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Last year I made many mods and additions, including a 143cc big bore top end from MNNTHBX. I already had a cold air intake, lightened primary, PC and auto tuner, and Graves exhaust. Dyno tuning had this set-up at 14.8 HP. As you might remember, I started experiencing an oil leak on the top of the crankcase so I knew I’d have to strip it down over the winter to figure it out. Since I would be splitting the cases, I decided to add the stroker crank and get the displacement up to 155cc.

Several other mods were done as well. Steady Garage has a newer high compression piston that coupled with the longer stroke gives a 12.2 to 1 compression ratio. I had noticed some power loss last summer in hot temperatures, so figuring that can only get worse with the new mods, I decided to upgrade the oil system with a Morin oil cooler, a high-volume oil pump, and a metal Scotts oil filter. Also added a Nology ignition coil and plug wire feeding an NGK iridium plug to aid with cold starts of the high compression mods.

There were a few issues that required mods. The bracket that holds the oil cooler to the top head bolts was fairly crooked and I had to redrill the head mount holes to get it straight. More importantly, the interface piece that connects the hoses to the block via the filter slot would not line up. One of the bolt buttresses on the block interfered with this piece. A few grinds with my Dremel tool fixed it in a few minutes, but this combined with the bracket was definitely annoying. And then when fitting it all together, the metal filter plain and simple would not fit with the new coupler and the original filter canister. Luckily I had a spare stock filter so I used that. For my first oil change I’ll try a K&N filter. Everything else went on fine and all fittings went on tightly.

Took it out a couple of weeks ago for a couple of shakedown runs, making very sure there were no oil leaks. The fact that the oil cooler lines are right behind the front wheel really has my attention and I want them perfect and reliable. Nothing wrong so far.

77 degrees last Friday so I got in a 17 mile run on the local back roads, stopping several times to again check for leaks. I would like to get it dyno tuned again after these mods, but the shop is closed due to the virus shutting virtually everything down here. But as to a seat-of-the-pants observation - this bike FLIES. The 155cc kit with all the other mods and the extensive weight savings (weighs 199 lbs with half a tank of gas) is extremely impressive, and I’m looking forward to some more riding soon this spring. When I do get a dyno run I’ll provide comparison numbers to before the crank and piston.

Till then, safe riding, and stay healthy...

JW
 

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Do you know of any good step by step guides on installing the 143cc kit (head, jug, and piston)? I want to do this for my bike, but I'm a little scared of screwing something up. It doesn't really seem to terribly tough though, from what I've read and seen so far. Congrats on your mods, you have a cool bike!
 

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Discussion Starter #3
Do you know of any good step by step guides on installing the 143cc kit (head, jug, and piston)? I want to do this for my bike, but I'm a little scared of screwing something up. It doesn't really seem to terribly tough though, from what I've read and seen so far. Congrats on your mods, you have a cool bike!
Thanks for the compliment. As I’ve said before, I like working on these small machines almost as much as riding them. Almost... :wink2:

And with that being said, this style of bike is as easy as it gets to work on theses days. Of course the old air-cooled 2-stroke dirt bikes I started on were the easiest, but these really aren’t much harder. Even removing the entire engine (which you don’t need to do for a top-end job) can be done easily without assistance.

Recommendations -
* Know your skill set and plan appropriately. If you’ve never done ANYTHING on an engine, try to team up with someone who can guide you in person. Get in good with your local dealer. You may have to bring them a box of parts. :surprise:
* All videos should be taken with a grain of salt. Way too many self-proclaimed experts out there that skip steps or don’t discuss pitfalls. There’s a classic out there of a Z125 top-end job where he LEAVES A PAPER TOWEL IN THE CRANKCASE under the piston and puts it all back together without noticing it!!! Worth looking this one up for the entertainment value alone.
* With the above comment, I personally don’t know of any good step-by-step Z125 videos. Maybe others here do. Although not the same, Hard Racing has some excellent Grom big bore videos on YouTube. I would recommend these if you just want an idea of what the process is like.
* Have a good selection of quality tools, including a torque wrench, at your disposal.
* Follow good mechanic practices. All moving parts need to be oiled as you assemble them. After assembly with your oil refilled, turn the crank several times by hand to make sure you have valve clearance and oil starts churning. Keep everything as clean as you possibly can.
* Buy your parts from a reliable place that we all know and love. I’ve bought various items from Hard Racing, Steady Garage, and MNNTHBX, the latter from where I got my big bore kit. All places have been good at answering any questions I have had. As you point out, there are virtually no instructions for any of this stuff, even reliable information on exactly what the valve clearances should be really could be easier to find. ASK FIRST IF IN DOUBT.
* And speaking of valve clearances, you should be very comfortable with reliably setting them. That and making sure you get the timing chain on properly. Your engine depends on you getting these perfectly correct the first time.
* The valves in my head kit were installed, but the cam and rockers were not. The tight fit of the cam bearing in the head required me to freeze the cam, and heat my head in the oven for 30 minutes before sliding it quickly in. Are you comfortable with this? If not, you should bring the pieces to your dealer.
* The one thing I still psych myself up to do is when it’s time to get the piston and rings into the cylinder for the first time. Take your time!

So hope this helps for you and anyone else thinking about doing work like this. Maybe a bit long-winded, but that’s my style and I’m stuck in my house now. It’s not “hard”, but that is a relative term for folks on this forum. And in the end, the only way to learn is to do...

JW
 

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Thanks JW. Just a quick couple of questions:

What did you set your valves to for your build and do you think that they will be the same spec as just the 143cc kit?

What temp did you set the oven to for the 30 mins and how long did you freeze the cam?


Thanks again for all your help and info, it's much appreciated!!
 

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Good info In here, the crank is well worth the upgrade, the little bike lack torque and the crank helps a lot in the department
 

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Discussion Starter #7
JW, Love the rear seat cowl. Where did you get it? Thanks in advance for the reply.
Hi, I bought it on eBay from an outfit in Thailand about a year and a half ago. Took a LONG time to arrive but it is nicely made and a very good color match. I am using a Dark Knight Street seat, also from Thailand. This caused some issues as it’s designed for use on the stock seat. Mods to both the seat and the cowl got it to fit and look good. I have not seen these folks on eBay for some time, and just looked now. I’d suggest monitoring eBay occasionally, searching on ‘Z125 seat cowl’. Good luck!

JW
 

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Discussion Starter #8
Thanks JW. Just a quick couple of questions:

What did you set your valves to for your build and do you think that they will be the same spec as just the 143cc kit?

What temp did you set the oven to for the 30 mins and how long did you freeze the cam?
Hi, the 143cc head I run has the valve clearances specified at .003" intake and .005" exhaust, verified with MNNTHBX at the time. I did the top end last year and when I rebuilt over the winter, the clearances had not changed at all after 1400 miles of running.

I had my head in the oven at 250 degrees for 30 min. (That does NOT sound good on proofreading...) Don’t overdo it or your oven and house will smell of motor oil, as there is a basic coating on the parts as shipped. Put the cam in your freezer for about the same amount of time. It’s much smaller than the head itself obviously, so it will cool fast. Note - this is real bachelor stuff here. Don’t blame me for any ‘unexpected consequences’... :grin2:

BTW - Can’t believe I wrote all that before and failed to mention a very important point - get the Z125 Service Manual! I have a hard copy ordered from the Kawasaki web site, but I believe it can be accessed online. Very helpful...

JW
 

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Discussion Starter #9
Good info In here, the crank is well worth the upgrade, the little bike lack torque and the crank helps a lot in the department
With the 143cc top end only, I had a bit of a flat spot in torque around 5-6K RPM. That is noticeably gone now. Hope to confirm on the dyno someday.

JW
 

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With the 143cc top end only, I had a bit of a flat spot in torque around 5-6K RPM. That is noticeably gone now. Hope to confirm on the dyno someday.

JW
Mine was similar with the 165cc and big throttle body, the crank definitely help, I ported the head on mine, added a v2-2 cam it showing some nice gains in the draggy really need to find a place local to tune it
 
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