Is the shear pin doing its job if it doesn't shear and hydro lock the engine and twist the crank ? Are we tightening the flywheel with too much torque ? If you use a lighter flywheel should less torque be used on the flywheel ? Or is the shear pin to strong to make a difference on the torque used to tighten the fly wheel ?

There is not much talk about the shear pin but I think this little inexpensive part is ment to prevent an upsetting expensive repair and your input and experience can help answer these questions .

Thanks
Enrique

Enrique, the flywheel "key" is NOT a shear pin. It is simply a means of aligning the flywheel magnets to the source coil, so the eng. fires at the right time. The flywheel is held on by means of the taper on the crank. The flywheel should be tightened very well using a steel washer and nut. After it won't tighten any more, the nut can be replaced with your normal starter pulley/pawl. The only protection from water injestion and subsequent damage, is a good cowling, a very fast "reverse" finger ( to close the carb ), and less rotational mass. If the eng. swallows even a little water at high RPM, damage will occur.

Definetely not meant to shear. If it was it would be made of brass so the keyway would not be destroyed as it sheared.

It wouldn't matter if the flywheel was gone you still can never take in one iota of water at any rpm.

You want the flywheel tight as it takes to feel the crush stop. When tightening it that tight you must hold the flywheel itself from turning.
DO NOT use a piston stop and especially don't hold it by the collet.

Hydro locks even at low rpm that don't even twist the crank can still ruin the entire rotating assembly. The crank and every bearing in the engine can be destroyed in just one single lockup.

And just because it might still run fine afterwards expect to experience catastrophic loss of the entire engine when a bearing fails shortly after the hydraulic. You might get another weekend you might even get a few but usually it will lead to a failure.

Zenoah cranks are very susceptible to hydolock damage. The journals of the rod are very soft comparatively to say a RCMK crank. One even light lockup indents the bearing needles into the journals and the bearing starts to skid or just create so much heat from the indented surfaces that it takes the temper out and it burns up.

Very easy to have a total loss from one small part of any bearing going through the engine.

I asked a afew question about a 1.00 part and get responses that can cost up 400.00 in repairs if you run high dollar engines. This would make some think twice about buying a pricy mod engine. If the cranks are most likely twisting on the flywheel end from its weight and momentum , WOULD it help at all if the fly wheel was allowed to free when the engine locks up, would at least cause less damage or with stand a few dunks that cost the pin replacement only ? It's probably wishful thinking on my part to dunk and not have to call it a day and have to tear down and check for problems.

I would guess that the pin would have to shear with less force than it would take to flatten a needle bearing? I like the brass idea. Maybe composite or some type of plastic. Think about it, if it dunks and breaks, it won't start and prevent further damage. If pin does not break , thank your trigger finger for shutting it down on time. If the pin takes the brunt of the flywheel force and breaks upon a hydro lock then the intenals only have to deal with their own weight compression force.

Thanks for replies , great feedback as usual,

It seems you have a couple of misconceptions. The flywheel, once tightened down on the taper of the crank, is locked onto the crank. At that point, there is no further need for the key. If you could be sure to maintain the proper alignment of flywheel and crank during the tightening of the flywheel, you could eliminate the key. The crank and key are steel, and the flywheel is much softer aluminum. Even a brass key is stronger than the keyway in the aluminum. The only way for a key of ANY material to shear, is if the "lock" of the tapers were to separate ( from coming lose ), The soft flywheel "keyway" will be damaged.

Water injestion will hurt a stock motor as easily as a fully modded one. Water ( even a small amount ) will not compress. The assembly will stop immediately with damage. The only way a very light rotating mass will help, is to shut down the rotation quicker, BEFORE water has a chance to enter the eng.

I get what you're sayin and get how it is working now but it can all ways be changed to work another way . We would still be getting cut in half by the hood of a car in front collision if changes were not made. Compression zones would make the hood fold in half to prevent catastrophic secondary damage.

The secondary damage from a hydro lock would be less if the pin would break Imo.

Just call me Mr misconception but good ideas have been realized from a misconception, just not mine, ....yet LOL.

I would never discourage someone from trying something new, if they felt the need was there. At the same time, we need to keep a "hobby engine" perspective to keep the costs available to the average model boater. Even the best of the marine model gas engines are still based on " utility eng. components", for that reason. A "fail safe" type of wrist pin would still most certainly destroy an aluminum piston, cyld. and/or button, and send pieces throughout the motor. Probably far easier/cheaper to develop a way to keep the water from doing the damage in the first place. I'VE GOT IT!! A water triggered air bag that would deploy into the carb venturi ... LOL, maybe not.

Ventury airbag, Dennis, LOL. Thought I would hear about some type of gyro failsafe that shuts off the engine when boat reaches an extreme angle from you lol. Keeping it cheap is what I'm about. I don't want anything in the engine to give way, strictly just the crank/ flywheel key way to be more specific. I would rather replace a key way that breaks and let's me know to check for secondary damage instead of replacing 75% of the engine. I can't stop my noodle upstairs from coming up with these crazy ideas but I already have starboard key way in a engine that's waiting for a dunk for testing. Starboard should shear without scratching the surface of the cranks taper. I just hope it's not to strong. I just hand tighten the pawl with blue lock tight making sure the flywheel is seated properly on taper with some luberiplate .

It won't make a difference if you used a shear pin, or a weak key that shears for the flywheel.The counter weights on the crank shaft carry enough inertia to damage the crank itself, bend the rod,break the piston etc.

Actually it can be more than that. In a bad hydraulic it can take out the engine 100% and it be non repairable where as it would be far cheaper to buy the whole thing new.

Prices of real racing engines today are getting closer to the $600- $1100 mark or more depending on what your running.


LIke Dennis, Bob and myself have mentioned the flywheel is not the only problem and you better have it bolted on tight enough that it doesn't need the key to stay in place even in a hydraulic. A properly tightened flywheel should never shear a key even when it is hydrauliced. If it wasn't tightened enough that it can't shear than you risk it coming off and doing more damage than you want to hear about. Like the source coil getting gouged into the flywheel destroying everything on that end of the engine.

You could run it with no flywheel at all and it still be damaged beyond repair with one single dunk.

The smartest thing is to not hydraulic the engine. The engine doesn't need any fix it needs YOIU to keep the water out of it.

I suggest you focus your efforts on preventing the water from entering in the first place because reciprocating piston engines are never going to be able to take in water and not do damage.

Kinda joking here but it is the only way I could think of simple $4 sparkplug could save a hydraulic from doing damage.
There is one idea I did think of a long time ago was a sparkplug that could blow the entire ceramic out of it at a pre-set blow out pressure. he plug is the only thing that is big enough to be able to open up and expel the water instead of trying to compress water that doesn't compress very well.

Maybe you could rig a airbox and ducting to keep the water from reaching the engine as easily. The best way is to just slam the butterfly shut at the first hint of the boat getting stupid.