Polaris 2-Stroke rebuild

Wiseco sells a replacement piston and ring kit for the Polaris Scrambler and Sport two-stroke ATVs. It is lighter-weight, better built, and less expensive than the stock replacement piston.

Polaris 2-Stroke Rebuild

One of the advantages that a two-stroke has over a four-stroke is that it generally produces more horsepower per cc. Because of their increased wear and tear, two-stroke quad en­gines require more reg­ular maintenance and replacement of the top-end components than their four-stroke counterparts.

The good news is that because you are not dealing with valves, springs, cam chains or pushrods, rebuilding your two-stroke is fairly simple.

To illustrate how quick and easy a top-end rebuild can be, we took our Po­laris 400 Sport and consulted en­gine tech specialist Jay Clark at Wiseco Piston for expert advice on how to change out the top-end on this popular sport machine.

We have never seen Wiseco Piston rep, Jay Clark, happier than when he can showcase one of his pistons vs. a stocker—especially when the stocker looks like ours did (the piston on the right).

Polaris 2-Stroke Rebuild

First of all, you need to make sure your machine is perfectly clean be­fore starting. Do not work on a dirty ma­chine! Remove the front plastic cowling on the Polaris Sport and be sure to put the lid back on the oil tank so you do not accidentally drop any tools into the tank.

Remove the mudflap bolts at the floorboard and all necessary fasteners, along with the fenders. Loosen the fuel tank bolts where the bracket is held to the frame. Turn off fuel and detach line. Remove the fuel tank.

Drain engine coolant into a clean bucket (if you want to reuse it).

Make sure you use a reputable shop to hone and bore your cylinder should you need to go up to the next larger size pis­ton. This will save you from having to return the wrong size piston.

Remove the exhaust system and carb.

Next, remove the head nuts in a crossing pattern so as to not warp the head. Remove the four nuts from the base of the cylinder and slide the cyl­inder up.

Place a clean paper towel in the cas­es to prevent any debris from fall­ing into the engine cases.

Remove the piston circlips and push out the wrist pin (if the pin is tight, use a 10mm T-handle to push it out as you hold the piston for support to not damage the rod).

You will want to make sure your ring end gap is correct before installing a new set of rings. Measure it and use a file to shave the gap if needed.

Now you will need to have your cyl­inder inspected by a reputable bor­ing shop to see which size re­place­ment piston you need (many peo­ple order a piston before checking this and have to return it). In most preventive maintenance cases, 0.25mm or .010” over will respond with a simple hone of the cylinder. A hone is where a metallic brush is used to remove the glazing-over that oc­curs on a cylinder under normal wear.

If you are dealing with the replacement of a piston that has failed, either by seizure or lower-end failure, you will need to go with the next-larger-size piston and have your cylinder bored. Cylinder boring should be done by a shop that has plenty of ex­pe­rience in the field. It is a good idea to bore it to .003” and hone the re­mainder on a stationary Sunnen-type hone (not a ball hone on a drill!). This will ensure that there is a good finish for the new rings to seat to.

STEP #10
Check the ring end gap. It should be between .012”–.014”. Use the piston to push the ring down, then check the gap. This is done with feeler gaug­es in between the ring tips when the ring is in the cylinder. If the ring is under this dimension, use a file to remove material off the tips until it is within the dimension.

 About the only trick you need to learn to install a new top-end on your two-stroke quad is getting the cylinder back over the piston and rings. Sometimes it is easier to do if you don’t have too many helping hands.

STEP #11
Install the rings on the piston after gapping is complete. Put one circlip on when the piston is on the bench (make sure the circlip is completely open and filling the groove).

STEP #12
Clean the old gasket off of the cyl­in­der surface (use a flat razor to re­move the old gasket and be careful not to gouge the surface). Install new gas­kets and then install a piston rod with a new top-end bearing. Small wire cutters can be very useful in in­stall­ing the remaining circlip.

STEP #13
Make sure to clean the cylinder in soap and water after the boring and hon­ing are complete. Use compressed air to dry cylinder off and WD-40 or something similar on cylinder sleeve to prevent rust and aid in installation.

STEP #14
When putting the cylinder on, in many cases people mess up because they have too many hands trying to help! In most cases, putting a single cylinder on by yourself is easier than with someone trying to help you. Rest the cylinder on top of the engine as you position yourself over the motor. Using both hands on the piston to collapse the rings, you can feel the cyl­in­der simply fall down. Practice and patience will be the keys to mastering this operation. On the Polaris, make sure you don’t pinch the oil feed lines when installing the cylinder (it runs from the front left of the motor to the reed cage area).

STEP #15
Tighten the four base nuts to the torque recommended in the owner’s manual. Put the new head gasket on and check to make sure there are no nicks or dings on the head. Put the head on and install washers and nuts. Tighten in a rotating crossings pattern to 20 pounds using a torque wrench.

STEP #16
Reinstall carb and exhaust system and hook up radiator hoses. Loosen the 10mm bolt on head. When cool­ant appears at this vent, all air is purged from the cylinder and head. Re­fill coolant with a new mix of 50/50 dis­tilled water and coolant if your old cool­ant has become contaminated. Fin­ish by tightening this bolt and re­in­stalling plastic.

STEP #17
For proper break-in of your new top-end, Jay at Wiseco recommends that you run it for five minutes with small blips of the throttle while on the stand. Then shut it off and let it cool down while you check to make sure everything is tight and no cool­ant is leaking out. It is also a good idea to retorque the head at this time. Next, you can take the machine off the stand and ride it around with small blips of the throttle for a good 30–60 minutes of break-in. This will give the rings time to seat into the bore.

STEP #18
The final step in our Polaris 2-Stroke Rebuild. Plan on doing the work yourself. That is if you have a fair amount of mechanical savvy.  If you do this, you have saved the downtime you would have lost by dropping it off at the deal­er’s.

Wiseco sells a replacement Pro-Lite piston kit for the Polaris Sport 400 (part #657). It includes rings, wrist pin and circlips, and sells for $89.00, which is $44 less than the stock Polaris piston kit. That al­lows you to save money and get a piston that is 20g lighter than stock!

Such a deal. For more information on Wiseco replacement pistons, see your local dealer.  http://www.wiseco.com

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