Hello all. My first post here. I recently received a CT1-C from my father-in-law. He purchased it new in '71. It has 8,500 miles on it and I know there hasn't been any major work on it. It hadn't run in 37 years so I went through it and got it running. It didn't take much - it had spark and all the jets were clear. The majority of the work was clean and sealing the tank and putting new friction plates in the clutch. I've been having fun riding it a bit. My father-in-law was in town for Thanksgiving and he got to ride it for the first time in 37 years. He's (a young) 78!

However, I do have a rattle and I'm wondering if the top end needs to be rebuilt. At first I thought it was the exhaust. But, after closer inspection that doesn't seem to be the case. I'm new to working on motorcycles and I'm looking for some guidance.

Here are a couple of videos:
1. The sound:
2. Inspecting the piston via the exhaust port:

Thanks in advance!

Here's a pic of the bike the day I picked it up:

Welcome from NZ. I'm 78 & don't feel young haha.
Your bike looks nice & original.
Top marks for your videos but it's piston rattle.
A new std kit would fix it for the next 50 years without reboring "guessing", or just carry on as sounds like the compression is reasonable by how it idles & responds.
Can see plenty of blowby down the past the rings though so yep a new kit is the best plan.
New rings wouldn't stop the rattle or not for long as it's rattle from skirt clearance at the skirt base or from there & up about 18mm then gradualy tapers towards the top so will always feel worse than bad if you have the head off & move the top.
Love CT1's. Mine is an AT1-125 1970 but grafted a TY175 top end on a few years back which rattles but luckily i'm pretty deaf lol.
Ok here's mine, poor old thing. It was used as a farm bike on a dairy farm most of its life. Dad used it until he was--umm--about my age.
Drag the video along as hard to watch.
I should be shot a dawn for a week for revving it that hard but that was when i was young a few years ago--like about 74--.Worked in the Yamaha shop right through those great times.

Was getting carried away there. Just to clarify, the main purpose of my video was to compare piston rattle. Mind you mine could expire anytime haha but has had a fair beating since that time.

Greetings from Tennessee. 
your bike looks great!

Oh man, that’s a beauty. I guess all my 175s must have piston rattle because they all sound the same as yours. My choice is to just ride and enjoy them.

imgur.com/a/eNpf7Yt

This is helpful, thanks. I think I'll stop worrying about it and just enjoy it until I decide to tear into it. It's been a lot of fun.

When I was 14 I had a CT2 in brilliant red. That bike never really ran right and I didn't know how to fix it at that time. That was the good ol' days before all this information was available at your fingertips. This has been a second chance for me.

Dogman I also had a Ct2 mine was a great bike. One thing you should do is replace your brake shoes 50 yr old shoes can and will delaminate for safety reasons. Ask me how I know 

Perhaps a standard oversize piston might be a better option for you .
That presumes it is still on standard piston size , and hasn't already been rebored .
If you were to just fit a standard piston  251-11631-00-96 , it's probably not going to quieten down much .
Looking at my parts book there are three more standard oversizes available too ,-97 , -98 and -99 .
All these take plain rings . There will be keystone pistons available too , though they may not be available in as many options .
You'll have to match correct rings for whichever piston you select .
That may depend on availability .
 

RT325 mentioned a new std kit. My first DT, I broke a cable to the autolube without knowing and ate the piston/rings. I simply hand sanded the cylinder and installed what I believe was a new Wiseco piston/rings at the time. It ran great and had lots more compression!!

With the high quality machine shops out there now, Swoop's idea looks good too...go oversize.

If you are uncomfortable boring the original, maybe buy a 2nd jug. I also wonder what shape that connecting rod bearing is in.

hi,
if you can rotate the piston, left to right, from inside the exhaust port, it would suggest that there is major play in either the little end bearing or the big end. possibly from corrosion caused by moisture in there from sitting too long.
this would mean a major engine rebuild with a reconditioned crankshaft. not an impossible job, even for a beginner with a good manual, but the end of the engine if it lets go!

my 2c,
cheers
les.