Stupid question: TM250 Crank Bearings

[ninjamk]

Hey guys, I have what I think is a stupid question but need reassurance.  My TM250 was running great other than a hanging idle and the plug showing a little lean.  No weird vibrations or anything.  So I pulled the LH crank seal to replace and found that the inner race of the bearing spins freely around the crank.  I am correct in that cannot be right? 

Thanks!

[David Lahey]

Not a stupid question. The magneto side main bearing should be a light interference fit on the crankshaft. Having a bearing that can slip on the crankshaft or slip inside the casing is relatively common, and can be caused by using main bearings with the wrong clearance number, which causes the bearings to be loaded up axially each time the motor heats up and cools down, which can wear the bearing inner and the crankshaft away a bit at a time. It can also be caused by the bearing ID being made slightly oversized or the crankshaft being made slightly undersized (but this would be very rare on a Suzuki motor)
If the crankshaft is now undersized (test the fit with a new bearing), it can be fixed by metal spraying and grinding to get the crank shaft back to the right diameter

[Tomas]

Should not spin freely. There should be interference of somewhere between 0.01-0.02 I believe. It may be because your motor is running a little lean and bearing is not geting enough oil. I think in TMs and early RMs only LH side bearing is affected.

[ninjamk]

Thanks David.  I have a 1 hour cross country race next week and a 3 hour endurance race in 3 weeks.  If this was your bike would you feel comfortable running as is?  I have no reason to think that the bottom end has ever been cracked open.  Here is a short vid.
https://onedrive.live.com/embed?cid=49B18A81B87EF46B&resid=49B18A81B87EF46B%212435&authkey=AIFosxaL8zmJQC4

Should also mention that the inner race spins with the crank when I kick it over.

[David Lahey]

As long as the main bearing is not failing then that looseness problem will probably get worse pretty slowly and maybe with a new (more flexible) crank seal there it won't leak air in but if there is perceptible up and down movement of the crankshaft even a new seal won't seal for long.
If the rolling elements in the bearing are still good, your main risk is that the motor might be damaged by it running lean due to air leakage

[LWC82PE]

I have no reason to think that the bottom end has ever been cracked open.

Gee if thats the case what bike would still be running good with 45+ year old seals? My original 1980 TS seals needed replacing in the early 2000's. Bike ran heaps better after that. The old Suzukis are pretty bullet proof but i would not be attempting any long distance events on a motor that has not had a complete tear down, inspection and freshen up.

[Oldfart]

What Lwc said ..

[OverTheHill]

Have seen a lot of that over the years & had 'some' success with lying the bike absolutely horizontal & running some loctite around it then wriggling the crank to get it to find it's way in. Probably arguments for and against that, as to does it end up central afterwards or a bit more glue on one half of centre than the other. Perhaps warm it up with a hair dryer to hurry up the cure [if in a hurry--or stop it in it's tracks if you think it's going to run out the otherside. Was common on small yamahas like V50 scooters & wore into the shaft quite bad by the time we foxed them so just bronzed up the shaft & machined it to an interferance fit [not sure how interferance but 'had a plan']. with bearing 'tightness', bearing clearance 'as i understand it, say C3 & C4 clearance is in referance to the internal fit of the balls 'not' the fit [tightness] of the bearing to the case or the shaft as such. Which is why the often stipulate C3 on the mag side & std on the other side due to being a tight fit on the mag end & a sliding fit on the other & bolted up tight with primary gear. Talking myself into a corner here--next please. Cheers.

[David Lahey]

Maybe I should have explained the C3 C4 business with crankshaft ball bearings a bit better.
The reason for using these bearings is to avoid the bearing being forced to move axially when the motor changes temperature (the crankshaft expands a different amount to the casings when it heats up). having clearance between the balls and the races means that the first time you run a rebuilt motor, the bearing (inner or outer) moves to a position where the axial force is relieved. It might move in the casing or along the crankshaft. Then when the motor is stopped and cools down, the clearance (C3 C4 etc) between the balls and the races allows for the differential contraction as it cools, to be taken up within the bearing, and avoids causing the bearing to move along the shaft or in the casing. Any movement there will eventually cause a loose fit in the casing or on the shaft

[Tomas]

Have seen a lot of that over the years & had 'some' success with lying the bike absolutely horizontal & running some loctite around it then wriggling the crank to get it to find it's way in. Probably arguments for and against that, as to does it end up central afterwards or a bit more glue on one half of centre than the other. Perhaps warm it up with a hair dryer to hurry up the cure [if in a hurry--or stop it in it's tracks if you think it's going to run out the otherside. Was common on small yamahas like V50 scooters & wore into the shaft quite bad by the time we foxed them so just bronzed up the shaft & machined it to an interferance fit [not sure how interferance but 'had a plan']. with bearing 'tightness', bearing clearance 'as i understand it, say C3 & C4 clearance is in referance to the internal fit of the balls 'not' the fit [tightness] of the bearing to the case or the shaft as such. Which is why the often stipulate C3 on the mag side & std on the other side due to being a tight fit on the mag end & a sliding fit on the other & bolted up tight with primary gear. Talking myself into a corner here--next please. Cheers.

Souns like lots of mucking around an your missus may end up having loctite all over her hairblower. Much easier to split the cases and check everything. There is a good chance that bearings are trashed and spin on crank and in cases. Whats with people being soo unwilling to split cases. It would only take a day to change all bearings ,seals and gaskets (have them ready) as long as the rest of your motor is in good shape

[ninjamk]

Thanks guys.  I replaced the seal and am going to run as is next weekend.  I just did the bottom end of my KX500 and will be doing the TM next week once I get all of the parts in.  Appreciate all of the advise!

[ninjamk]

Just to update, I split the cases and replaced the bearings and seals.  The new crank bearings fit much snugger on the crank so all is well.  Anyone know if there is a transmission vent?  Does it even need one?  I am using TS cases if that matters.

[Doc]

no vent that I recall

[pokey]

i had always thought suzuki hid one in the oil pump section.

[LWC82PE]

The 77 onwards 250 models have the vent in the oil pump housing section. The 77 onwards 185 has the vent behind the magneto cover towards the 1 O’clock position just where the wires come out The PE's have the vent here too but its a brass nipple that a hose then attaches to it. If you have the cases split you can see all the hollow cut out sections and you can follow the path of the breather through a sort of ‘maze’ and find where it vents to atmosphere.
I would have expected the TM to have one somewhere.