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Lower unit of an outboard engine

3 Ways to Loosen a Stuck Lower Unit Drain/Fill Screw

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I have trouble loosening my lower unit drain screw whenever I procrastinate winterizing my boat. So…almost every year, sadly.

I usually get the screw to release with elbow grease and sustained pressure. But not this time. That sucker was stuck, and no amount of pressure by hand would free it up. Even worse, the slot in the head was beginning to strip.

So I began to look for alternative methods. I found three things to try – but only one did the job.

The Stuck Lower Unit Drain Screw

Lower unit drain screws have an oversized shoulder and screwdriver slot. The shoulder allows for the use of a big fat sealing washer. I suppose the oversized slot is supposed to prevent overtightening of the screw.

But when the screw is stuck, that big slot works against you, not giving much surface area to apply leverage to the screw. And if you try and fail a few times, the slot starts to wear out, making things worse.

So before I stripped out the head and was forced to bring it to the marine mechanic, I looked into other ways to get the screw out myself.

Method 1 – Rubber Mallet and Screwdriver

The first thing to try was gently tapping the flathead screwdriver on the end with a rubber mallet and imparting enough force to free the threads on the screw.

This didn’t seem like it would work on my dead stuck drain screw, but it was worth a shot. You don’t want to overdo it in a place like the lower unit, but I gave it a few tries.

The result? Nothing. On to the next idea.

Method 2 – Impact Screwdriver

I saw some people on forums having success with a Harbor Freight impact screwdriver. So I ran to the store and picked up one for under $20.

The instructions were terrible, and it was unclear how to set the handle for loosening a screw. There was no way to know if I would be helping or hurting my efforts to break it free.

I took my best guess and rapped on the driver with my rubber mallet. I hammered progressively harder with each blow, but still, the screw would not budge.

Worse, I was worried I was impacting the screw tighter yet.

Method 3 – Screwdriver Bit and Wrench

Frustrated with the impact screwdriver failure – I took a break to regain my composure. I was down to one final option and wasn’t optimistic it would work. I figured the slot was too worn out to get enough torque to loosen.

I grabbed the thickest, fattest flathead bit I could find. There was a good one that came with the impact screwdriver and then a 3/16-inch box wrench.

Placing the closed end of the wrench onto the end of the bit, I pushed in on the screw and applied pressure on the wrench handle.

And just like that, the screw gave way! Sweet relief.

With the screw out, I finished the job, draining and refilling the gear oil, stabilizing the fuel, fogging the cylinders, and swapping the spark plugs.

Replacing the Stripped Lower Unit Screws

The final step was to resupply. I don’t want to mess around next fall trying to winterize the boat again with frozen hands.

I bought some replacement screws to use immediately and a few extras if this should happen again. Get the magnetic version for the bottom screw, regular screws for the top, and a pack of replacement seals.

Quicksilver Lower Unit Screw (Magnetic)
Magnetic screw and seal washer for the bottom lower unit drain port.
Quicksilver Lower Unit Screw (Non-Magnetic)
Regular screw and seal washer for lower unit vent hole.
Quicksilver Drain Screw Seals
It’s good to have spare sealing washers on hand. Replace the seals every year even if the screw is fine.

My oil pump tubing cracked at the end, so I got a replacement pump for next year. I also bought a fresh can of engine fogging oil for the cylinders and another set of spark plugs.

Shoreline Marine Pump Lower Unit Fill
  • Use With 32 ounce (946 ml) Gear Lube Bottle
  • Yamaha Tohatsu Adapter
  • Fits Any Outboard or Stern Drive with A 3/8 inch (10 mm) – 16 Drain Screw

With a new lube pump and new screws, I’m all set for the next few years of winterizing.

Last updated 2024-02-22 at 16:02 / Affiliate Links & Images from Amazon Product Advertising API

Tom Harkman

Tom has been fishing for over 17 years, occasionally competitively in tournaments, and has been a longtime B.A.S.S. and FLW member. His home lake is currently Lake Eufaula, AL.

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