B58 Baron from water contamination of the fuel past a poorly maintained cap and o-ring:

The right engine sputtered to a stop just after takeoff. Here is the fuel cap center shaft from the above splashed Baron, you decide!

See the whole article on this B58 engine failure HERE

Here is a conventional nitrile rubber Fuel Cap O-Ring (the larger outer one) after a while in service:

See more on fuel caps at CSOBeech HERE

See a Shaw 531 Fuel Cap O-Ring Change Article written by BeechTalker, David M. HERE

Thanks for the contribution David

Listings of my favorite B55 & Other O-Rings (Check your parts catalog to be sure they will work for your SN)

Military Nitrile Rubber O-Rings

• MS28775-222 (4) = AN6227B-27 = Cleveland 30-66 Brake Caliper Pistons

• MS28775-011 (4) = M83461/1-011 = AN62227B-6 = Cleveland 30-66 Brake Line & Bleeder

• MS28775-012 (4) = M83461/1-012 = AN6227B-7 = Cleveland 30-66 Brake Line & Bleeder

• MS29513-010 (4) = Shaw Fuel Cap Center Shaft

• MS29513-338 (4) = Shaw #431-9 Fuel Cap Large 3.5" Outer

• MS29513-232 (4) = Shaw Fuel Cap Large 3.0" Outer

• MS29513-339 = Gabb Cap Large Outer

• MS29513-110 = Gabb Cap Small Seal for Center Shaft

• MS29513-010 (4) = B55 Fuel Selector Shaft w/Separate Aux Tanks

• MS29513-136 (2) = B55 Fuel Selector

O-Ring Reference Guide PDF HERE

Description: Standard Nitrile is also known as Buna-N. Excellent resistance to petroleum-based oils and fuels, water and alcohols. Nitrile also has good resistance to acids and bases, except those with a strong oxidizing effect.

Limitations: Avoid highly polar solvents (Acetone, MEK, etc.) and direct exposure to ozone and sunlight.

Chemistry: Copolymer of butadiene and acrylonitrile. By varying the acrylonitrile content, elastomers with improved oil/fuel swell or with improved low-temperature performance can be achieved. Specialty versions of carboxylated high-acrylonitrile butadiene copolymers (XNBR) provide improved abrasion resistance. And hydrogenated versions of these copolymers (HNBR) provide improve chemical and ozone resistance elastomers.

Trade Names: CHEMIGUM ® , HYCAR ® , PARACRIL ® , PERBUNAN ®

ASTM D1418 Designation: NBR, XNBR, HNBR

ASTM D2000/SAE J200 Type, Class: BF, BG, BK, CH

Temperature Range: –55° to 120°C (–65° to 248°F)

Typical Uses: Ion Implant, PVD

• M25988/3-010 (4) = Shaw Fuel Cap Center Shaft (Fluoro Silicone Blue - Long Life)

/3 = 60 Durometer Available from Genuine Aircraft Hardware (List Price $4.50/each WOW)

• M25988/1-010 (4) = Shaw Fuel Cap Center Shaft (Fluoro Silicone Blue - Long Life)

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $0.50/each)

• M25988/3-338 (4) = Shaw Fuel Cap Large 3.5" Outer (Fluoro Silicone Blue - Long Life)

/3 = 60 Durometer Available from Genuine Aircraft Hardware (List Price $28.00/each WOW)

• M25988/1-338 (4) = Shaw Fuel Cap Large 3.5" Outer (Fluoro Silicone Blue - Long Life)

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $6.96/each)

• M25988/1-232 (4) = Shaw Fuel Cap Large 3.0" Outer

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $4.25/each)

• M25988/1-222 (4) = AN6227B-27 = Cleveland 30-66 Brake Caliper Pistons

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $2.75/each)

• M25988/1-339 = Gabb Cap Large Outer

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $8.25/each)

• M25988/1-110 = Gabb Cap Small Seal for Center Shaft

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $1.00/each)

• M25988/1-011 (4) = M83461/1-011 = AN62227B-6 = Cleveland 30-66 Brake Line & Bleeder

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $0.50/each)

• M25988/1-012 (4) = M83461/1-012 = AN6227B-7 = Cleveland 30-66 Brake Line & Bleeder

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $0.50/each)

• M25988/1-010 (4) = B55 Fuel Selector Shaft w/Separate Aux Tanks

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $0.50/each)

• M25988/1-136 (2) = B55 Fuel Selectors

/1 = 70 Durometer Available from O-Rings, Inc. HERE (List Price $2.50/each)

*Fluorosilicone o-rings, whether 60 or 70 Durometer, have less tear resistance than the military Nitrile rings being crossed. They may or may not be appropriate for your particular application. Notice the tensile strength and elongation spec differences in these two materials. Be sure your mechanic approves return to service.

O-Ring Reference Guide PDF HERE

NEWS FLASH 6/21/2009:

The GROUP BUY on these super long lasting Fluro Silicone O-Rings from Genuine Aircraft Hardware and O-Rings, Inc. resulted in participation to convert 108 fuel caps! Great participation folks!

Stay tuned for more Group Buys from CSOBeech.com

Baron Owner & IA, Stuart S., pirep on Fluoro Silicone: "Mine are like new and they were installed in 1996."

Description: Combines excellent low-temperature performance of silicone with improved
chemical resistance. Very good resistance to military and aerospace fuels. Excellent performance
in oxygen plasma environments.

Limitations: Avoid polar solvents, hydro-carbon fluids and phosphate ester brake fluids.
Susceptible to hydrolysis by acids and bases. Limited abrasion resistance.

Chemistry: Methyl disiloxane with CF3 groups (Methyl-trifluoropropyl siloxane)
(CH 3 )(CH 2 CH 2 CF 3 ) –SiO.

Trade Names: SILASTIC LS ® , FSE ®

Temperature Range: –60° to 180°C (–76° to 356°F)

Typical Uses: Dry Etch, Resist Stripping

Fluoro Silicone PNs Available at Genuine Aircraft Hardware & O-Rings, Inc.

No O-rings for this "Old School" Fuel Cap for very early Bonanza airframes. Rumor has it these puppies are quite pricey!

• Flap Decal 20 Degree 35-000066 Placement Instructions HERE