HELP! Stihl MS650 PTO bearing play

[popopboat]

Once again a post about my MS650 !

In my last thread ( https://opeforum.com/threads/stihl-ms650-new-mysterious-behaviour.30751/#post-1582731 ), I described how my MS650 started acting up when hot, with the PTO bearing the culprit in the end.

I replaced it with a new OEM NTN bearing from the dealer (replaced oil seals also with new OEM), after what i used it for maybe an hour or so and it was resting on the shelf since then.

Today, after some cutting, the issue resurfaced. After just a few cuts, the saw started behaving erratically again. I checked the bearing, and sure enough, there was already some slight play.


Why? What causes this? Has anyone encountered this ever?


To be clear :

- Im letting the saw doing the work, rarely use dogs.
- Chain is sharp, depth gauge is set to 0.6mm (0.025" ) with a grinder.
- Bar is smooth and not disfigured.
- Firstly used Castrol Power 1 Racing at 40:1 (or even more), tried switching to oil that local forestry used.
- Carb is set on rich side.
- Clutch drum, clutch, clutch bearing and crankshaft are OEM in very good condition.
- Saw is slightly "ported", widened and polished but port timings were not touched.
- Mild muffler mod.
- Ignition timing is stock.
- Bearing is NOT loose on crankshaft nor it spun in the crankcase.

Id really appreciate if someone could help me as im not planning to replace the saw.

 

[hacskaroly]

Id really appreciate if someone could help me as im not planning to replace the saw.

What was your process for replacing the bearing? Tools used? Not sure if that would have any effect since your saw exhibited this issue with the original bearing. Sounds to me you are doing everything correctly.

I just replaced a bearing on the flywheel side of an 034 that had a slight wobble to it, but have not run the saw enough to see if the issue came back.

 

[popopboat]

Forgot to mention that!

Thought that I may have damaged the bearing during the initial install, so I decided to be extra careful this time and used the hot/cold method.

I left the crank in the deep freezer, warmed up the bearing (but not too hot to hold), then placed the bearing onto the crank. After that, I put both back in the freezer before heating the crankcase and assembling everything—ensuring axial forces on bearings were absolutely minimal.

One of the possible reasons is that bearing might be overheating, but why?

 

[heimannm]

It is possible that the bearing pocket in the crankcase has been worn oversize from the previous bearing failure. In that case you will need a new crankcase or a be creative and have a sleeve installed in the existing one to positively hold the bearing.

Mark

 

[Motorka]

Look for air leaks vacum - pressure test.

 

[popopboat]

Dont have proper equipment for an air leak, but ive tested it with a "homemade" tool and it showed no leaks.

Bearing doesnt spin/move in the pocket, so i dont think it ever spun a bearing in the crankcase.
But, honestly when I was installing a new bearing, it went suprisingly easy in.
I guess its a bit on a looser side, but it doesnt move in the pocket.

 

[Vintage Engine Repairs]

Dont have proper equipment for an air leak, but ive tested it with a "homemade" tool and it showed no leaks.

Bearing doesnt spin/move in the pocket, so i dont think it ever spun a bearing in the crankcase.
But, honestly when I was installing a new bearing, it went suprisingly easy in.
I guess its a bit on a looser side, but it doesnt move in the pocket.

If in doubt use some 609 retaining compound by loctite between the bearing and pocket.

 

[hacskaroly]

If in doubt use some 609 retaining compound by loctite between the bearing and pocket.

How much harder is it to remove the bearing later after using the 609?

 

[Vintage Engine Repairs]

How much harder is it to remove the bearing later after using the 609?

It’s not hard, a bit of heat and a few firm taps and the bond breaks.

 

[hacskaroly]

It’s not hard, a bit of heat and a few firm taps and the bond breaks.

Awesome, I just didn't want to get it, use it and find that it is stronger than epoxy...lol

 

[Vintage Engine Repairs]

Awesome, I just didn't want to get it, use it and find that it is stronger than epoxy...lol

Haha na. Ultimately there is very little if any resistance on the outer race, the retaining compound will be everything you need, so long as there is no slop. If it just goes in a bit easy, retaining compound is a perfect solution to what otherwise may need a new case.

 

[MG porting]

Are you leaving the bar and chain on the saw when you're not using it and if you are are you loosening bar tension before you put it away??

 

[popopboat]

Are you leaving the bar and chain on the saw when you're not using it and if you are are you loosening bar tension before you put it away??

Usually yes, I leave the bar and chain very loose (hanging by an inch or two off the bar), but this time i took it off.

I mean, its literally breaking my heart, I treat her like a queen and still gives me troubles.

 

[MG porting]

Usually yes, I leave the bar and chain very loose (hanging by an inch or two off the bar), but this time i took it off.

I mean, its literally breaking my heart, I treat her like a queen and still gives me troubles.

I forgot to ask is the bearing a plastic cage bearing?

 

[popopboat]

Yes it is a plastic cage bearing (NTN), but it is not damaged.