I need to change the bearings in my ST 4500. The parts are available from Tower. Is this a difficult operation to do myself or should I send it in for repairs($$$). Thanks

Doesn't the front of the engine come off on the 4500 ? (4 bolts)

The thing about replacing bearings is having the tools needed. It changes with the engine. The roughest part of some is getting the prop drive off the crank. Some are locked with a tapered ring, IE Saito for one, and others just slide on and are keyed like the Magnum. Looking at the parts list for the engine, yours is held by the tapered ring/cone. It also looks as if you have a clear shot at getting a small gear puller on the back of the prop drive washer without damaging anything, so you should be good to go in replacing the bearings.

1. remove the four screws holding the front crank housing on the crankcase.

2. Turn the housing in the crankcase to loosen it up so it will slip out. DON"T pry with a screwdriver or knife, it will damage the seat and screw up the clearances.

3. After turning the housing, it should pull straight out. Use some feeler gages and measure the clearance between the crank and the housing. You will use this measurement during assembly to verify things are going good.

4. Use a small gear puller and pull the prop drive washer and then remove the cone. When the washer comes off, it will pop loose and the sudden release of tension may cause you to drop things. Don't let the housing and crank drop to the floor. Be careful not to damage the threads on the crank, leave the nut on the shaft to help protect the threads. Note the position of the front bearing, When you have things back together, it should be the same.

5. Using a setup that will support the housing but not interfer with the crank, you will push the crankshaft out the back of the housing. Same warning as above about the threads. I use large sockets if they will fit. You want to make sure the socket supports the machined part of the housing, not just the ears. Pipe couplings are another source. You want to be able to support the housing and have the crankshaft exactly perpenducilar to the table. I use my large drill press for a bench press on the engines. They usually don't require a lot to get them apart or back together. Just be careful not to damage the drill press by applying to much preasure.

6. once the crank is pushed out of the housing, you may have a couple situations. The first is that the rear bearing stuck on the crank and you will have to pull it with the bearing puller. The other is that the crank came clear and the bearings are still in the housing. In this case, use a rod that will just slip in the larger rear bearing and push out the front bearing. I would recommend that you use a gloved hand holding the housing and tap the bearing out using the rod and a hammer. There isn't much room to support the housing on it's nose. If the bearing doesn't want to come out easily, you can apply heat with a covering heat gun, but be sure you paint the heat on, not high heat on it stationary. The gun can get very hot and may get hot enough to soften the aluminum. Don't burn your hand.

7. once the front bearing is out, push the rear out using a larger rod. You can use the same setup used to remove the crank. Be careful not to damage the housing.

8. Clean everything up. Make sure all the shelac is removed from the bearing area, and polish the bearing surfaces with 600 grit wet or dry with a bit of oil so the bearings slip on to the crank. Be sure to clean everything after using the wet or dry so you don't have grit running around in the engine when you are done.

9. Things go back together in reverse. Here though, put the bearings in the freezer for an hour first. Get your setup so you can press in the rear bearing first. Here you can put the nose of the housing on a flat and use the drill chuck to press the bearing. You want the pressure to be on the outer race. If you have to use a lot of push, applying it to the inside race can damage the races and balls. When you have the setup ready, warm up the housing a bit and quickly remove the rear bearing from the freezer and push it in place. You should be able to push it in by hand, but may need a bit of help from the drill press. You need to get it seated quickly because as soon as the temp difference is gone, the shrinkage of the bearing is gone and things get tight and hard to move. You need to pay very close attention to getting the bearing started straight. If it is slightly cocked, you will shave some aluminum from the housing and it will not let the bearing seat correctly.

10. After the rear bearing is in place, push the crank shaft through the bearing and out the front. You can use the prop nut to pull the front bearing in place. Use washers close to the OD of the bearing and enough of them so the nut will not bottom on the threads before the bearing is seated. Once you have the setup OK, remove the bearing from the freezer and quickly slip it on the shaft, place the washers, and tighten down the nut. You should be able to hold the crank with a gloved hand while tightening the nut to push the bearing home.

11. At this point, the bearings should be in place and turn freely. If you feel a little bumping, you don't have the bearings seated flat, or you have contaminated them. Check the clearances to be sure they are what you had before taking things apart. You now put on the cone and thrust washer and using a plastic prop, screw on the prop nut as tight as you can get it, This will set the cone and washer. Does the crank still turn free and not lumps. If not resolve this now.

12. Now you are ready to put the housing back in the engine. align things up and push it in by hand. With the glow plug removed crank it over. Every thing OK? If it turns smoothe, now you can screw it down. Screw in the four screws only finger tight. Does the crank still turn freely? OK then tighten 1/4 turn at a time starting at the uppr left, lower right, uppr right, lower left. Use this sequence to get the housing tight. Check for the crank turning free after each sequence.

At the end of this, the crand should turn easily, with no lumps or grinding when turned. If it doesn't , the bearings are not seated correctly and the crank isn't in the correct position. Resolve this problem before trying to fly. If there is the slighest sign of a problem it will only get worse running when things heat up.

It a long list, but once you get your setups for pushing off and on the bearings, you can do the whole process in less than 15 minutes.

Don Rogers

http://www.rc-bearings.com has great prices on that stuff, might want to check with him before you spend double with Tower.

I have ordered 3 sets of bearings from these guys. All are great. I need to put ceramics in my Saito 100 I just do not feel like taking it apart!! LOL!!

LATER

Last week I ordered four sets of bearings from rc-bearings.com and they arrived within 5 days. Not bad - especially when you consider that I'm way down here in New Zealand!

What's more, they worked out (even including postage) to be less than 1/3 the price of locally sourced bearings.

Paul at RC-bearings provides A1 service and a great quality product -- who could ask for more?

Stan,


As was mentioned, Paul at [link=http://rc-bearings.com/]RC-Bearings[/link] could supply the bearings you need at a fraction of the cost at Tower.

You need the following:

A 6000 bearing (10x26x8 mm) ST number 22123000

A 16004 bearing (20x42x8 mm) ST number 22123002

A 6203 bearing (17x40x12 mm) ST number 22123001 (which I did not find in Paul's site, but I am sure he can get it too for you)

Take the measurements yourself, with vernier calipers, since I read from the web and did not measure them by myself.


All will cost about $12 together... Unless you want to upgrade to ceramics...