Blue Printing my 08 Yz125 Top End

[Ringdading]

I got this 125 on trade near the end of the season. I rode it at the last couple races and have been practicing at the local indoor track this winter. I just logged 25 hours on it so it's time to do a top end. While I'm in there I'm going to blue print the top end and get it ready for the coming season. Over the next week or two I'm going to post what I get done so let me know what you guys think and any tip, tricks or suggestions you might have!

I tore the bike down Tuesday and before I started I did a compression and leak down tests on it to see how health it was.

Comp: 172psi

Leak down: 1 pound in 30s (this is way to much it was leaking around reed cage on both sides)

Once I got it apart I found a couple things I didn't like!

First off the powervalve pin rattled loose and the right side powervalve stopped working and came down and hit the piston causing some scaring on the piston on two of the corners. As bad as the piston looks it still ran greAt and the cylinder survived.

Second is the what I'm tore between to be to lean or just right for peak power wash and piston carbon build up.

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On Wednesday I started to clean everything up and measure things. I didn't bother to measure the piston because there is no way I was to use it.

Cylinder: I cleaned it all up and ran a hone down through it to break the glaze. Then I used my dial bore gauge to check to out of round and taper. They both are well within the .010mm spec.

Out of round: .004mm

Taper: .006mm

Bottom end: I checked all the bearing for play and they all feel good. I also measure the big end rod side clearance which Was also in spec.

Side clearance: .017"

Since the cylinder looked good I ordered a 05-13 YZ125 Wiseco GP racers choice piston kit.

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Now the fun started today. I got to start cleaning up the ports. I'm not going to be changing the shape or heights just cleaning everything up. I have pictures of the stock castings and what they look like after the rough work is done. I'm going to have to wait until the piston and gaskets get here before I can make sure all the ports are even and the gaskets are all matched up.

The last two picture are of my two stroke porting set up. It works pretty good.

[Ringdading]

Okay so time for an update. I've been working on it little by little and here is where it's at.

First off I received my wiesco GP piston kit the other day. The thing looks awesome! I'm amazed at the quality of forging done on the GP compared to the pro lite pistons!

The recommended ring end gap is .004" per inch of bore and since my bore is a little over 2" perfection would be .009". Although I am blue printing the top end I am stuck with the ring I got so I am a little wide at .012"

Ring end gap- .012"

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Next since I got the piston kit I was ready to start taking all kinds of measurements such as bore, stroke, combustion chamber cc, crankcase cc, piston pin offset, port timing, duct length and area, dimensions of the ports, reeds, pipe Etc etc.

As I gather the info I input it into MOTA which is a two stroke tuning software. Once I gather all the information I am able to run a simulated dyno pull and get all sorts of info such as pressure wave activity and timing in the exhaust, piston speed, hp, torque, port duration and so much more.

So I ran the stock info and got a nice torque and hp curve shown below. In nice yz fashion it had 1500 rpms of usable power. That's great for a 125.

Next I address the couple problems that I ran into during the beginning rough port job. I didn't like the lip on the exhaust and I wanted to true up the base and top. So I input the changes I wanted to make to see what effect it would have on the motor. Of course since I'm raising the exhaust port there was a slight lose of bottom end but man does the top come alive! 1/2 hp gain at peak with almost 1 hp gain at sign off! I will definitely be performing these mods. You can see the comparitive graphs bellow.

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I machined the bare minimum off the base and the top of the cylinder to make it true. I use an expanding mandrel on the lathe.

It went very well so next I assembled the top end to check the squish. The minimum squish for a 125 is .7-.9mm. Again with very minimal machining a measurement of .84mm was achieved.

Squish clearance- .84mm

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It's been a couple weeks since I made a post. I got it back together and was to busy riding it and rebuilding the suspension to finish the post.

So here is the rest of the build. The next thing I did was did the finish work on the ports by polishing them with 60 grit.

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The last step before assembly was to match the cases to the gasket and the cylinder. As you can see they were way off!!

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Now for reassembly. The first problem I had to take care of was the pin that fell out of the powervalve. I had a friend of mine find a pin that was .002" bigger than the one that was in it. Then I pressed it together an it seem to be pretty solid. I'll give an update of how long it stays in.

Once I got the cylinder on I measured the final port heights and I was very close to my estimated measurement and found the graphs to be identical to the ones above.

I finished the assembly and richend the pilot jet two sizes from the lack of piston wash. I also repacked the muffler.

During break in it seemed to run good . I'm in pa so I'm stuck riding the indoor track until the snow goes away but hey some riding is better than none!

If anyone has any comments on the build let me know what you think! Thanks

[souliog]

Thanks for sharing... Looks good so far.

After talking to a 125GP engine tuner/racer, I wish I went tighter than 0.050" squish on my 250. He recommended 0.035". Maybe next rebuild.

[SmokinJoe90]

So is your bike all nothing but top end power now looks like it's been overly ported the s outta it.

[adam728]

So is your bike all nothing but top end power now looks like it's been overly ported the s outta it.

How is it overly ported? He said he hardly changed the porting at all, just cleaned things up, and the simulation he showed has almost equal power until the top end, where his setup now should pull a little higher number and more over rev.

[Ringdading]

The guidelines in thousands I go by for squish are:

100-150- .028"-.036"

150-250-.040"- .055"

Mine is at .084mm which is .033".

And no my bike pulls pretty well from the bottom for a 125. The ports are all stock with minimal metal removed to clean it up. The exhaust port is raised around .40mm which only has an effect when the exhaust valve opens.

[Bremer120]

The last step before assembly was to match the cases to the gasket and the cylinder. As you can see they were way off!! 📎ImageUploadedByThumper Talk1394164551.399031.jpg📎ImageUploadedByThumper Talk1394164567.355889.jpg📎ImageUploadedByThumper Talk1394164594.820309.jpg📎ImageUploadedByThumper Talk1394164607.317654.jpg

I was gonna gasket match the cases on my 250 on the last build because, like yours, it was WAY off. However I talked to a guy in the area that ports all the Yamaha's and he told me to leave it alone. He said he has tried it in the past on several bikes without much gain and I think he mentioned the lip serves a purpose to create a swirl or dead spot? I don't know. But obviously if your bike runs good it must be fine. Nice build. Not very man people blue-print or play with things.

[naturaledge]

Great thread - thanks for sharing ?

[souliog]

The guidelines in thousands I go by for squish are:

100-150- .028"-.036"

150-250-.040"- .055"

Mine is at .084mm which is .033".

And no my bike pulls pretty well from the bottom for a 125. The ports are all stock with minimal metal removed to clean it up. The exhaust port is raised around .40mm which only has an effect when the exhaust valve opens.

 

How about squish band width? My math comes out to 9.724mm on a 250. Advice?

(50% area of 66.4mm Bore)

[Ringdading]

That's a different view on gasket matching. The theory I was going off of it the the lip covered an 1/8 of an inch of my ports. To me that decreases flow to an extreme. My second thought is that was a lip would cause swirl but in my opinion to much. The man that taught me how to port cylinder heads built very high hp drag strip door cars. He said that most people get to carried away with the swirl idea and make the ports to rough. Air will swirl without disrupting flow from a 60 grit polish job.

Now with saying that I didn't run the bike on a dyno before and after just doing that mod so it's possible that it the lip is there for a reason that is beyond me and only Yamaha knows the secrets of. I would love to see someone run a bike just performing that mod and letting us know the results.

Then the guidelines for squish band width I go off of is a very broad 40-60%. I try not to mess with it to much because Yamaha put a lot of r&r into the width and shape.

When I do mess with it is when I build a big bore or switch from a dome piston to a flat top. I know there is a lot more to it but I use it as guide. I get in between 40 and 60 and make sure the comp ratio is right. I also stick with the stock angle or decrease it if going with a flat top.

There is a program that some company makes that is around 200 bucks that figures out the science behind cylinder head design and helps solve the width and angle mystery. I don't have it yet but will probably get my hands on it some time or another.

[Bremer120]

That's a different view on gasket matching. The theory I was going off of it the the lip covered an 1/8 of an inch of my ports. To me that decreases flow to an extreme. My second thought is that was a lip would cause swirl but in my opinion to much. The man that taught me how to port cylinder heads built very high hp drag strip door cars. He said that most people get to carried away with the swirl idea and make the ports to rough. Air will swirl without disrupting flow from a 60 grit polish job.

Now with saying that I didn't run the bike on a dyno before and after just doing that mod so it's possible that it the lip is there for a reason that is beyond me and only Yamaha knows the secrets of. I would love to see someone run a bike just performing that mod and letting us know the results.

Then the guidelines for squish band width I go off of is a very broad 40-60%. I try not to mess with it to much because Yamaha put a lot of r&r into the width and shape.

When I do mess with it is when I build a big bore or switch from a dome piston to a flat top. I know there is a lot more to it but I use it as guide. I get in between 40 and 60 and make sure the comp ratio is right. I also stick with the stock angle or decrease it if going with a flat top.

There is a program that some company makes that is around 200 bucks that figures out the science behind cylinder head design and helps solve the width and angle mystery. I don't have it yet but will probably get my hands on it some time or another.

I completely agree with you. I believe matching ports is free horsepower and should be done. I too build V8 motors and gasket matching intake and exhaust is a must on any build. I too would like to see a dyno comparison with just grinding off the case.