LiftMaster 178B34 Straight Door Arm Section
The LiftMaster 178B34 straight door arm section is a replacement straight arm used with select LiftMaster and Chamberlain garage door opener rail assemblies. It is commonly paired with the curved door arm to form the full door-arm connection between the trolley and the garage door.
Common Applications
- Replacement for a bent, missing, or damaged straight door arm
- Trolley-style residential garage door opener rail assemblies
- LiftMaster and Chamberlain openers that specify 178B34 or 4178B0034B
- Door arm repairs where the curved arm and hardware are reused or ordered separately
Features & Specifications
| Feature / Specification | Details |
|---|
| Manufacturer | LiftMaster / Chamberlain Group |
| Product Type | Straight door arm section |
| Part Number | 178B34 |
| Full / Current Part Number | 4178B0034B |
| Application | Garage door opener door-arm assembly |
| Compatible Operator Type | Select trolley-style residential garage door openers |
| Related Part | 041B0035B curved door arm, when required |
| Hardware Note | Mounting hardware and curved arm may be separate depending on kit configuration |
Also Known As / Related Part Numbers
- 4178B0034B
- 178B34
- Straight door arm replacement section
- Related curved arm reference: 041B0035B
Technician Note
This is the straight arm section, not a complete opener, rail, trolley, or full door-arm kit. Reuse only hardware that is not worn or distorted, and replace clevis pins or cotter rings if they are loose or damaged.
Before Ordering
- Confirm the existing part is the straight arm, not the curved arm.
- Check whether the installation also needs the curved arm, door bracket, clevis pins, or cotter rings.
- Inspect the garage door top section and reinforcement bracket before reconnecting the opener.
- Make sure the opener rail and trolley are not damaged from the same impact.
- Do not operate the opener if the door is off balance or binding.
Professional Advisory
A bent or broken door arm can be a symptom of a heavy, unbalanced, or stuck garage door. The opener should not be used to force a door that does not move smoothly by hand.
Product Questions & Answers
Is 178B34 the complete garage door opener arm assembly?
No. It is the straight door arm section. The curved arm, clevis pins, cotter rings, and door bracket may need to be reused or ordered separately.
What part number should I look for in the parts diagram?
The straight arm may be listed as 178B34 or 4178B0034B depending on the parts reference.
Does the straight arm connect directly to the trolley?
In most trolley-style setups, the straight arm connects between the trolley and the curved door arm or door bracket assembly, depending on the opener layout.
Should I replace a bent door arm or straighten it?
A bent door arm should normally be replaced. Straightening damaged opener hardware can weaken the part and may cause poor door travel.
What should I check before reconnecting the opener arm?
Check door balance, top-section reinforcement, trolley condition, and all pins and fasteners before reconnecting the opener.
