Hi all,

Currently restoring a '72 CT2 and splitting the motor to replace the main bearings, found some interesting 'bits' at the bottom of the case....Look's like a needle bearing  - Where could these be from, I thought?

 

 

Ah - Bugger - New RHS crankcase it seems....
 


Found a RHS crankcase from a DT with the same part number so not too bad.

Any advice on what to check/look out for when replacing/using matching a different crancase?

Yep, all those problems started for us in NZ when the CT2 arrived as they uprated the power & lightened the mag flywheel which was all good on the 125 but caused grief on the 175. Keep the timing at no more than 1.8mm btdc which usually means definitely no more than 15 thou. By memory they used to quote 12-15 so in between that just reduce timing a bit. You never know, it might've broken a flywheel key sometime then got kicked lots with kicking back. Other possibility is if it started & ran backwards once, & the rider touched the kickstart lever even accidentally caused it to started to engage it--the motor then grabs the gear & forces the lever back to the stop with nowhere to go except split the case. We did a few back then. Think there was an inline diode just before the plug coil to stop the possibility of running backwards. Wouldn't stop it kicking back but cant start. Might be wrong on 'that' comment if someone knows!!.

Well the kickstart was knackered due to the end stop spinning (See my initial post on the forum) as it's got the old part - Probably engaged as you said and hit the sprocked causing the break...

When I saw your first photo I was thinking that could only be the caged roller for the primary trans shaft, and then saw your broken case & confirmed it. Bummer...where are you located? I have a few AT2 case halves I'd be willing to part with. PM if interested.

Thanks for the offer Apex -but I've already ordered one from the states (I'm in the uk) so hopefully that one will be okay (if not I might be contacting you!)

I hope the "DT" case half you bought was from a US 74-76 DT125?  Otherwise it might be the wrong part.  So many mislabeled parts for sale out there.

I have seen unmatched case halves that match perfectly and others where the cylinder base gasket surface is a different height.  I don't know why sometimes they don't line up but the good news is usually they do.

Hi Mark - thanks and I'll just need to see if they match up! It's from a DT125 1974 from the US, part number matches so I hope its the right one...

Bad luck with the broken case, hope you get this issue solved.
Very interesting thread, i hope the case you bought, fits.
With my 75 DT, i faced also some problems with the LH case and wanted to buy an intact half. This was before i found this site. 
It starts like most rhumors start: ...an old Yamaha mechanic told me...Maybe he was wrong, but he told me in the factory, they bored both case-halfes of each motor together, to line up correctly all the axles . In the ´70ies , machines were not as precise as today, so tolerances were bigger.. Finally the production workers even chosed indivitualy for each motor primary drive gear wheel/clutch basked drive gear to fit the right distance between crancshaft axle and clutch axle bore.
So even if by chance one half seems to fit to another, not all bearing holes may have exactly the same center left/right case and soon or later the bearings could fail
I will not agree or disagree this, but this thoughts let me go the safer way to a welding shop and repair my case-half.
Maybe the experts here on the side know more about this production theory, could be a fairy tale...but in fact, at least at my model, the primary gear wheel and clutch basket wheel have numbers on it, written /engraved by hand, so they match.
Sorry to leave a bad taste to your Ebay purchase, i hope, i am wrong..
Regards from Germany
 

The broken case is a bumer, but the topic of 1970s vs. now maching technology is fascinating.  While rebuilding my 360 I was blown away by steel inserts molded into the cases and other nuances that led me to believe that cases were set up and machined by machinists.

Did a quick search on eBay for "DT125 engine cases" out of curiosity and because I am thinking about restoring one and a bunch of sets pop up.  Not sure these are the precise ones needed you this case, but seems like there might be a used matched set to be had.

Yamaha sold individual case halves for these bikes...  many people have replaced only a half without issues.  My guess is Yamaha used a precision manufacturing of each half so that they did not need to be matched.

The "line boring" theory is not without merit...  as some manufacturers did use that process.  On 1970's Husqvarna cases for example, you'll find matching numbers stamped in each case half near the parting line to identify they are a matched set.

A good machine shop can check the bearing bore alignment if there's a concern.

Husqvarna cases showing the number match.



   




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