Thrust bearings

[Daniel's Racing]

A little bit off topic but...
I've never seen the inside of these gas engines being run in these r/c boats (nitro fan). Are ANY of them running four bearings? If so, why???
Don't recall ever seeing four bearings in anything but four cycle engines. Most where bushings.

Yes many have four bearings. The Quickdraw, the Tiger King, RCMK engines just to name a few. For crank stability. When your using cranks with same dimensions as a 20 year old 23cc engine making 1.5 horsepower and you put it in a modified 30.5cc making over 8 horsepower. Most do it to stiffen up the assembly and keep the flywheel off the source coil. We have run 3 and 4 bearings in regular Zenoahs. To work properly the crank must have straight ends and most are tapered so the crank may not even be touching the inner race of outer bearings except when crank is flexing.
Then you need seals on top of that at each end.

The inlines run two sets of needle caged needle bearings in the center and 4 more deep groove ball bearings 2 at each end. The center section that splits horizontally is water cooled.

Here is a front outer bearing on a inline that decided to come out of it's bore. Unfortunately it is a little too common of an occurrence. Usually it is from factory piss poor fit and/or the fact they press them in cold. Twisted crank and overheating are two other reasons one might spit a bearing.

 

[Tom Foley]

OK , I've read it all , heard all the theory and I do agree with most of it but here is the deal from my real world experience . Running my Twin with both MAC 84's and VAC 91 's in X and Twin Hydro in many multi day events beating them like circus animals the rear or main bearing ALWAYS shows wear first not the small or front bearing due to the horrible thrust loads . It doesn't matter ! Run the boat using conventional flex collet setup and enjoy the simplicity .
I will replace the rear bearing 3 : 1 to the front bearing . The Mac and the VAC share the same size bearings . I change every 4 - 5 outings redundantly . Just my opinion , do what you want .

 

[Daniel's Racing]

You have to remember that most of the small nitro engine designs were originally airplane engines. Airplane engines deal with the thrust on the crank in either direction comfortably.
Even the Zenoah gas engine will run a airplane prop hub with same engine design we use in the boats. So thrust on crank was already designed and factored in in the majority of the engine designs we have.
Not only capable of the push pull thrust on the crank but also the forces a propeller places on the crankshaft in all directions that we don't have in the boats so bearings are usually up for the task.
Then most of your purpose built marine engine designs have even beefier bearings because full throttle high rpm non stop in marine duty takes bearings up to that task.

 

[K F Wheeler]

I think it's kind of obvious (or maybe not...) why I brought this subject up. We're ALL trying to get the most torque and RPM to our favorite prop(s)!!!
I'm always looking for ways to make my boat faster! If I can find a way to make the drive line more efficient I'm willing to try it! Never liked the cable set up. Been out of boats close to 20 years. With the internet growing so fast. And this pandemic problem pretty much makes it a necessity for me to shop on line now... I've found it very difficult to find parts I needed to put together a boat to race. I've used Teflon coated bushings in the strut before. I've seen the brass sleeves with the holes drilled in them. But just stuck with the Teflon coated bushings. That was then... Got to KEEP LOOKING for a possible better way for anything!!!
Things have changed in almost 20 years! NOT just RADIOS!!