Mercury 40 Two-Stroke Outboard Powerhead Rebuild: Part 2
Welcome to Part 2 of our Mercury Tracker 40HP two-stroke engine rebuild project, in which we’ll be assembling the pistons and connector rods, then installing them onto a brand new engine block.
Our Mercury 40 outboard was severely damaged, so we ordered a brand new engine block for this rebuild. Watch the video above or read on below to continue with our Mercury 40 two-stroke outboard engine rebuild.
NOTE: We’re breaking this rebuild project up into three posts for length purposes, since the engine rebuild project video above is more than 40 minutes long. This article is part 2 of 3, which covers assembling and installing the pistons.
Tools and Parts - Mercury 40 Tracker 2-stroke Outboard Engine Rebuild
- Snap ring pliers
- Emery cloth
- 600 grit lapping compound
- Contact-brake cleaner
- Assembly grease
- Connector rod
- Connector rod bolt
- Piston assembly
With the crankshaft ready to go (see Part 1), we’re now working on the piston and connecting rod assemblies. These are a little bit different compared to anything that you may have worked on before, as we have to build up the bearings inside the small end of the connecting rod.
The Mercury manual suggests a special tool with a bushing that can go in there, and you can build the bearings around it, then bring it up inside the piston and put the wrist pin through. However, the tool is a bit pricey for something you’ll probably only use once, so you can save some money by doing this without the tool.
We’re building it up using assembly grease to carefully hold the little needle bearings in place, then we’re driving the wrist pin through the piston.
NOTE: We’re using a thick grease from Pro Circuit to do this. It’s an assembly grease designed to break down with gasoline, because that’s what this is going to get immersed in eventually. DON’T USE the Mercury 2-4-C grease like you would use to grease a bearing in a lower unit. It’s not going to break down like it’s supposed to and it’s going to cause damage. Make sure whatever grease you use is soluble with gasoline.
Mercury 40 Tracker Two-Stroke Piston Assembly & Installation
Step 1. Clean the connector rod with contact-brake cleaner.
NOTE: Even though the connector rod is brand new and just came out of a box, Mercury sends out the part with a light coating of oil so it doesn’t rust. It’s recommended you get all of that oil cleaned out before you start putting it together.
Step 2. Add a little layer of assembly grease to the small end of the connecting rod before installing the needle bearings.
Step 3. Install the new needle bearings one at a time, then use the wrist pin to get them squared up.
NOTE: The needle bearings don’t come as a set; they’re sold individually, unless you order the connecting rod assembly, which includes the correct amount.
Step 4. Install the lock rings (circlips) into the pistons, starting each one from the edge and working it around the slot until it pops into place.
NOTE: Mercury also makes a special tool to remove and install the lock rings (see Step 5), but we’re installing these on one side without it. These lock rings are not very forgiving if you push them over too far. Once they crimp, they’re done. So you may want to order a few extra just in case you damage one. You can use the wrist pin to help guide them in if you’re not comfortable installing them without it.
Step 5. Line up the connecting rod with the piston to get everything centered, then install the wrist pin and the circlip on the other side. Repeat this step for the other piston.
NOTE: As mentioned, Mercury makes a lock ring installer tool to install the circlips. We used it to install the circlip on the other side for illustration purposes.
Step 6. Clean up the engine block, then oil the pistons, their rings and the cylinders for each piston.
NOTE: You don’t have to glob a lot of oil into the cylinders, just add a thin layer so that they’re not completely dry on start-up.
Step 7. Install each piston into the cylinders on the engine block, then remove the connecting rod end caps from each piston.
NOTE: You’ll notice a tapered edge at the bottom of the cylinders which helps compress the piston rings as you’re installing them. If you have to push too hard to get the piston to seat properly, you’re doing it wrong.
NOTE: Mark the orientation of each connecting rod end cap before removing it. If you’re reusing a connecting rod, you’ll need to order new connecting rod bolts.
With the pistons installed into the engine block, the next step is to install the crankshaft and the crankcase bottom. Continue to Part 3 of our Mercury 40HP two-stroke engine rebuild.
