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Rebuilding Johnson 70 hp

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  • #16
    Have you determined the cause of the failure , yes or no ?-----If no then the " rebuild " will likely fail again.-------I would simply replace the block / find another motor / install a replacement sleeve.-----Every one works to different standards I suppose.----Does the water pump telltale come off the side of the block or the top ?

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    • #17
      The bores need attention and are you sure that piston is a Siezco as rings look wrong???
      When in doubt,Trim it out!!!
      Business Owner & Certifed Technician
      Boats I own & use
      20ft Javlin w/250 Junkrude
      17ft Bullet w/200 Merc (SOLD!!)
      13ft Allison w/150 I-6
      17ft Hydra-Sport 175GT
      15ft Allison w/75 Stinger(project)
      14 x 48 Drifter with 1100SS Merc

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      • #18
        racerone :
        As a matter of fact I got the engine in boxes, so I have no clue. My Theory is that the bottom carburettor got clogged up.
        If it was the water pump or an air pocket, the top cylinder would have been toast. The other two pistons look okay.

        To me it looks like the Telltale comes from the area of the lifting eye.

        Finding a donor engine without a blown powerhead over here is really difficult. Lots of 49ers, but the 56ci version is pretty rare to find. Also the prices are typpically horrendous.

        If I went 0.40 over the grooves should be gone I think. The one on the top of the cylinder might be there still, but its really small.

        Faztbullet : Yeah Im pretty sure its a Wiseco. On top of the pistons there is the the number 3022P3, below that KD and 0.30
        If you type that into Google you will find those Pistons as NOS Wiseco pistons. Seems like an older version of the currently available ones. Maybe like an early generation?

        What is interesting is that only the blown piston has those holes drilled to oil the bridge. They seem to be drilled by the factory. I cant see how they contributed to the failure though....

        I guess all in all going 0.40 over would be the safest thing to do. Any concerns about the chipped of waterjacket?

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        • #19
          Another question that just came to my mind: am I comprimising anything when going 0.40 over?
          Am I still okay regarding the wall thickness of the sleeve? I figured these OS pistons wouldnt be sold
          if it was too much of an oversize, but better save than sorry

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          • #20
            Ok, so here is some news:

            I found a used 93' crank that looks great. I just gave the journals a very light polish with some 1200 grit and WD40.
            No Pitting, no grooves on any journals, so this problem is solved. Its a different Part Nr. however, so I might have to modify something along the road?

            Also a couple days ago my WSM rebuild kit with 0.040 OS pistons arrived. Will have the block bored accordingly in mid January.

            There are three questions that came up:

            1) There are no holes drilled to oil the bridge, so I wonder if I it would be beneficial to them by hand. I read that it is recommended for modified outboards, but if it helps reliability its worth a thought for me. What do you think?

            2) The small end bearings were originally loose needles, the new Bearings in the kit are Metall caged rollers. Is that a problem, or is there a benefit to one or the other?

            3) I noticed that there were some very tiny chips broken of 2 of the con rods on the inside (At the parting line of rod and cap). It seems that they are not where the rollers touch but I'm still concerned a little. There is so much money going into this motor that I don't want to take risks. Do you think I will have to replace them as well?



            Merry Christmas everybody and thank for your help!
            Attached Files

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            • #21
              Rods are fractured when they are made.-----Caps will only fit on one rod and only one way.----match the dots and tighten rod bolts a bit.----They can / should be lined up perfectly.-----The rod in the second picture appears to be scrap to me.----Different part # on the crank is due to different / various seal arrangements on the splines for the driveshaft.-----Study those parts arrangements.

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              • #22
                That one rod is iffy and take a piece of emery cloth to the bridge in block. Sand just the bridge so it about .0005 bigger than bore measurement. Do water updates to block and prop it to the upper RPM range and it will be good to go. Rejet mains .002 bigger.
                When in doubt,Trim it out!!!
                Business Owner & Certifed Technician
                Boats I own & use
                20ft Javlin w/250 Junkrude
                17ft Bullet w/200 Merc (SOLD!!)
                13ft Allison w/150 I-6
                17ft Hydra-Sport 175GT
                15ft Allison w/75 Stinger(project)
                14 x 48 Drifter with 1100SS Merc

                Comment


                • #23
                  Hi guys,

                  After waiting for 3 weeks on the machinist I'm back on it.
                  The guy from the machine shop told me that he would like to know the piston to wall clearance for the pistons, to decide whether to use a cutting bit or taking of the material with a hone.
                  Unfortunately the pistons didn't come with any specs (WSM cast pistons). On my internet search I found various numbers ranging from 0.002-0.007 inch.

                  I'd also like to hear your opinion on WSM aftermarket conrods vs used OEM ones. I spent enough money that I'm willing to go the extra mile to make everything as good as it can be, so I'm wondering if a fresh set of rods is a good idea?
                  I can get used ones for about half the price, but having trust in my motor is also important.

                  Thank you for your support!

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                  • #24
                    Where did you find 0.007" clearance for fitting new pistons ?----And O/S pistons require boring !

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                    • #25
                      I think it was on PWC. Yes they need boring for sure. The machinist told me that the step from 0.030 to 0.040 over isn't that big. Because the top piston seized it did open up the bore a little bit already, so he said that, depending on the piston clearance, he would rather like to use a certain type of hone to carefully remove the rest of the material.

                      I hope that I translated this good enough for it to make sense. Anyways, it's not a regular automotive shop and he's done a couple of 2 stroke OB's, so I trust his judgement.

                      Does anyone have a spec sheet for the pistons or knows the tolerance?

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                      • #26
                        An example for you to think about and talk to the machinist about.------A motor with a standard factory bore is say 3.500 " and is worn out damaged.------A 0.020" o/s piston is fitted in a bore that is opened up to 3.520 "-----It is that simple.

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                        • #27
                          I understand what you mean and I thought about it the same way before he told me he needs the clearance. I guess he wants to measure the pistons individually to make sure he is doing the right amount. Maybe a good idea, should the piston have a different size than what it should have...

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                          • #28
                            ??----Factory today make very good , accurate parts.---------Not the same as 75 years ago.

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                            • #29
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                              I am pretty new to the marine world, but it seems to me that you would benefit much by having a set of both inside and outside micrometers. With them you can quickly answer some of the questions in this thread. IMHO, they are priceless for any mechanic. Do you have Harbor freight in Germany? If not, here is their online selection:

                              https://www.harborfreight.com/catalo...eatured+Weight %2Cf%2CSale+Rank%2Cf&q=calipers+%26+dial+indic ators+%26+micrometers

                              ​​​​​​​...Mark I hope that this is ok, I am not a lawyer and I read the terms and conditions and my untrained brain seems to think that this is not prohibited. I also checked and could not find any micrometers on the Iboats site...

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