Water Getting in Top Cylinder Yamaha 40hp Twin

[joesnuffy]

You were right sheared flywheel key. When you mentioned that this morning a light went off in my head.

Thank you so much. You don’t know how relieved I am. I also feel blessed it sheared or I wouldn’t have known my reeds and reed stop was getting ready to be ingested by the engine with only 1 of 3 screws holding it in place. Luckily the 1 screw that was holding it in place was bent so it didn’t back off. If all goes well I can sell the replacement cylinder head I ordered and the 1 good reed valve assy that was on lower cylinder. I will keep the extra head gasket. I ordered extra manifold gaskets and I intend to shim the reed assy out a bit to be safe. The new/used rod cap did not look to be damaged so I must have just missed it when I re-assembled the motor. Possibly the screw heads were cracked and came of when running but the actual rod cap look like new I think it would have showed signs of damage if it had struck the reed assy.

I will post a video when I am finished of it running. You can see a few of my old motors here running.

https://www.youtube.com/channel/UC8AbVA … 3Ag2T1FlWQ

Thanks Again,
Joe

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Attachments:

[fleetwin]

COOL!!
Be sure to torque that flywheel in place with a new OEM key….
Are the flywheel/crank tapers bunged up? If so, you may want to use some fine lapping compound and lap the flywheel while the key is missing. Please be very careful not to get any compound in the upper seal.
Keep all the new gaskets, I’m sure you will want to recheck stuff inside after running the engine a bit, so you will feel more confident about things.
Thank YOU for doing your best to answer all our questions and provide meaningful information so we could help out….

[joesnuffy]

The tapper looks pretty good it got messed up just a hair toward the top near threrads. The key slot is like new on crank and flywheel. I did replace the crankshaft with a good in spec used crankshaft when I rebuilt the engine because the top journal for rod after I polished it was right at the limit for specs so I decided to get a good used crank not wanting to use the old one which would have given me trouble soon. I will lap the flywheel to crank as you said possibly the flywheel tapper might not match the used replacement crankshaft as-well-as it did the original crankshaft?

I will try to get the oem key at Yamaha today they might have to order it they mainly do motorcycles around here but that’s a pretty common part. Should I use loctite on the flywheel nut? I have the red and the blue loctite. One thing I don’t like is instead of a lock washer they have the moon washer or beveled washer concave not sure what you call it. The flywheel nut didn’t take much pressure to break which concerned me.

Thanks Again,
Joe

[fleetwin]

Well, usually the flywheel nuts don’t require any Loctite. But, a few drops on the threads only surely can’t hurt. I’m thinking you are correct in suspecting the new crank tapers weren’t mating up nicely with the flywheel tapers. I would surely try some fine lapping compound to see just how the surfaces are mating. Again, please protect the upper seal area, it is probably best to apply the compound to the flywheel tapers only so excess compound will be forced up the flywheel tapers instead of down towards the upper seal area. Just like doing a valve job, you will know pretty quickly how the two surfaces are mating. Needless to say, the lapping compound must be carefully removed, don’t spray clean the excess down into the seal area. The tapers must be clean and dry for installation, no Loctite, grease or anything else. The flywheel key is really only needed to properly locate the flywheel initially. Once properly torqued, those tapers should lock in place. Please find the correct torque spec and use it. Tightening flywheel nuts with an air gun/impact gun is a mistake most of us are guilty of from time to time. Not enough torque, the tapers don’t lock, and the flywheel key will shear. Too much torque will distort the taper mating surfaces, resulting in a sheared flywheel key and unusable flywheel.
I can’t comment about whatever washer Yamaha might use/require for this set up. So, please investigate and follow the OEM instructions, it may be that this washer must be replaced every time, don’t know. As a rule, OMC rarely ever used any washers between the flywheel and nut.

[joesnuffy]

Great Advice. Thanks for the help. Got most of the parts now to put her back together. I will get a video when done. I picked up some lapping compound yesterday the torque is 116 ft lbs if I remember right but I will look it up again and torque it with my old snap on torque wrench.

Joe

[joesnuffy]

Thanks for all the help. I finished the motor up Wed and we took the pontoon out for the evening and again all day yesterday for over 7 hours run time. The engine ran superfantastic. I used the valve grinding compound to ensure the crankshaft tapper and flywheel tapper matched. It worked like a charm. I used masking tape cutting like 2 inch strips of it working my way around crankshaft and seal to create a barrier for when I used the valve grinding compound to make sure it didn’t get into the top seal on crankshaft. Some did get on tape but that kept it from getting to seal. I had to do the valve grinding compound 2 times to get the burs off the crankshaft and make it fit like it should and rotate without binding. I noticed that the flywheel nut had a red residue on it so I decided to put the red loctite on it when I re-assembled it and torqued it to 116 ft lbs is what the manual said. The manual was just genric on the how-to put flywheel back on. I couldn’t find in manual whether or not to change the washer but I did face it in the direction I was taught at 13 years old when I built my first engine in night vocational school. I put some carb cleaner on a rag and used that to clean up the crankshaft where the valve grinding compound had been being careful not to get any on seal and used it to clean up the flywheel tapper to get rid of any oil or left over compound/oil it worked very well.

I did put 2 gaskets where the reed assy bolts to case to shim it out a little to be safe the new gaskets seemed thicker than the old one that was on there but it could have been smashed a bit but the new ones looked considerably thicker.

Here is a video of it running

https://youtu.be/DcdlFUCQ_Jc

Thanks for the help,
Joe 😀

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Attachments:

flywheel reassembly.pdf

my40hpYamahaOutboardTorqueChart.pdf

[fleetwin]

Great job Joe!
I’m sure the flywheel will be fine now, and the two intake gaskets should keep the reeds safe. Sounds like she is running great now….
All that is left to do is to get out there and enjoy the outcome of all your hard work!
Thanks again for communicating with us so well, makes it much easier to help out. D

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