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Replacing a piston in 3 and 4 cylinder engines.

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  • Replacing a piston in 3 and 4 cylinder engines.

    This one is for Spinkick:

    It is really rather easy to replace a piston in these engines. Even though you have a 1998 Force and I have not yet done one, the block is the same as earlier Force engines and replacement will be the same.

    The .020 stamped on the top of your piston indicates it is .020 oversized. If the cylinder has not been scored, then buy a replacement the same size. If it is scored, then you have no choice but to disassemble the engine and have it rebored to the next larger size.

    I prefer Wiseco because they are forged pistons. The stock and Sierra replacements (in older Forces) are cast. Forging produces a superior piston, stronger and able to resist damage better. Many people will state that Wiseco pistons are "junk" and will refuse to use them. Personally, I have had good luck with them. However, the reason I recommend them is because they are way easier to install on the rod with no special tools. Cost? around 150 complete--piston, rings, wrist pin, and bearing kit.

    Usually when a piston melts, the rod and bearings are not affected. However, with the Wiseco pistons, because the wrist pin is included, you must buy a special bearing kit for the small end. It only costs about 15 bucks though.

    Now, any melted aluminum throw off that passes into the exhaust will just pass out the snout and not harm anything. Just check to see that some pieces have not melted to the port webs so they could possibly work their way back into the cylinder. No need to remove the exhaust chamber cover.

    If the engine shows any evidence of aluminum throw off in the bypass ports, then you MUST remove the bypass cover and clean the whole area of any metal there.

    As I said before, the head gasket is re-usable as long as it is not damaged and it is actually preferable to re-use it if possible: A new head gasket will compress the first time the engine is run long enough to heat and expand the aluminum. After the first run, the head bolts MUST be re-torqued. Since the old head gasket is already compressed, generally you will not need to re-torque the head bolts after test running.

    To replace the piston, you remove the head, then remove the carb and manifold servicing that cylinder. Remove the rod big end capscrews and push the piston out the back with the rod attached to it. Do not lose any of the 16 roller bearings. Press out the wrist pin to remove the rod. Take note of the orientation of the rod--there are special grooves in it-- and reassemble in the same direction. The end cap only goes on one way also so be certain you put it on correctly after you install the piston and big end bearings. Also be certain the ground flats on the rod are perfectly matched. If correct, you should be able to run a pencil point or scratch awl, or small screwdriver over the crack without feeling it catch or jump. Do not tighten the big end capscrews until alignment is perfect. Take your time with this step: Good enough is not good enough. It must be perfect.

    Since stock gaskets are glued they will be destroyed (in most instances) when you disassemble parts. SO: you will most likely need a carb gasket, manifold gasket, and a couple of bypass gaskets. If you are cheap like me, you will buy a 4 buck roll of 1/32 grey gasket paper from Pep Boy's and make your own. Child's hole punch makes the correct sized bolt holes and scissors does the rest with a little bit of time invested. You will need to remove the reed Vee blocks to replace the gaskets. Be certain to use threadlocker on the screws. It is also a good idea to check the reed screws for tightness at this time. If you find any loose ones, remove the screws one at a time and use threadlock compound on them. Beats having the engine eat a screw or reed.

    To prevent melting another piston, find out why that carb went lean. Clean both carbs, set the float levels and set the idle needles. Put a cheap 3 buck plastic fuel filter between the fuel pump and the carbs. Sometimes when a diaphragm starts to go bad it will shed disintegrated pieces into the fuel. They will clog the carb, it will go lean, and the piston(s) associated with that carb will melt. The filter I use is Purolator F50153 available at Pep Boy's and probably most other auto stores. It costs around 7 bucks but you can back-flush it to clean it, you can see its condition, and it has a fine screen instead of a paper element. It also has a 90 degree fitting on one end so it generally fits nicely below the bottom carb on the 4 cylinder engines.

    Since you have never replaced a piston before, figure on a whole weekend, but you will probably be able to do it in one day. Takes me a couple of hours taking my time. I could probably do it in an hour if pressed.

    Tools needed: 3/8, 7/16, and 1/2 inch sockets. Torque wrench capable of 225 inch pounds, 1/4 inch drive 12 point 1/4 inch socket (Sears Lowes etc.), couple of open end hand wrenches, and a couple of screwdrivers.

    Head bolts are torqued to 225 inch pounds and rod big end 12 point capscrews are torqued to 190 inch pounds UNLESS Mercury has changed the specs--Get a manual.

  • #2
    Re: Replacing a piston in 3 and 4 cylinder engines.

    outstanding. Between your guide and the manual, I just might survive it.

    Last question before venturing off... the source of the cylinder running lean. What should I look for?


    • #3
      Re: Replacing a piston in 3 and 4 cylinder engines.

      Re-read! I was editing when you posted.


      • #4
        Re: Replacing a piston in 3 and 4 cylinder engines.

        Got it. I'm sure i'll have a question later. Already have the cheap filter you described. Where's a good place for the carb kits?


        • #5
          Re: Replacing a piston in 3 and 4 cylinder engines.

          I also agree with you Frank, on forged pistons being stronger than a cast piston.And I think your post is an excellent how to ,and should be listed as a sticky at the top of this forum.You give alot of excellent advice,and I learn quite alot from you.


          • #6
            Re: Replacing a piston in 3 and 4 cylinder engines.

            I haven't done this yet as I'm waiting on the piston kit and gasket set. It appears I will have to remove the powerhead though. The intake manifold is full-length of the block. And, there appears to be a bold under the flyweel that attaches the bearing cap to the intake manifold.

            Think there's a chance that bolt could be removed/replaced without removing the flywheel?


            • #7
              Re: Replacing a piston in 3 and 4 cylinder engines.

              What year is this engine again? Up until 1990 at least, there were no special bolts and it was not necessary to remove the engine block or flywheel to replace a piston if the block did not need overboring.

              The bottom two bolts that hold the neutral interlock switch mounting plate are the lowest bolts. There are four bolts between the carbs. If you look closely, you will see that there are four bolts around each carb, holding the manifold to the block. DO NOT remove the angled bolts on each side of the carbs. These six bolts hold the reed vee plates and only need to be removed if the manifold gaskets were destroyed in removing it.

              Depending upon age of the engine and ignition type, the bearing cap and stator mount will be held with four or five bolts. These do nothing but hold the bearing cap.

              The stock gaskets on the manifold are self gluing and the manifold frequently needs to be pryed off. Once you break the gasket free, you must then pull the manifold straight out. If you cock it and try to pull the top out first, for example, the reed plates will jam in the openings.


              • #8
                Re: Replacing a piston in 3 and 4 cylinder engines.

                It is a 1998 model. I have some close up pictures of both port/starboard sides of the intake. It is one piece. True, the carbs have 4 bolts each, but I don't think I could get to the rod bolts. The Clymer manual I have doesn't really show a good picture of the 1998 120hp block. It has an "early 4 cylinder" diagram, but it is close. However, in that exploded diagram i don't see the reed valve location. It does show a similar one-piece intake though.

                I would SURE like to avoid pulling the powerhead if possible, so you have my undivided attention.


                • #9
                  Re: Replacing a piston in 3 and 4 cylinder engines.

                  Well, you see, they will NEVER tell you in the manual to replace a piston without removing the block. That's one of my little tricks, because enquiring minds want to know. I look at things and wonder about how they are assembled. Then I figure out different ways to take them apart. Sometimes I end up with extra pieces--No! only kidding! I know, I'm a geek.

                  ANYWAY: I really don't remember what I had for breakfast so I gather you have a four cylinder engine.

                  While the four cylinder engines are a bit more difficult than the three cylinder engines, it is still do-able. The reed openings in these blocks are vertical and at first blush you would think "No way will I be able to get to the rod bolts". Think outside the box! You need to get one bolt at a time, then move the crank a bit. Use a six inch 1/4 drive extension bar with a 1/4 drive 1/4 inch 12 point socket. Use an adapter to 3/8 drive so you will have enough leverage to break free the rod cap screws. Once the bolts are off, you need to finesse the cap a bit to pull it out unless you decide to push it along with the piston. However to re-install it, you really need a long needle-nose pliers and you need to coax it a bit. But it is do-able. The first time I did it on a four cylinder Chrysler--the one in the avatar, I said a couple of "prayers" that burned my wife's ears. By the time I got to the fourth piston, I was an old hand at it. ---For some reason I don't remember, I had the timing set a 42 degrees advanced--melted the crowns of all four pistons slightly. Engine was still runable but I just didn't like those ragged looking pistons but I didn't want to disassemble a brand new rebuild so I replaced them through the reed openings. Wiseco pistons too. Damage was confined to the edges of the baffles and I think if the pistons had not been forged, there might have been more damage

                  So just work slowly and carefully, be patient, and analyze what you are doing before you do it. There are 16 rollers on the big end. account for all of them If they go down the exhaust, no harm, but you may never find them. If one or two go into the bypass, they will blow into the engine while running and really do some damage. So, as I said, account for all of them. One fellow on the forum replaced a piston and used some type of bearings from a "Rude. They are captive rollers and easier to install. If you research past threads on this subject you may find it.


                  • #10
                    Re: Replacing a piston in 3 and 4 cylinder engines.

                    I may have missed a clue.... Are you saying replace the piston by removing a carburetor and accessing the cap bolts from that opening?

                    Here's the Clymer drawing i referenced earlier... bolt #3 was my concern about the flywheel.


                    • #11
                      Re: Replacing a piston in 3 and 4 cylinder engines.

                      I got a little bit of an education and I found the clue you mentioned. I see how to get to the rod cap from the intake. I'll be a bit nervous about dropping the bearings, but will try it.

                      I pulled off the top carb and accessed the reed valves for #1 and #2 cylinders (yes, it is 4-cylinder). Much to my disappointment - yet satisfaction - the reed valves were very "new". They all laid flat like they are supposed to. I also tapped them open and had no trouble. So the disappointing part... how could this have affected only one cylinder? The top carb had a little more oil on it that I would have expected to see.

                      I understand about the jets being potential culprits, but i'm still at a loss understanding (but thankful) it was only one piston affected. If it had an air leak, say in the head gasket, would there be anything to look for?

                      Oh! I did notice when removing the carburetor, there was a bolt not completely seated at the bottom of the reed assembly (for lack of educated description). That bolt is offset to the right...toward the bottom of the #2 reed set. Do you think a leak may have formed there? None of those bolts were particularly tight. In this picture it is the lower left bolt.

                      Thanks again.

                      And I did order the Wiseco piston kit.


                      • #12
                        Re: Replacing a piston in 3 and 4 cylinder engines.

                        I did find a couple of posts regarding the rollers. One guy used TransGel (or something like that)... If i cannot find that stuff can I use packing grease or what is the other recommended alternative?

                        And how do you remove the rod and endcap without dropping these little jewels? Pressure on the cage?


                        • #13
                          Re: Replacing a piston in 3 and 4 cylinder engines.

                          You will drop at least a couple of them. Do not be tempted to retrieve them with a magnet. They are fully hardened and will take magnetism. Then they will tend to trap steel particles.

                          Do not worry about dropping them since most will fall onto the crank cheeks. One or two MAY go under the crank cheek but they can not get lost or stuck there. A bent piece of wire will get them out. Just be careful not to let them go into the ports while retrieving them.

                          Regular grease will be ok to install the rollers but apparently, transgel will hold them better.

                          While you have the manifold off, remove the reed Vee blocks. Remove the reed screws and replace them using Loctite compound. Then replace the Vee blocks and use Locktite on the flat head screws holding them. Since you found loose ones in the top manifold, remove the bottom manifold and do the same. Ounce of prevention!

                          Reeds do not cause a melted piston. Lean running causes melted pistons. Junk can pass through the fuel pump and clog the inlet of one carb. That carb will then run lean while the other two pistons carry the engine. That is why I harp on always installing a fuel filter BETWEEN the fuel pump and carbs.

                          Why only one piston? Because one of the pair serviced by one carb usually runs a little leaner; when both are starved for fuel, the leaner one detonates and melts.

                          So, what you need to do is to clean both carbs, and set the float level to factory specs. Buy a Purolator F50153 fuel filter--7 bucks. Also, note that because both carbs are fed from the bottom, if the fuel pump is not up to snuff, it is unusual, but the top carb can run a bit lean. Therefore, replace the diaphragm and valves in your fuel pump.


                          • #14
                            Re: Replacing a piston in 3 and 4 cylinder engines.

                            Ok. I've never rebuilt a carb, so this is going to be fun. I'm waiting on parts as we speak so the adventure will start soon.

                            My carb kit choices range from $14 (gasket only) to $66 (complete kit less float). Which do you think I should do?

                            Thanks again for the advice.

                            Next day... piston kit and gasket set just arrived. Show begins soon. Do I need gasket sealant of any type on an outboard?


                            • #15
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                              Re: Replacing a piston in 3 and 4 cylinder engines.

                              Piston has been replaced! Generally speaking, it was quite easy. I see what you meant, Frank, about the vertical reed spaces. Definitely required a little finesse, but I survived it. Only two things made this a pain in the lower lumbar region...and it all had to do with the flippin' wrist pin. The pin itself was a breeze, but those retainer rings and bearing pins were testing the limit of my vulgarity.

                              I finally got the bearing pins in after about 20 tries. Unfortunately, I could not use the ones that were included with the piston. They were too long. The old ones were in good shape so I reused them.

                              Now here's a picture (3 angles) of the piston I removed. I was a bit perplexed... the "topside" of the piston walls were bone dry. However, behind the rings had plenty of oil. most of the scoring and throwoff were on the sides of the piston. Is that because of the direction of airflow in the cylinder?

                              Another bummer... the gasket set I received today is not the one I ordered. Shop was closed but wonder if you can tell me, I ordered the Force 120 gasket set (#2875-1 for 1995 and up), but received the one for the older Force 120's (#2875 through 1994). I'll have to wait until they are open Monday. Do you know how much difference there is in the gaskets? Would there just be extra/fewer gaskets or are the castings close enough to the same?