YZ 100 Rebuild: Bad Crankcase???

[Joe]

I took got a good look at the right side(side with clutch) of my 78 yz 100 before starting to assemble the motor and noticed a problem with the bearing for the crankshaft.  The bearing itself is ok but the chunk of aluminum that the bearing is pressed into is not attached to the rest of the crankcase.  There is a gap between that piece of aluminum and the rest of the crankcase all the way around that's big enough to put a piece of paper into. It can move a little but will not slide out.  On the other side it appears that there were 3 holes that were filled with J B weld.  Anybody got any ideas what is going on with this?  I will post a picture latter if needed.  How should it be fixed?

[Coop]

Someone probably rode that bike a long time with bad bearings and that's how the case got broken on both sides. That or they were too aggressive with splitting/assembling the cases. That pice won't slide out because the bearings are a friction fit and get pressed into the case. You will have to split the cases and use a puller to get it off. I would start looking on eBay for new cases unless you know a very good welder. either way the cases will need split so the bar cases with case parts can be worked with without risk of damaging any of the other parts.

[George]

...How should it be fixed?

With a yz125 engine shoe-horned in

[SachsGS]

The cases could be welded/machined and inserts fitted if you have access to a machine shop.

[Joe]

Here are some pictures of it.  On the inside the crack I mentioned is clearly visable, on the outside there appears to be 3 globs of J B around the seal.  I plan on having it welded if there is no other way to fix it.  Will welding it ruin the bearing and/or seal? If so how can I remove them without damaging them? Thanks  
Links
http://imageupload.org/getlinks.php?s=1daacd339ba41f0ad32a5c71a8542ed2
http://imageupload.org/getlinks.php?s=de0fee9d993e3f565a6ce8bad96ae33b
http://imageupload.org/getlinks.php?s=a3728006e28756d1bc1ac90f5deeb591
http://imageupload.org/getlinks.php?s=21570d8fa4d7060727d0a6661653ffdd

[TotalNZ]

You're gonna need to take the bearings and seals out.
Put the case in the oven, inside facing down if that makes sense. Then crank it up high on bake and wait.
The bearings will drop out as the case expands.
Take the cases to your local engineer to get TIG'd up.

[Coop]

Like NZ says, the bearings and seals will have to be out. That means splitting the cases because you can't have it welded with the bottom end together.

[dogger315]

I ran into a similar situation with the 73 Honda CR250 I restored.
Those engines were notorious for breaking the kickstart return
spring bolt.  When I opened up the engine, there was a big
hole in the right side crankcase half where the bolt used to be.
I ended up replacing that half with an intact replacement I
found on Ebay.  I could have opted for a weld repair, but I
wanted the engine to be factory fresh, not rigged together.

Welding will probably work in your situation, but it won't be
cheap because of the precision nature and the fact that it's
Aluminum.  Replacement cases can probably be had for less
than the weld repair bill.  Make sure to ask the right questions
of the seller so you don't get stuck with another set of bad
cases.

Just another option to consider.

dogger 

[Joe]

Thanks guys for all your help. It wasn't until I had the cases split that I noticed the problem, so they are already apart, just like in the pictures I tried to post.  I included a link to each picture just in case anybody wants to go back and see them.  I just got a dumb question, due to the crack going all the way around, the whole bearing moves.  Should I just tell the machine shop to center it before they weld it or am I going to have to assemble the crank cases with the crankshaft and then somehow mark the position?

[SachsGS]

The problem with welding is that regardless of what you do the parts being welded will always "pull" or distort and ideally the crankshaft mainbearing bore would need to be machined oversize and an insert pressed in to reestablish concentricity.What I would do is reassemble the cases with the crankshaft main bearings and crankshaft installed to find your location again.The engine will be o.k. (it's amazing just how tough 2T engines are).