Will Standard 3.5K Brake Assemblies Fit A Backing Plate Bolt Spacing Of 2-3/4" And 2-7/8"?

Updated 08/15/2025 | Published 07/25/2025

Question:

Want to put electric drum brakes on my 2003 Karavan boat trailer, not hydraulic brakes. It has either a 3k or 3.5k axle with tapered spindle using 1-1/16 and 1-3/8 bearings. Tires are 175/80r13 d load rated. It has a brake flange mounting plate. I measured the hole distance using a calipers and they are exactly 2-3/4 not 2-7/8. I see some of the freebacking hydraulic brakes state 2-3/4 mounting but all the electric brake assemblies show a 2-7/8. Please let me know if their is an electric brake assembly that will work. Thank you!

asked by: Jason J

Expert Reply:

Hello Jason, thanks for reaching out. All of the 3.5K electric brake assemblies we offer use the standard bolt spacing which is 2-7/8" apart on center. The dimensions of the brake mounting flange from bolt hole to bolt hole is an industry standard, however some companies such as "Demco" refer to this measurement as a 2-3/4 instead of the more common 2-7/8".
The actual dimension is really more like 2-13/16", but different brake manufacturers will round to either 2-3/4" or 2-7/8". Either way, it is an industry standard size on 3,500 lb. axles.

expert reply by: Jesse M

Jesse M.

8/15/2025

Hey Jason, did you get a chance to try fitting the electric brake assembly on your Karavan boat trailer? I'm curious to know if the industry standard size worked for your setup, as it could help others in a similar situation.