Do you have to deglaze cylinder? Or run as is? (Yz125)

[HERA]

57 minutes ago, morrison250r said:

    Then why does yamaha make 4 different size pistons if "they all work fine"? Clearly, there is only one right answer.

    Yz125 piston to wall clearance is

.0016-.0018 std with a .0039" changeout.

That's a fact. It's from yamaha.

That means there is only ONE piston that is going to fit right. 

.0018-.0016=.0002" or half the step between sizes.  

D might get you .0017" while A will be .0033" One is right. The other is wrong and awfully close to the .0039" changeout spec. 

OR

D will get you .0018" and A will get you .0002" does that sound "really does not matter.....they all work fine"? One is nearly already worn out while the other will stick as soon as it gets any heat.

 

On my B cylinder with 55 hours, i needed a D piston.

It's not very hard to measure. It is extremely easy. Even with a mic that only reads to .001", I can easily to half thousands. With a .0001" mic, I can read half tenthousands, easily. I have eyes.  You can get a mic for $30. Idk what if the shelf equipment is. Off your shelf? The average person's shelf? A yardstick or tape measure?

 The skirt clearance and oil ratio directly relate to sealing ability.  You cant take a 100 hour top end, re-ring it and push the same cranking psi. 

All this is right in the manual. The amount of bad information floating around this board is shocking.  I'm done helping. You guys refuse to help yoursel (oem manual+4th grade math) while at the same time shooting down my grade A advice. Good luck. 

Look man.. You have your opinion, I have mine, Yamaha has theirs... Whatever works for you... It's all good!

I did not say it was impossible to measure.. now did I?   But a snap gauge will NEVER get you even close to within .0005" in accuracy.. You need a dial bore gauge to get that kind of accuracy and those you do not pick up at Walmart.

AGAIN.. the RINGS do ALL the sealing.. truth be known, the rings never touch the cylinder wall directly.. And the RINGS are the SAME SIZE from the A piston to the D piston.. NO DIFFERENCE!  

The piston size difference from A-D will not effect the compression or running.  Did I mention the rings handle this task and they are all the same size?

The different size pistons (again.. very small in actual size difference) are present simply to handle plating tolerances to keep all in good specs and if any wear did occur from foreign material ingestion etc. etc.

Piston thrust loads and rocking are effected via the piston size. Runability is not effected by piston size at those small differences.

Anyway, not here to argue just helping with some known facts.

[2stroke>4stroke]

1 minute ago, HERA said:

Look man.. You have your opinion, I have mine, Yamaha has theirs... Whatever works for you... It's all good!

I did not say it was impossible to measure.. now did I?   But a snap gauge will NEVER get you even close to within .0005" in accuracy.. You need a dial bore gauge to get that kind of accuracy and those you do not pick up at Walmart.

AGAIN.. the RINGS do ALL the sealing.. truth be known, the rings never touch the cylinder wall directly.. And the RINGS are the SAME SIZE from the A piston to the D piston.. NO DIFFERENCE!  

The piston size difference from A-D will not effect the compression or running.  Did I mention the rings handle this task and they are all the same size?

The different size pistons (again.. very small in actual size difference) are present simply to handle plating tolerances to keep all in good specs and if any wear did occur from foreign material ingestion etc. etc.

Piston thrust loads and rocking are effected via the piston size. Runability is not effected by piston size at those small differences.

Anyway, not here to argue just helping with some known facts.

Mostly agree on that, you’d also say I’m fine with a c?

[YZDOC]

Wow, what an up and down thread.

Listen to reason, clean it up with oil and scotchbrite, then rinse, do this with bare cylinder PV removed. Cleanliness will be key upon startup. No Left over debris in any ports or passageways, use a little favorite lube to assemble. 

[2stroke>4stroke]

Just now, YZDOC said:

Wow, what an up and down thread.

Listen to reason, clean it up with oil and scotchbrite, then rinse, do this with bare cylinder PV removed. Cleanliness will be key upon startup. No Left over debris in any ports or passageways, use a little favorite lube to assemble. 

So I’ll clean it with an abrasive sponge with oil on it, then rinse it with warm soapy water, then put 2T oil on the bore (will it rust in the coolant passages?) and then reinstall the power valve. Done. That correct? 

[YZDOC]

That’s how I do it, but I use the blue Sponge pads. It doesn’t take a lot of elbow grease to clean them up. On the 125, you just want to make sure around exhaust ports, there is still nikaseal. Upper area is the most vulnerable on those little screamers. 

[2stroke>4stroke]

Just now, YZDOC said:

That’s how I do it, but I use the blue Sponge pads. It doesn’t take a lot of elbow grease to clean them up. On the 125, you just want to make sure around exhaust ports, there is still nikaseal. Upper area is the most vulnerable on those little screamers. 

will the green one also work it’s all I have rn? And I think the Nikasil is good the cylinder only got 60 hours. 

[YZDOC]

With a lot of lube you be fine. This area is what needs to look good. If it’s polished and aluminum looking instead of chrome shinny and no cross hatching, that’s the cylinder lining wearing out. 

[2stroke>4stroke]

1 hour ago, YZDOC said:

With a lot of lube you be fine. This area is what needs to look good. If it’s polished and aluminum looking instead of chrome shinny and no cross hatching, that’s the cylinder lining wearing out. 

Photos always make it look worse haha. Yeah it should be good but how come so much wear at 60 hours. 32:1 with good oil except once I mixed at a Gase station guessing, always clean air filter, jetting mostly right (cylinder head is nice and brown) always let the bike warm up except once. Broke it in nicely. Is it normal wear or did I get unlucky at some point...

Edited June 14, 2020 by 2stroke>4stroke

[YZDOC]

49 minutes ago, 2stroke>4stroke said:

Photos always make it look worse haha. Yeah it should be good but how come so much wear at 60 hours. 32:1 with good oil except once I mixed at a Gase station guessing, always clean air filter, jetting mostly right (cylinder head is nice and brown) always let the bike warm up except once. Broke it in nicely. Is it normal wear or did I get unlucky at some point...

125’s are a hirevengine, so it breaks down faster than the bigger cc bikes.

Should be fine, as photos never do anything justice. 

[2stroke>4stroke]

Just now, YZDOC said:

125’s are a hirevengine, so it breaks down faster than the bigger cc bikes.

Should be fine, as photos never do anything justice. 

Yeah I’ll clean it up tomorrow with the green side of an abrasive sponge and then just ride it. ?

[MrKTMrider]

On 6/13/2020 at 5:42 AM, 2stroke>4stroke said:

Hi,

So I am doing a top end on my yz125. The bike/cylinder has 61 hours. I have seen a post by Kevin from wiseco where he says that a cylinder has to ALWAYS be honed or deglazed for a new piston or else it will blow up in the first 20 hours. I have however also heard of people who don’t do anything to their cylinder if they can see cross hatching and there are no major scratches you can see or feel. What do you guys think/do?

You can still se cross hatching on my cylinder since it only has 60 hours. There are some vertical lines you can see but no scratches or chips you can feel. 
 

Thanks in advance!

I wouldn't loose my time cleaning it. As soon as the motor start it will be covered in oil anyway. One of my buddy did a few rebuild and never touch the cylinder. Just good inspection first hand. 

[rpt50]

On 6/14/2020 at 1:55 PM, YZDOC said:

Wow, what an up and down thread.

Listen to reason, clean it up with oil and scotchbrite, then rinse, do this with bare cylinder PV removed. Cleanliness will be key upon startup. No Left over debris in any ports or passageways, use a little favorite lube to assemble. 

I was just looking over this thread this morning.  The poor OP's head must be ready to explode!

[rpt50]

I think there's a larger issue here, and you guys are just killing the OP.  The larger issue is that it is impossible for regular guys to have a perfect bike every time we ride.  That only happens for factory MX stars who have a team that disassembles the bike every time it comes back in and they go over and check and/or replace every wear item.  For regular riders, it's impossible to maintain this kind of perfection, unless you never want to ride your bike.  I'm going riding today, even though my bike has wheels that are not perfectly round, tires that are not perfectly new, a cylinder with over 400 hours that is no doubt no longer perfectly round, linkage that has some slop, a radiator that used to be rectangular, brake fluid that needs to be flushed, and probably a bunch of other stuff that would give a factory mechanic heart palpitations!  I'm going to ride that bike, and I'm going to have fun!  Wash, rinse, repeat (and fix what breaks).  

The OP has a bike with 62 hours!  Let him put it back together and ride it!

[2stroke>4stroke]

1 hour ago, rpt50 said:

I think there's a larger issue here, and you guys are just killing the OP.  The larger issue is that it is impossible for regular guys to have a perfect bike every time we ride.  That only happens for factory MX stars who have a team that disassembles the bike every time it comes back in and they go over and check and/or replace every wear item.  For regular riders, it's impossible to maintain this kind of perfection, unless you never want to ride your bike.  I'm going riding today, even though my bike has wheels that are not perfectly round, tires that are not perfectly new, a cylinder with over 400 hours that is no doubt no longer perfectly round, linkage that has some slop, a radiator that used to be rectangular, brake fluid that needs to be flushed, and probably a bunch of other stuff that would give a factory mechanic heart palpitations!  I'm going to ride that bike, and I'm going to have fun!  Wash, rinse, repeat (and fix what breaks).  

The OP has a bike with 62 hours!  Let him put it back together and ride it!

Well written, thanks! Fully agree ?

Just puttin it back together rn ?

[2stroke>4stroke]

So it runs ? , I ran it just above idle for 2-3 minuets for the heat cycle. but I still have a few question:

when I cleaned the cylinder head I noticed ever so slight detonation on it, what can be the cause of this so I fix it before it gets worse? I run 32:1 with 95 pump gas, I run a br9evx or something like that it’s the one the manual recommends. Or did compression just get too low at the end?

when I put the cylinder on it was quite hard to compress the rings and put it over at once, the rings have to be compressed at a certain spot at the back and I am not sure if I perfectly did that. Like if it was perfectly compressed and the gap Should be at that spot but I don’t know if it was exactly there. But the cylinder went on just fine, it has compression and startet so it’s probably good right?

is it normal for the forged piston to run a bit louder than the cast piston, in the 3 minutes I ran it I noticed that the engine was just making louder noises than before, like the same sound of the cylinder but louder. I switched from the stock single ring cast piston to a forged double ring piston in case anyone is wondering. 

also I’m planing to do 2 more heat cycles, then ride it easy for 30 mins, medium/normal for 20 mins and then go through all the gears pinned to make sure all is good. So 1 hour brake in. Is that good enough?

 

sorry just want to make sure all is right 

Edited June 16, 2020 by 2stroke>4stroke