GSJames

In August, I blew a rear bearing in my 424 engine. I foolishly didn't realize that it had the old style rear bearing (lesson learned). At the time it was a very good engine, peaking at about 17.2-17.3 on an APC 9-6. In rebuilding it, I replaced the Piston/Liner, cleaned up the head and of course the bearings. I replaced both bearings with a set from RCBearings.com http://www.rc-bearings.com/catalog/p...roducts_id=748

Prior to the rebuild the engine had noticeable endplay. Now it does not. I even bought a bearing seating tool from Darrol Cady and re-set them again. The bottom end is very free, and the crank rocks back and forth nicely. I have about a gallon of 15% through it at this time and 4 heat races. It was re-broken in on an APC 8-7 running rich at between 16.8 and 17.3K and then with a 9-6 at best mixture. The engine seems fairly strong, though not a world beater. HOWEVER, as the engine heats up, it slows down noticeably. For the first minute after starting, it will run strong at about 17-17.2 and then as it heats up more, it will slow down to 16.7 or so. I noticed this during the heat races as well, it started out well but slowed down as the race progressed.

The RC Bearings replacement front bearing is listed as being .281 inches wide, Tower shows the stock front bearing as .28 wide. http://www3.towerhobbies.com/cgi-bin...&I=LXAHZW&P=ML There is also text saying that it is 7mm wide. According to my conversion program, 7 mm is 0.2755906 not .281.

Since the coefficient of linear thermal expansion of aluminum (the case) is almost twice what it is for steel (the crank), it is possible that I am getting a bind as the case heats up and that it is what is causing the engine to slow down?

Other than buying a stock replacement front bearing from Tower, I can't think of any "legal" solution to this problem (it's not legal to mill out the front bearing recess). Any thoughts on other ways to fix this? Thanks!

HighPlains

My Feedback: (1)

Gary,

I would go back to the stock front bearing, since it's failure should not cause the type of engine destruction that failure of the rear bearing will cause.

When everything has proper clearance, the rear bearing has all the axial thrust loads, while the front bearing is floating. The difficultly is the expansion coefficients of steel and aluminum as you noted, so it should be set up for running conditions. Pop it in the oven at different temperatures (under 200 degrees) to check the end play. If the bottom end is running hotter than that, you have other problems.

[email protected]

have to agree wth the previous reply.....reseat the bearings, the problem is most certainly in the bottomend. and also check the clearance type of bearing- C1,C2, C3, etc. Darol's bearing tool is very good but you have to know how to use it. trevor Henderson,nz

djlyon

Gary your description of the problem does not suggest milling the front bearing seat. It sounds like you need a shim between the crankshaft and front bearing inner race. Or maybe there was a shim there and you forgot it when you reassembled the engine. Now when you tighten down the prop nut you squeeze the inner races of the bearings toward each other which is the worst kind of no end play.

Denis

GSJames

Thunder Tiger 40 Pro engines do not have shims on the crankshaft. That is one of the first things I looked for.

jaka

Hi!
Try C4 bearings from Swiss company WIB (same used in Nelson and Jett engines) or reheat the case and see if the play returns.

GSJames

I received a new, TT-branded front bearing. It mikes out exactly the same as the after market bearing so, unfortunately, that wasn't the problem. Oh well.

Scorpion Racing

Gary, I have had issues with some of my Thunder Tiger PRO 40's doing this same thing. In my case, it was the tapered drive washer sliding down the crankshaft bolt and not locking tight. What I did was take a thin strip of .014 Mylar sheet and place it under the tapered nut before tightening the prop down. I then wiggled the prop back and forth and tugged on the Mylar till it cam out. It took a couple of times, but I eventually got it.

Another method is to tighten the prop nut and prop down good, and give the prop nut a solid tap with a hammer. this should knock the crankshaft back some, but still leave it tight. Be sure to use a prop nut you don't care about if you try this, because it will end up with a flat spot on the tip. I am sure you realize this could potentially damage the engine, but I know you are smart enough to know that!

Best of luck...

jaka

Hi!
You should not replace the TT bearing with just any bearing!!!
The bearing to use is from the swiss company WIB! It's a C4 bearing with plastic retainers!
That is the bearing sold by Dave Shadel and that is used in all Nelson and JETT engines.

fizzwater2

My Feedback: (61)

I have had excellent luck and good performance with the bearing from RC-bearings.com. It's the correct size, with a plastic retainer, and costs a whole lot less and is easier to get.

I've been buying the stainless ones, but might switch back to regular old steel.

jaka

Hi!
It,s vital that you get a lose fit bearing (C3 or C4)! Not an ordinary standard steel bearing! C3 or C4 means that the play between the balls and the inner and outer surface is less than a standard bearing...that means that the bearing produce less friction then a standard bearing.

fizzwater2

My Feedback: (61)

Last time I looked at performance specialties website, I didn't see any bearings for sale. Do you have knowledge of any other US sources for the Swiss bearings?

djlyon

http://aeroracingengines.com/

Is the current manufacture of the Nelson engines. The rear bearing is the same. I am not sure about the front bearing.

Denis

GSJames

Scott: THAT is EXACTLY what is happening. I'll try your trick. Thanks!

djlyon

It is a trick and it might work but it is not the right answer. If the tolerance buildup between the crankshaft and the crankcase is such that the inner race of the front bearings end play is used up and the inner race is pulled tight against the balls by the Colet then the only good safe answer is a shim between the crank shaft and the inner race of the bearing even if you have to make one.

Denis

lfinney

My Feedback: (44)

the higher the c rating on bearings the looser the fit......a thorough cleaning of the bearing surface of the crank at the rear bearing area is vital, i use 1000 grit wet or dry paper with water or wd-40 to polish and clean the bearing area on crankshafts, and then make absolutely sure that the case is clean..., i also polish the front bearing area of the crank as well, then check the fit of the bearings, if they take alot of force then the clearings will tighten up during assembly. i have had fellow modelers ask me to examine their engines after a bearing change, and they often fail to properly seat the rear bearing, resulting in zero endplay and also they have some tension as well, by reseating the rear bearing it usually produces some slight endplay at room temp, if the engine has no endplay at running temps thats bad.. i always reseat thunder tiger/magnum/supertiger engine before running.. if you cant find the WIB swiss bearings the plastic retainer types with a C3 clearance will work well too, they come stock in most OS and YS heli engines FWIW

SmokinJoe

This subject is dear to my heart, as Big Bruce will testify. I've grappled with this problem on and off since I started running the TT engine. Most of the time, you just tighten the prop nut gently against the prop, remove the prop and give the crank a light tap with a mallett, which moves the crank a few thou in the collet and there is your end play.

However, sometimes the collet then slips on the crankshaft when the prop nut is retightened. Sometimes I've swapped the collett and fixed the problem, sometimes a different crankshaft fixes the problem, sometimes even washing everything in alcohol to remove all traces of lubrication between the collett and shaft. Sometimes nothing I try will stop the collett slipping.

I must admit that I have not yet tried the shim trick. However, even with a shim between the shoulder on the crankshaft and the back of the front bearing, if the collett is slipping, you will still tighten the collett hard up against the bearing - which then compresses the front bearing - while leaving clearance between the rear bearing and the crankshaft.

If anyone has found a bullet proof way of preventing the collett from slipping, I would like to hear it.

Smokin Joe

djlyon

think about it. With the correctly sized shim the collett will go hard against the bearing inner race which will go hard against the shim which will be hard against the shaft shoulder without PRE loading the bearings AXIALY. A properly installed collett should be hard against the inner race of the front bearing. You should not depend on the colletts grip on the crank shaft to do any thing but prevent the prop and drive hub from rotating on the crank shaft.

Denis

jaka

Hi!
Remember that the front bearing is only taking axial loads (forward load)! That means the crankshaft is actually hold by the front bearing surfaces!
The rear bearing is only taking radial loads (the piston pounding up and down).

kommander

I can't believe this post died with so much mis-information! The only one that is correct is dlyon.

racerxwest

Kommander, it can't be fixed.

jaka

Hi!
????
Are you referring to my statement?