YZ125K Rebuild

[alexbrown64]

Just dropped the cylinder etc... of at the bike shop.  He said he has seen it all before. Should be a looser fit because of the steel lined cylinder and forged piston.  Its going to the machine shop to be checked out and worked on. 
Thanks for all the input fella's and i will get back with results and new ride report when i get it all back.

Cheers,

[sleepy]

Those will do the trick.

Yes the bore can be honed to size. When a cylinder is bored it should be left .003" under the finished size and then honed to final size. 

[Lozza]

Go to your local tool supplier and buy a cheap 50-75mm micrometer and matching telescopic gauge. The feeler gauge method is quite inacurate.

Telescopic gauge in the hands of 3 different people will give 3 different measurements. I can measure a 0.01mm difference in a bore with feeler gauges, that tells me a measurement(of then clearance) the cylinder bore gauge tells if the cylinder is round and parallel.

http://www.ebay.com.au/itm/Metric-Dial-Bore-Gauge-50-160mm-Cylinder-Internal-Bore-Measuring-Engine-Gage-/161452232262?hash=item25974d8e46:g:ekMAAOSwwbdWPbgZ

Had a couple o cylinders done by Greg Ball when I lived in Sydney an both have clearance around 0.9mm. Actual bore sizes are 55.55 and 56.05. I checked both with bore gage just to know what the cylinder guy did. Never had any issues. Cylinders are runnig wossner pistons.  If you can just push in 0.8 feeler around top of the piston you cylinder is too tight at the bottom (skirt). Take any piston a measure it with a micrometer when you get a chance. You wil be suprised to see where the size of the piston that is printed on the box comes from.

You don't measure the piston at the top, as it has both cam and taper on it. The piston is sized 10mm up from the bottom of the skirt front to back. If you have to run large clearances your giving away free hp.

[Tomas]

That micrometer is of very good quality. It is Russian made. I have a few of that brand.

[alexbrown64]

Its been a long time...

Finally got back to the bike after work, health and other project commitments.  It was honed, piston and ring checked to be in spec, and run in again.  Got the old girl out for a run and it seized again.

I have taken it up to Ray Easson in Perth and he has the cylinder and everything now.  I told him to put a ProX cast piston in and replace the forged Wossner.  He is going fit a new piston and make sure it is all in spec.
He also said it sounds like i have to go up some on the main jet.  So, i will check my settings and go up as far as i can go on the main.

Merry Xmas to everyone and hope you all have a fun filled and accident free holidays...

Cheers,

[Lozza]

  It was honed, piston and ring checked to be in spec, and run in again.  Got the old girl out for a run and it seized again.

  ....................... put a ProX cast piston in and replace the forged Wossner. 

There endeth your piston drama's, any pics of the piston? If you have lots of vertical lines on the skirt(exhaust side mainly) with the crown in perfect or not even coloured condition you have been affected by the "wossner curse"

[evo550]

  It was honed, piston and ring checked to be in spec, and run in again.  Got the old girl out for a run and it seized again.

  ....................... put a ProX cast piston in and replace the forged Wossner. 

There endeth your piston drama's, any pics of the piston? If you have lots of vertical lines on the skirt(exhaust side mainly) with the crown in perfect or not even coloured condition you have been affected by the "wossner curse"

What is this Lozza ? I have a 10 hr Wossner in my sons yz 125 that fits this description...

[alexbrown64]

Piston looked like new except for the vertical lines on the 4 sides.  Maybe it was the Wossner curse... anyway, i hope going to a larger main jet and new ProX piston will solve my problem and i can ring the bikes neck without it seizing on me....

[Tomas]

Had the same issue with Wossner piston on my RM 125. It was not seized but had scoring on piston wher exhaust port is. Just pulled the cylinder after 3 tanks off to see how it looks and notised scoring on piston on exhaust side. Bike is running on ritched side because it is spitting some oil out. Using sintetic Repsol two stroke oil at 25:1. Would this be a bad batch of pistons or is this something that Wossner pistons normally do?

[80-85 husky]

woosners known to need 1 - 2 thou more than stated when final honing. std issue on café husky...lots of woosner siezes

[skypig]

Two things:

1. This thread cost me $18 for a set of the telescoping gauges. (Who knows what quality -$18 delivered!)

2. As I'm sure is mentioned somewhere in the thread: Forged Pistons generally need different (more?) clearance than cast, and sometimes a lot longer to warm up before hard use. Not long enough warm up was blamed for the Wiscos seizing in a Jetski I owned. Maybe BS. But possible due to expansion rates. Cast Pistons seem popular in two strokes, even at the high end. Forged are favored in "Diesels".

[80-85 husky]

in my experience, oriental bikes used cast pistons and lots of them while euro's used forged at great expense but more life.
eg kdx 250 raced in 84 used two pistons and ring sets for the season. the next year I raced a husky and didn't replace the piston until the end of 86 just a set of rings in late 85. and I didn't  need to bore it, just a new piston.

this comes unstuck if you shag an expensive piston. ol mate on an 84 a/c 250 husky did 3 pistons and rebores in 3 months until we found 1 head bolt bottoming out in the barrel before the head was tight. much unhappiness....

[alexbrown64]

Some interesting comments fella's and all taken onboard.  I had a call from Ray Easson and he said he had measured everything up and it was actually a loose fit.  So i was OK on that side of it.  He can't get the ProX from his supplier anymore, so i am going to send him a NOS Yamaha piston kit. 
Now, as stated before, i can run this bike with my other cylinder, Wiseco piston (biggest oversize), and cylinder head, and it will run all day.

I put the new sleeved cylinder on with the Wossner as a loose fit, and it seizes. Ray said its apples and oranges as the two cylinders are different as the new one is ported.
So, my question is, what would be the best way to go to make this new cylinder safe from seizing. I am going to put in a larger main jet, but what about timing.  It has the digital PVL at 1.2mm btdc.  I was thinking of retarding the timing to 1.0mm btdc as Eric Gorr states..

 "Retarding the timing will make the power band smoother in the mid-range and give more top end over rev. When the spark fires closer to TDC, the pressure rise in the cylinder isn't as great. The emphasis is on gaining more degrees of retard at high rpm. This causes a shift of the heat from the cylinder to the pipe. This can prevent the piston from melting at high rpm"

Anyway, thoughts gentlemen?

Cheers

[Lozza]

woosners known to need 1 - 2 thou more than stated when final honing. std issue on café husky...lots of woosner siezes

Here in lies the problem, when wossner state the clearance should be 0.05mm why the fork would you need to give a piston WAY MORE  clearance Wossner themselves state just so it doesn't seize? Other manufacturers seem to be able to make piston that run perfectly with the clearance they state on the box.

  It was honed, piston and ring checked to be in spec, and run in again.  Got the old girl out for a run and it seized again.

  ....................... put a ProX cast piston in and replace the forged Wossner. 

There endeth your piston drama's, any pics of the piston? If you have lots of vertical lines on the skirt(exhaust side mainly) with the crown in perfect or not even coloured condition you have been affected by the "wossner curse"

What is this Lozza ? I have a 10 hr Wossner in my sons yz 125 that fits this description...

If it had a mild to full seizure the first or every time it gets hot even with rich jetting it's the Wossner Curse'

Had the same issue with Wossner piston on my RM 125. It was not seized but had scoring on piston wher exhaust port is. Just pulled the cylinder after 3 tanks off to see how it looks and notised scoring on piston on exhaust side. Bike is running on ritched side because it is spitting some oil out. Using sintetic Repsol two stroke oil at 25:1. Would this be a bad batch of pistons or is this something that Wossner pistons normally do?

Would seem so

[rocketfrog]

Is there any known issues with Woosner 250 pistons in Nikasil cylinders that I should know about or is the curse only on the smaller bore sizes? I have never used one but I have one to fit my YZ250D1 (1992) should I refrain and go a OEM or Pro-X cast piston? I have not heard too much good or bad about Woosner but this sounds like a road I dont want to go down with a Nikasil barrel - or at all for that matter!!

For what it is worth, I am tipping that the OEM piston will work fine Alex. I have never had any issue with a stock piston.