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Thread: Tohatsu M40D Build 40 50

  1. #1
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    Default Tohatsu M40D Build 40 50

    My first rebuild a M40d2. Already done the following:

    -Bored .30 over
    -wiescos in hand
    -Ported and polished
    -crank rebuilt and welded
    -flywheel lightened
    -head cut

    I think I'm ready to reassemble. What is the best adhesive to seal the crank case to the block? Any advice will be appreciated since this is my first build.
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    Jerry Wienandt
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    Here is a document I wrote up for the 4 cyl. Mercs, but this info applies to your question as well. Hope it helps. Copied in below:

    Crankcase Sealer… if some is good, more is better, then way too much must be just right.

    DON”T DO THAT!

    So, you need to seal those machined castings, where there is no paper gasket, on your freshly built or restored pride and joy… what to use?

    This info applies to any motor with a two piece block, like the Mercury’s which have half the crankcase cavity cast into the block and the other half of the crankcase cast into a matching cover. Think Mark 55, Merc 500 and the jillion similar models, 2, 3, 4 and 6 cylinders.

    I say ‘matching’ because the cover is matched to the block early in the manufacturing process, and a 2 or 3 digit ID number is stamped on both parts, then that assembly is finished machined as one. The really critical operation, here, is the crankshaft linebore and then facing off of the top and bottom end cap surfaces.

    I don’t EVER recommend mismatching covers and blocks. It can be done, but not unless you know exactly what to check, have access to a mill and have a whole pile of covers to choose from… don’t go there.

    Sealer: Some is good, more is very bad., but its absolutely critical that this joint be well sealed, otherwise you get a nasty drool of leaking fuel and oil, and in extreme cases, a cylinder can even run lean from a crankcase compression leak.

    Back in the day, way back, Mercury used white lead as a sealer. Then we went to various goos, some of which worked fairly well, some not so well. I recall needing to tear down perfectly good running race motors for no other reason than to reseal the case cover, as the sealer would slowly dissolve over several seasons. Must have been really good fuel! Never use RTV Silicone sealer here, as gas breaks that stuff down, BTW.

    I recommend LocTite 518 sealer. I put a row of small dots of sealer up and down the mating surface. Then I tap, tap, tap with a fingertip to evenly spread the dots into a uniform, very faint coating. If you can tell its red, you have too much… just a faint pattern of pinkish fingerprints is the goal. If any squeezes out as you torque the cover, you used too much.

    The danger of too much is a thicker coating will squeeze out at the edges but will leave enough in the gap that the cover is ‘floating’ on a layer of sealer several thousandths thick. (I’ve seen motors, built by others, where this coating was over .010”) This makes the line bore oversize in one direction, no longer round, so now the crankshaft with center main bearing, reed cages, and bearing caps has clearance to rattle around in the line bore. You see that as shiny burnished mating surfaces in badly built motors. What we really want is a perfectly mated metal-to-metal joint with just enough sealer to fill the microscopic imperfection in the mating surfaces to prevent leakage. NO MORE!

    The good news is the LocTite sealers, 518, 515, 514, 510, will stay sealed until you mechanically separate the parts. This stuff is an anaerobic resin that cures in the presence of metal and the absence of air. It is fuel proof indefinitely. The different product numbers reflect the hardness and elasticity of the cured product. For our purposes, I’d recommend 518, as the ‘one size fits all’ answer.

    Cured LocTite sealer is easily removed with Zip Strip Original formula paint remover, or any similar methylene chloride based paint remover product. Lots of ventilation, then wash. You know
    the drill.
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    Administrator Ron Hill's Avatar
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    Default Engine Building for Yamaha, Tohatsu, and Mercury??

    Trident, do you do engine work for people? I have some Tiller Handle Racer looks to build some motors for them.

    Thanks for your posts they are always very informative. You are a great asset to BRF!

  4. #4
    Jerry Wienandt
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    Thank you for the kind words.

    Long since retired as a driver, more recently retired from building motors. It was fun while it lasted, but age and infirmity is sneaking up on me!

    I still try to post stuff where I think it can help, as these same questions come up over and over as we get new guys working on their stuff.

    Jerry
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    Thanks Jerry that's some good info. I see u are in WI. Pete Nydahl, from WI, rebuilt the crank on this motor and did the block work.

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    If you want to meet one of the great engine builders of all time, come to the BSOA 75th on March 18th. Trident (aka: Jerry Wienandt) hopefully will be there.
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    I finally got back on this project. I dropped in the crankshaft and bolted it up last night. Afterwards, I realized I had not checked the Wiesco piston rings to see if they were of different types for the top and. Ottom groove. I did not notice any difference and can not tell any difference when viewing the rings through the exhaust ports.

    Does Wiesco use different rings for top and bottom?

    Thanks,
    Justin

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    Default Crank ready to be bolted in

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    Painted last night

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    Team Member Roflhat's Avatar
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    Looks really sharp in blue!
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