HONDA CT125 Big Bore Kit

Samg7 said:

Anyone got the spec for the yuminashi 143 ring gap?

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Here's the guide from JE Pistons that I used. Take your 2.205" Bore X Gap per Inch you choose and go with it. I used the High-Perf. Street-Strip as my target.

Thanks guys.

Stock rings measured 0.20mm top and bottom.

Yuminashi rings measured 0.20mm top and 0.18mm bottom. I left the top alone and gapped the bottom to 0.24mm.

Started her up last night with no problems.
EFIE today and I should be set.

Stock Piston vs Yuminashi 143

I've got 110 miles on the yuminashi 143 + S-238 Cam + EFIE @ 225mV

The initial 30 miles (@ 100-250mV on EFIE) I felt that the engine was getting too hot.
I changed the plug from NGK CPR6E to NGK CPR8E. Its a plug that should handle hotter conditions vs stock plug
When I made this change I also started carrying an infrared thermometer and a multi-meter thermocouple with me on rides

I don't know if it's in my head but the plug seemed to make a big difference. In power and temperatures
Could have been the ECM slowly learning as well

Anyway

Today after 30 minutes of 30-50MPH highway and gravel, my oil temp was about 190°F (the thermocouple was dipped in the engine oil via dipstick opening)
Following that, after 20 minutes of 1st/2nd slow speed but decent load trails, the engine oil temp was 215°F. Ambient temp was 76°F today

The grom guys seem to be saying 190-220 is a good range, with 240 the warning temp that something isn't right (too lean)
I figured they're measuring oil temp, not cylinder or head temp but @m in sc will know for shore

I also wonder if the CT125 has a higher heat tolerance than the Grom just from their intended uses
I think an actual oil temp gauge cloud be very helpful.. wish I could read the stock sensor


.

Very cool observations!

its fine. opmid does make an oil temp gauge that reads off the stock sensor. sounds like its doing just fine to me. theres other sources as well, webike, hard racing, etc. runnign oil temp will be slightly different as well, but not much.

I would like to take a look at my piston just for fun, but need to wait on new gaskets from Yuminashi. I have some new stock head gaskets, but they're for the original bore, not the 56mm.

Pretty happy with how the bike pulls now. I will wait until after 300 miles till I really beat on her and see the top speed.

bore scope camera, do it all the time w my 2 strokes

Yep. Off to harbor freight

Samg7 said:

Yep. Off to harbor freight

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If you want to save some money and reuse a smartphone screen, I've had good luck with this one: https://www.amazon.com/gp/product/B07C9C6P5D/
Higher resolution camera that captures more fine detail than the AMES ones at Harbor Freight.

home depot has one
as well for like 39 bucks.

Pulled the cylinder. Check this out. Just the hips of the skirt have worn down. Both sides. Cylinder has some markings but I think they will hone out. Can’t feel any edges on the skirt or the cylinder with my finger.

No wear on the piston above the oil ring. Only wear on the edges of the skirt

hats a typical 4 point (almost) seizure. thats too much heat.

Thanks for the info. The new piston I have measures 59.96mm at the skirt, whereas my old piston is 59.84. Measurement at the center of the skirt, where there is no damage.

feel free to tell me I’m high, but is there any value in gently sanding and reusing the old piston since it’s perfectly deformed now?

For a while I heard what I thought was very light piston slap. Started early on when I had the EFIE set to 100mV and the stock plug.
After the hone and new piston that sound is gone.

no problem doing that, we do it on 2 strokes all the time.

Are there big bore kits that keep a stock lower (9-10) compression ratio? I'd like to be able to run standard octane fuel.

Another question, what businesses produce large valve performance cylinder heads? Is there such a thing as a 125 4 valve head? ( Yes I know you might as well go big bore at that point. )

You can get the Finbro Big Valve Head, its a 2 valve. I got one on my 143 with takegawa n15 CAM and everything clears.

Originally I was going to go bigger than the 143 so i got the big valve head but decided to do 143 and avoid all the supporting mods that are suggested for the bigger BBKs. It probably helps around redline but im not sure it helps a whole lot in the rest of the rev range without the displacement.

I think Ive got another almost seizure. Felt a loss of power and waited for the bike to cool off and started the ride home. With the light piston slap noise. Happened at about 60 miles into the new piston, while riding at about 1/2 throttle going around 50mph. Measure the case temperature didn’t show excessive temps, just around 180F.

Will take apart tonight and get some pics of the damage.

Here is the 2nd piston with 65 miles.
Felt great until it tried to seize at the 60 mile mark.
The last piston that did this, it was also at about the 60 mile mark…