YZ50 engine rebuild - PROBLEM

[YZ250H]

Hi Guys,

Just putting the bottom end of the latest project together.  Now that I have the case halves back together the crank is tight .  I assume this is not right 

The crank turned freely when installed in half a case (if that makes sense).  Now back together and very stiff.  Still able to be turned, but way too tight for what I think it should be.  Ideas ??  Wrong size bearing on the second half ??  Pretty sure they are right.  Got me buggered.  Gearbox shafts all turn nice and freely.  Gears seem to shift OK.

Cheers

[vandy010]

did you fit a new rod kit to the crank?
and if so, was the crank pressed back together to the correct measurement?
is the crank balanced correctly?
if it's out of balance, it'll usually go, tight,free,tight,free when rotated {keep it clean folks}.
sure your gonna handle that big bore 50 Tony?
be mindfull of the gearlever also

[Billet YZ]

Hi Guys,

Just putting the bottom end of the latest project together.  Now that I have the case halves back together the crank is tight .  I assume this is not right 

The crank turned freely when installed in half a case (if that makes sense).  Now back together and very stiff.  Still able to be turned, but way too tight for what I think it should be.  Ideas ??  Wrong size bearing on the second half ??  Pretty sure they are right.  Got me buggered.  Gearbox shafts all turn nice and freely.  Gears seem to shift OK.

Cheers

Hi Tony,
             give the crank a decent tap with a rubber or plastic mallet directly on each end and it will free up, the crank will be firm up against one of the crank bearings making it harder to turn. I would suggest hit the right hand end first and then check the movement of the crank again. It won`t need much to centre the crank.

[YZ250H]

I wish I lived next door to you Billet 

Mick,  I didn't rebuild the crank as the bigend was good.  There's probably a whole topic on wehter that is wise or not but .....

It is tight all the way around, so I'll just duck to the garage and give it a wack (the bike that is) and let you know.

I love this forum.  You guys rock.

[YZ250H]

Hmmm  No good. I think what you are saying is right thought Peter.  I don't suppose it would right itself when it is running.  I am imagining a seizure and a lot more work.  Better to put the work in now 

It's a bit odd - the cases went together fairly smoothly.  Only needed a bit of a firm tap with the rubber hammer.  As I said I can turn it by hand, but it is ceratinly not running freely like I think it should.

Should I give it another wack, or just go and have a beer (it's only 11 am over here)

[Hoony]

G'day Tony,

no it won't come good when running.

did you install new Main bearings? if so did they go all the way up to the crank shoulder.

if not, give it a few more wacks each side and check after each one. may need a deadblow hammer (heavy soft face hammer). if you don't have one use a piece of soft aluminium or copper between the shaft and a steel hammer

does it feel like its rubbing on the crankcase anywhere?

[YZ250H]

Yes, new mains Hoony.  I'll give it a few more hits with the deadblow.  I've got a little butane torch, would a bit of gentle heat help you think ?  If that doesn't help I might have to split the cases and have another look.  The LHS main was definitely pressed in nicely.  The RHS I can't be certain.

[Hoony]

assuming the seals are in case don't use heat.  are they genuine bearings or aftermarket?

i have seen some crank bearings have a large radius on the ID ( to mate to the radius on the crank) where aftermarket ones don't.

does it feel like its rubbing on a crankcase half?

i would pull it apart and check at this stage.

I never press bearings in, i use a small torch to gently heat the case and drop them in. same when i remove them i heat the case as alloy expands faster than steel and tap the case onto a block of wood the shock drops them out.

[YZ250H]

Aftermarket bearings.  Could be my downfall 

Doesn't feel like it is rubbing.

Will pull it apart again and see what's up.

Thanks for your help guys. 

[Hoony]

no worries. i hope you have used C3 clearance bearings for the tolerance due to heat.

Where in WA are you ?

[oldfart]

While it's still together ....spin the crank slowly while applying some duct tap or electrical tap thru one revolution ... and then the other side. This will give an indication of crank binding with cases.

[YZ250H]

Yes C3 bearings Hoony.  Based in Perth now, been here since beginning of July.  Just in time for the coldest July on record    Already trying to work out how to get a bike to CD8.

That is a great tip thanks Stew.  Unfortunately the manuals only show you what to do when everything goes right.  I am not an experinced mechanic (as you all know), but I get there eventually.  I learn a lot as I go along too (what life's all about really).

I'll give it a go.  I have parked it up for later.  Time to get things a bit organised.

I will let you know how I go, as it is a bit odd 

[Hoony]

Tony,

Sorry if i have asked the obvious, but i thought better to ask than not to !

[YZ250H]

Nothing is ever too obvious when you are talking to me Hoony

Always good to have all bases covered.  It is usually the most obvious thing that gets forgotten 

Thanks for your constant help    (my missus made me put that in )

[smed]

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