Here's a picture of the compression reducing "hole" modification that Yamaha used on some of the 1973 CT3 cylinders. There was a bulletin released on exactly how and where to place it. They also explained that it has no effect on the performance after the motor starts and reaches the recommended idling rpm's. If you look closely at the cylinder I did for YamaRod, you can see it.
This mod should prove useful for anyone who upgrades their 125 to 175 specs, and still wishes to use their electric start.
It looks so sweet. I just bought a head To go with it. Thanks again for all your knowledge which has educated me on the direction to go with this bike. Top notch work. I have a small lathe and mill but not a boring bar.
The cylinder will pretty much clean up with a 1st oversize piston set, and .0015" clearance.
I read on another site where you ask a question about running a DT piston in a CT cylinder .... but no one really answered you. Can you either give me the Yamaha part number of your DT piston ... and/or , post a few pictures of it.
I'm not sure where you are seeing the same part numbers for these two heads, but perhaps you have come across a mistake or error. The AT3 model has a three digit prefix number of 316, whereas the CT3 models begin with the model code 314.
So ... if you see a cylinder head part number that starts with the prefix number of "316"-11111-00, it means it's for the AT3 125 model. The CT3 head would be "314"-11111-00
The smaller AT3 and DT125's motors have a smaller bore of 56mm, whereas the 175cc models have a larger bore of 66mm. Naturally you wouldn't want to interchange these two different sized heads between models, or you'd run into compression problems (too much or to little) not to mention the damage that would likely happen ...... such as detonation.
So, sounds like you have come across an error in part numbers . . . which to tell you the truth, is not all that uncommen in Yamaha parts books and online parts sites.
I hope that makes sense and clears up any confusion created by erroneous part numbers that you may have stumbled upon.
i retrieved the cylinder today, but need to clean it up, measure, and determine if it will need a 1st or 2nd size overbore. I'll get back to you about it in a few days.
I checked my inventory sheets, and can pretty much confirm that I have one .....
The problem is the snow up here is very deep, and I probably won't get to it anytime soon. I'll post again whenever I have a chance to retrieve, examine and measure it.
I did this very same mod on the exact same bike and cylinder, however, I had ported the cylinder, which lowered the compression a bit.
it would turn it over and start fine, but a stock cylinder and head would likely struggle in turning the motor over fast enough to start. A manually operated compression release would then be the answer if you have trouble spinning it over fast enough.
Thanks for the explanation and education on the modification. Looking forward to getting this bike back together. Thanks again Enduronut
Here's a picture of the compression reducing "hole" modification that Yamaha used on some of the 1973 CT3 cylinders. There was a bulletin released on exactly how and where to place it. They also explained that it has no effect on the performance after the motor starts and reaches the recommended idling rpm's. If you look closely at the cylinder I did for YamaRod, you can see it.
This mod should prove useful for anyone who upgrades their 125 to 175 specs, and still wishes to use their electric start.
Here you go Rod .... The complete conversion with fresh 1st oversize and moderate stage 2 porting as requested.
I'm located in the Black Hills of So. Dak.
The cylinder will pretty much clean up with a 1st oversize piston set, and .0015" clearance.
I read on another site where you ask a question about running a DT piston in a CT cylinder .... but no one really answered you. Can you either give me the Yamaha part number of your DT piston ... and/or , post a few pictures of it.
Where are you located,I'm in Oregon. Lots of snow here now sounds like you deal with it also
Thank you sir
i retrieved the cylinder today, but need to clean it up, measure, and determine if it will need a 1st or 2nd size overbore. I'll get back to you about it in a few days.
I checked my inventory sheets, and can pretty much confirm that I have one .....
The problem is the snow up here is very deep, and I probably won't get to it anytime soon. I'll post again whenever I have a chance to retrieve, examine and measure it.
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Give me a few days, and I'll check if I have one.
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