I installed the missing coil and main switch that i purchased at Mid-Ohio last weekend.  Fixed the rest of the wiring someone hacked up, installed a new battery, rebuilt the carburetor and fuel petcock. Put new gas in it. Kicked it about a dozen times and after a few pops and backfires it started up and ran!

My wife looked in the garage with a smile and gave me a thumbs up while i was revving the hell out of it trying to keep it running. But all smiles faded away after about 5 minutes. It died and now the wife is running out of the garage and i am trying to see thru all the blue smoke.... Damn! Oh well! Still was a good feeling bringing it back to life after all these years. 

So i decided to pull the cylinder head off and take a look inside. What a mess! I am surprised this thing even started at all. Rust on the cylinder wall everywhere. Rings didn't even touch it while it was running. Lol! Pulled the cylinder, did a ring gap check and piston clearance check. Ring gaps are over .060. Piston to wall clearance is all but laughable! 

I will have to take the cylinder to work to get some measurements but doesn't the cylinder wall port side look awful thin? I don't remember seeing any that thin with a stock bore? 

I also notice some good play in the left crank bearing. Looking like a big major overhaul on this engine. Not good.

Here are some pics of the cylinder and piston.

 
 
  



 

Probably only stopped due to all the old oil & rubbish thrown up into the plug. Still on standard bore after 50 years. Not too bad!!.

 Plug still looked good. Don't know for sure, but it died when i was letting it come down to an idle for the 2nd time. One last pop and that's all she wrote. Still has good spark and fuel. 

My concern now is how thin the skirt looks being on the stock bore? I don't think i've seen one that thin at the port side. I will know more after measurements.  
Crank bearings concern me too. I was really hoping to "not" have to split the center cases but the left bearing has some decent up and down movement. I may end up having to go thru the whole engine which is going to get costly. 

Yeah, decent up & down sounds very in-decent unfortunately. Keep that flywheel tight too on assembly as they all eat crank keys at the slightest provocation. Great these early models keep turning up still. Wish we had the same luck down here.

I wonder if it's popped one of the crank seals out.
My CT1-B did a similar thing, ran fine then wouldn't start again.
I pushed the seal back in and she ran.

Anyone have a CT1 cylinder laying around and can look at the bottom of the cylinder and see how thin it is to the ports? Or take a picture from the bottom like i did in a earlier post and post it here.

thanks 

I just looked at my 78 DT 400 cylinder here on the bench just back from a third over bore. Lots of meat left where yours is paper thin.  Curious on what the experts will say. Obviously not right. Is that a DT piston? LOL 

edit…Oops looks like Pete has confirmed they are paper thin. My bad…who’d a thunk it!

Haven't been on the Forum for awhile. Had my Enduro spares scattered about the garage since moving them out of the shed to accommodate a friend whom was relocating to my area and needed some additional space. Today I reclaimed all my shelving and organized everything. Found the pictured CT1 cylinder and piston in the garage. I miked the bore and it comes out to something like 2.598-2.600 inch I.D. (or 66.03-66.04 mm.). Also found this older link which addresses this:
www.yamaha-enduros.com/index.php/forums/...and-ct1-piston-sizes
The original cylinder/piston pictured seems to be like that described in the above URL...
Your piston looks like "968" so not sure what that last digit is about...

 

You can see mine in thread #55
It's on original bore.
www.yamaha-enduros.com/index.php/forums/...restoration?start=50

I wonder if the sides are so thin because it’s the same cylinder used on the 125’s and by the time they got it bored to the size of a 175 that’s all the meat it has left