YZ50 engine rebuild - PROBLEM

[vandy010]

actually, there's an idea right there!
just back the case screws off a tad to relieve any possible compression on the crank/bearing and see if that makes a difference before you just go ahead and re~split the cases.

[Lozza]

I have come across a similar problem rebuiding wr/dt200 motors,The genuine yam bearings are a poofteenth narrower than aftermarket jobs,although the i.d & o.d were the same! It had me knackered until i measured them,the crank turned fine until you done the case screws all the way home! I,m not saying that is your problem but it,s worth checking! Instead of splitting the cases to check measure the old bearings & then go to the bearing shop & ask to measure the same bearing they supplied to you or i think you can get bearing dimensions on the net,but dont know where.cheers smed

I doubt this to be true. A bearing number is a industry standard, unless the manufacturer puts a unique number on the bearing. A 6206 for example is exactly 30 X 62 X16  from both OEM and aftermarket. If they are I would be very surprised but I have never found this to be the case and I rarely use OEM bearings.

Check the cases have not been welded that had me in the same dilema once.

[smed]

G,day Lozza,Yes I agree a 6206 is a 6206, I never said anything about bearing numbers,what i said was wr/dt200 motors run a narrower main bearing than whats available aftermarket but has the same i.d. & o.d as a commonly available bearing,it was years ago i came across this problem so i cannot recall what the original bearings had written on them,I must have measured the i.d & o.d & grabbed the bearings off the shelf that matched & of course they fitted o.k, until it came time to assemble the cases & torque down the screws,the crank would not turn,after careful inspection & going over everything again i realized the new bearings were wider,i rang the local yam shop & quized the mechanic & he confirmed that was the case & he had also been caught out himself! Honda sometimes run strange size bearings in there motors,G,box especially & you cannot buy aftermarket,genuine only.cheers Smed

[torm92]

SMED ,s reply is interesting. I had the same issue with my yz125. Was tight when i put the cases together, I also used aftermarket bearings. I ended up taking 0.5mm off the thickness of the outer race. The aftermarket must be wider , very strange.
You are better off doing something about it as it will not fix itself. Hope this is a help.

[Lozza]

That will be something I'll be interested to check next time smed ,thanks for the tip. I assume you surface ground them down?

[smed]

I think from memory i got some genuine ones Lozza & chalked it up to experience,the next time i rebuilt one of those motors i found Bruce from yamwreck in vic had allready had some surface ground so i got a set off him as they were heaps cheaper than yam jobs, cheers Smed.   

[yamaico]

Try checking the big end end float as it has a tolerance (the manual will tell you what it should be). If it is at the high end you could just press the crank together to the low end - might save you having to surface grind the bearings or buying new genuine ones. Sounds like it's only going to be a few thou and you are going to have to split the cases again either way so it might be worth checking. Don't forget to make sure that the wheels are true after you press it closed.

[YZ250H]

Thanks for all the advice here lads.  I will have a crack at it tomorrow night.

I'm not sure if I kep the old mains or not.  I'll have a look.

[pmc57]

Bearings will bind and seem tight if they are assembled with preload, in otherwords if the OD of the bearings are a neat fit in the cases ie you had to heat the housing or press the bearing into the cases and the ID is also a similar fit (ie not a sliding fit) on the crank shaft then I suspect when the two cases were assembled the bearings may have some axial preload present hense giving the tight feel.

If this is the case, I would suggest removing the left hand seal and giving the inner race of the bearing a slight nudge under a press to remove any axial pre-load that may exist as a result of tightening the two case up. If you study all the workshop manuals they all recommend using special pullers for removal and re-assembly of cases, cranks etc so this does not happen. Unless you are experienced in bearing fitting procedures, these little obstacles sometimes present themselves.

If either the OD or ID were loose sliding fits then disregard the above and go back to clearence issues.
P.S. Talk of grinding bearings to suit is a recipe for disaster. Unless the bearings are throughly cleaned (and I mean cleaned, cleaned and cleaned some more) of all grinding residue, your bearing will have a short life.

Hope this helps

[TonyB]

I had the same prob with the 465 so i heated the case halves really hot. you think they are going to melt but thats when the clearances are enough to let the bearings slip in without them getting side preload. as the bearing is forced into the case the inner shell is being pushed opposite to the outer shell causing preload. After i heated the fu*& out of them i had to use an old tee shirt to hold them and rest the cases on. After tightening all the screws i lifted the engine up only to find the tee shirt hanging from the bottom of the engine. Got pinched in the case halves joint. I wasnt going to pull the barstards apart again to i ran a stanley knife ofer the joint trimmed the tee shirt off and she has never leaked in 3 years ha ha 

[Stan S]

I had the same prob with the 465 so i heated the case halves really hot. you think they are going to melt but thats when the clearances are enough to let the bearings slip in without them getting side preload. as the bearing is forced into the case the inner shell is being pushed opposite to the outer shell causing preload. After i heated the fu*& out of them i had to use an old tee shirt to hold them and rest the cases on. After tightening all the screws i lifted the engine up only to find the tee shirt hanging from the bottom of the engine. Got pinched in the case halves joint. I wasnt going to pull the barstards apart again to i ran a stanley knife ofer the joint trimmed the tee shirt off and she has never leaked in 3 years ha ha 

I betcha the next bloke who pulls it apart will get a bit shirty.

[wmc83]

How did the 50 end up? I had a similar experence today with my rmx250 i had big trouble fitting the crank to the lhs case even after heating the case and freezing the crank it still wouldn't go and in the end I had to use a old socket and hammer to hit it home ( I know it's wrong but I don't have a case puller). Anyhow when I fitted the rhs case and tightened it up the crank started to stick so I gave it one more firm tap and it  came good I think the combination of pulling the cases with bolts and a tap with hammer was enough to do it. Forgive my mechanical ignorance I'm a plumber not a mechanic!

[YZ250H]

Didn't get to it during the week and most saturdays are working these days  :'(

Tomorrow is play day 

[wmc83]

Know the feeling I snuck home from work at 1pm for a couple of hours quality time in the garage before the    Family gets home 

[YZ250H]

OK - pulled the little sucker apart.  As suspected once the pressure was released from the screws surrounding the crank and the cases separated by a hair thickness it all freed up again.  It is just that little bit of pressure at the end that does it, so there must be only a poofteenth in it.  A bit frustrating .  The LH bearing seems pretty solid on the crank and stayed on the crank when I separated the cases.  Do you think perhaps it is pushed onto the crank  too far ??  Maybe the bearing on the other side isn't far enough in ??  FIIK

There is a minor mark on the LH case where it looks like it has been dragging.

It runs free in one case (as before).  Picture coming.  Also some minor marks on the crank.

Don't worry WMC83 - I am an enjuneer not a mechanic (obviously ).

Should I just admit defeat and take it to Mr Yamaha at $1000 an hour