Baby Lock Upper Knife 428910101B

This replacement upper knife, part number 428-9101-01B, fits many Baby Lock sergers in the BL3, BL4, and BL5 series, as well as the Simplicity SL850 and SL880. The upper knife works against the lower knife to trim the fabric edge as each stitch forms. A worn or nicked blade leaves fuzzy, uneven edges and can drag the fabric. Installing a new knife restores a clean, consistent trim.

These older serger models remain popular workhorses, and a fresh blade is one of the easiest ways to keep them running well. Replacing the upper knife protects your fabric and helps the stitch form evenly along the edge. It is an inexpensive part that is worth keeping in your sewing kit. Check the compatible models below before ordering.

Specifications

Installation Tips

• Unplug the serger before starting any blade maintenance.
• Follow the upper-knife replacement procedure in your exact model's instruction manual; access and knife positioning vary by model.
• Set the blade height and overlap to your model's specifications before tightening the mounting screw; seating the blade alone does not establish correct alignment.
• Once alignment is correct, turn the handwheel slowly by hand to check clearance and cutting action before sewing at speed.
• Test on scrap fabric. If you are unsure about blade alignment or cutting remains poor, contact our team or a qualified service technician.

Signs It Is Time to Replace

• Fabric edges look chewed, frayed, or uneven instead of cleanly trimmed.
• Threads catch or the fabric drags as it passes the knives.
• The blade has visible nicks, dull spots, or damage after sewing over a pin.
• Inspect both cutting edges and replace a worn or damaged blade as needed. The fixed lower blade generally lasts longer and does not automatically need replacement when the upper blade is changed.

Model compatibility is listed from published cross-reference information. If your exact model is not listed, please contact our team before ordering so we can help confirm fit.

Why Buy From World Weidner?

Buying this upper knife from World Weidner means getting a correctly identified replacement part from a team that stocks and services serger parts every day. We list the part number and compatible models up front, ship quickly from Oklahoma, and back every order with the perks below. If you are unsure about fit, our support team can help you check your model before you order.

• 60-Day Money-Back Guarantee: We offer a generous 60-day full satisfaction and money-back guarantee for risk-free shopping on your purchase.*
• Low Free Shipping Threshold: Enjoy free shipping within the contiguous United States on all qualifying orders over our low minimum threshold.
• Fast Order Delivery: Orders ship quickly from our central Oklahoma warehouse, usually arriving at your doorstep within 1 to 4 business days.
• Loyalty Rewards Program: Earn 5% back on every purchase online and in-store. These points can be saved up and redeemed for discounts on anything we carry.
• Referral Bonus Perks: Take advantage of our "Give $10, Get $10" program that rewards both you and a friend with coupons for future orders and accessories.
• Interest-Free Financing: We partner with popular payment platforms to offer flexible, interest-free installment payments at checkout so you can start creating today.
• In-House Customer Support: Our dedicated, knowledgeable team, including factory-trained service technicians, is available by phone, live chat, or email to assist with part identification, model compatibility questions, installation guidance, and project troubleshooting.
• Massive Interactive Showroom: Check out our 18,000-square-foot physical Creative Center in Oklahoma City for hands-on product demos, direct support, and community classes.