125 Crankcase Bearing Pocket Repair

I'm rebuilding another RS125 motor and need to sleeve the cases to make the main bearings fit tight again.

I turned some 3mm thick sleeves out of 6061 aluminum. I figured aluminum was the material of choice since it would expand/contract at the same rate as the cases, but a friend has me second guessing this. I've also seen random photos online of repaired cases using brass sleeves.

Any reason these aluminum sleeves I made won't work? I was going to do a light interference fit (say .001") and then pin them in place. I would then bore/hone the ID to final dimension. I figured once the bearings are then pressed in, everything will be pretty snug without over-stressing the cases.

Thoughts?

 

You may have issues, when you need to replace the bearings again. Or not. Not sure if you are going to pin the sleeves to prevent rotation, or prevent them from coming out of the case. The sleeves might come out with the bearings depending on which is held in better after you are done. Most sleeves I have seen, are also retained with screws so they won't come out. They are "top hat" shaped in cross-section, and the screws go in the brim.

Not saying this won't work, but it may only work once until the bearings need replacing again. YMMV

 

I see your point. I'll use flat head countersink screws instead of pins. That way it will not only keep the sleeves from rotating but also retain them in the cases for future bearing replacement.

 

Loctite makes a couple of different retaining compounds that might work to hold the bearing in place without having to go the sleeve route. I used the 635 compound on KTM 85/105 RH crank bearings in several builds. The LH side used a roller bearing and the RH bearing would "walk" when the cases got hot (and in short time ruin the big end bearing).

 

If the current fit is just slightly over, I would consider epoxy if I could get stuff that handles high temperatures. The manufacturer web sites have compounds you don't see on the auto parts store shelf. Some include epoxy with metal particle mixed in. The joint would need put together so that are no voids. If the current joint actually had clearance where the bearing is loose, I would not use epoxy.

 

Here is another completely different idea. SKF and other bearing companies make "speedy sleeves" for multiple ID and ODs. They are designed to go over a shaft to renew the sear surface, but the correct size can be used as a bore shim or bearing OD shim. They are typically very thin like .010 . They could be heated and put over the bearing outer race to increase the bearing OD.

I would only recommend this if the bearing bore in the case is round and not ovaled out.

Back to the filler option, this stuff seems like a possibility at a max temperature of 1000F: https://www.tractorsupply.com/tsc/p...MI6MiWmf3h-AIVevHjBx3eLAtvEAYYAyABEgIaZvD_BwE

If I was going with an insert that required machining the cases, I would use higher press fits. It is pretty easy to get parts up to 300 to 500 degree F in an oven so I would look at the relative expansion rates then design the insert to a "line to line" fit into the case when the case is hot and the insert is cool. After these parts are together, the insert with cases needs to be heated so that they slip over the crank bearing OD when the bearing is cool. Obviously the fits must be considered as to which is the final assembly one. The set ups differ if the assembly fit is the crank to bearing ID verses the bearing OD to case ID.

 

I see no reason why your sleeve technique wouldn’t work. Most shrink fits are about 0.001” per inch of diameter. The “top hat” sleeve with a flat head would be added insurance. In order for a shrink fit to be effective the sleeve must be a drop in at time of installation .

 

The thermal expansion of common aluminum alloys is about .000013 inch/inch DegF. For a 2 inch diameter item it works out to about .008 inches of growth for a 300 degree delta. The .001 per inch would probably be OK but when it is easy to heat and handle parts I like more interference. Of course we are talking about shrink fits and not press fits. Press fits typically tolerate less interference for assembly. Here is a very nice thermal expansion calculator. https://www.engineeringtoolbox.com/linear-thermal-expansion-d_1379.html

 

look at the big brain on Dave
This is why I come here,hoping to learn something.Sometimes it even happens

 

Thanks for all the feedback. I appreciate it.

Long story short, I had to sleeve the cases because some bonehead had previously installed screws in an attempt to keep the bearings from spinning. Said screw holes were drilled all the way through the cases, and not even square to anything. So I had to weld those holes up to begin with. With weld now protruding into the bearing pocket, sleeving was the best/cleanest way to restore the bore in my opinion.

So I welded up the holes, then machined the welds down, then bored out the pockets and dropped the sleeves I made in. I used loctite 603 between the sleeves and cases and went with a light press fit (like only .001" interference). Then I drilled and installed two M6 flat head screws onto each side of the sleeves for extra insurance.

Finally, I carefully bored the sleeves' ID to right around 2.4385", a hair under what new/good cases measure out at. The bearing OD is 2.441" (62mm), so that will make for a ~.0025" press fit. Should be nice and tight for many future rebuilds... I hope!

Here are some pics from along the way:

Previous owner's work:


With bearing and screws removed:


Holes welded up (both sides):




Machined, bored, and sleeved, with new retaining screws installed:

 

Looking at your first two pics just sums up the fact that some people shouldn't be allowed anywhere near an engine.

 

Thanks a bunch for the link! It’s great to keep on learning !!QUOTE="dave3593, post: 5998891, member: 61496"]The thermal expansion of common aluminum alloys is about .000013 inch/inch DegF. For a 2 inch diameter item it works out to about .008 inches of growth for a 300 degree delta. The .001 per inch would probably be OK but when it is easy to heat and handle parts I like more interference. Of course we are talking about shrink fits and not press fits. Press fits typically tolerate less interference for assembly. Here is a very nice thermal expansion calculator. https://www.engineeringtoolbox.com/linear-thermal-expansion-d_1379.html[/QUOTE]