YZ250 Piston Instead of Early 250R Piston?

[rustyATV]

I picked up a YZ250 recently and a friend handed me a piston (it's an '02 and later) he thought I could use in it. Just for grins, I compared it to my 250R pistons and was rather surprised by how close it is to the early style piston.

(250R on Left, YZ250 on Right, with YZ wrist pin holding them together)

http://img.photobucket.com/albums/v11/hoppy/250R/YZ250Piston1.jpg

http://img.photobucket.com/albums/v11/hoppy/250R/YZ250Piston3.jpg

Other than that, they seem to nearly be identical. The YZ250 piston is ever so slightly taller (maybe 0.010, but just eyeballing) and the wrist pin makes a tight fit in some of my 250R pistons.

http://img.photobucket.com/albums/v11/hoppy/250R/YZ250Piston2.jpg

Think it's worth using? I'm considering it when I go to put a new piston in my 250R because you can get YZ pistons up to 68.50mm's and they have the boost port that Duncan puts in his pistons for his PC2000 kits (which I have and I think are based on YZ250 cylinders anyway). Also the shorter wrist pin makes for less reciprocating mass (obviously). Seems to make a good fit onto the top end of the stock 250R crank, as well.

[Witz]

There are a number of engine builders who have used YZ pistons in the past. I think one of the concerns is the location of the anti-rotate pins for the piston rings. I think on a stock cylinder there is a concern of where those pins are in relation to the ports.

I've got an LA Sleeve powervalve cylinder built by LRD that uses the YZ piston. I'm not sure of all the differences, but I do know the spacer used for the YZ is thinner than the spacer for the 86 style 250R piston when using a long rod.

[rustyATV]

Originally posted by Witz
I've got an LA Sleeve powervalve cylinder built by LRD that uses the YZ piston. I'm not sure of all the differences, but I do know the spacer used for the YZ is thinner than the spacer for the 86 style 250R piston when using a long rod.

Which YZ piston is that? Is it the early 80's that's used for the big bore kits, or a late model like above?

I ask because in the pictures above I have a wrist pin going through both pistons and the difference in height is only about 0.010in.

I was wondering about the rotation pin location as well. I'll have to mark up the piston and try them out in my cylinders, but at first glance they look to be very nearly the same place, though the second piston ring is lower than on the 250R.

[C-LEIGH RACING]

The YZ piston Witz more than likely has in the LRD cylinder is the old 1986 model.
By looking at your compairison pics, I can tell you the old 86 model YZ piston is taller from the wrist up, than the late model YZ piston you are showing.

Normally when your useing the old model YZ piston to build a 265 or either a slite bigger bore on top of a long rod TRX crank, a 2.5mm spacer plate & two base gaskets would be needed.

To add a little more confussion, if your building a bigger bore engine, with the long rod as well, & going to use the old model 85/86 TRX piston, a 3mm spacer plate is needed & four base gaskets.
This is providing you wanting the piston to come up flat with the deck & not concerned with where the ports will end up being located down in the cylinder.


Rusty, I think you have found something that will help us all in the long run, thanks for that.
Neil

[Witz]

The YZ piston used in my 265 is from a '92 - '98 YZ250. The part number I'm using is Wiseco 677M06800.

[leatherface]

great thread. good information.

[Honda 250r 001]

so does it make better horsepower or something? for the stock cylinder. will it work without the rings tips getting caught in the ports?

[factoryX]

no, it was not a matter of making more hp, it was the fact of a replacement piston when 250r pistons are hard to get...

[wes350x]

I recently rebuilt my motor but my setup was a 265 with an 86' model yz piston, I believed was a 68 or 68.5mm, 2.5 mm spacer, decked cylinder. The piston was a Pro-X brand. It was strong for a 265 when I had it jetted half a$s. I recently bought a new carb to fix that problem for my ESR 310.

[Honda 250r 001]

hel pistons are not hard to find, at least for me. Aftermarkets are everywhere. But i can see pistons getting hard to find getting between 68.00 and 72.00mm

[rustyATV]

Took some more pictures. On the top of the two pistons I etched the approximate locations of the rotation pins and set them in a Duncan and stock 86 cylinder. My cylinders are all set up for 68 or 68.5mm pistons, so the stock YZ might not line up good enough. Regardless, the pins on the YZ piston do get close to the boost port. Too close?

250R piston in the Duncan.

http://img.photobucket.com/albums/v11/hoppy/250R/YZ250Piston4.jpg

YZ piston in the Duncan

http://img.photobucket.com/albums/v11/hoppy/250R/YZ250Piston5.jpg

250R piston in 86 jug

http://img.photobucket.com/albums/v11/hoppy/250R/YZ250Piston6.jpg

YZ piston in 86 jug

http://img.photobucket.com/albums/v11/hoppy/250R/YZ250Piston7.jpg


Originally posted by factoryX
no, it was not a matter of making more hp, it was the fact of a replacement piston when 250r pistons are hard to get...

Actually, I'm told that the "boost port" in the intake side helps top end power. Also, the shorter wrist pin would free up some energy in the form of reciprocating mass, which is the mass most detrimental to engine performance. Not a whole lot, mind you, but a little here and there can make a difference.

[C-LEIGH RACING]

The ring pin location on the right side dont look to bad, would probably work, but the left side is mighty close. Still though, you probably could use it.

Both those cylinders your showing are ported, right.
If you had one of the YZ piston in hand, before any porting was done, would be easy to not cut as much on the left side of the boost port to allow for the pin location.
On that same note, if you think having the main boost port smaller would be a problem on your build, you could drill the two Mac Dizzy port holes up through the corner of the secondary transfers for added boost.
Remember though, good bottom end pull port, you need to wad as much intake as you can in the back top side of the cylinder. More pop, more pull down low rpms.
Neil

[Langbolt]

Now if only we had a YZ cylinder to see how much overlap (distance from ring pin to edge of port) the Yamaha engineers allowed for.

:devil:

[rustyATV]

Originally posted by C-LEIGH RACING
The ring pin location on the right side dont look to bad, would probably work, but the left side is mighty close. Still though, you probably could use it.


Keep in mind the marks were just eyeballed, along with placing the pistons in the cylinders; they're just sitting in there, not assembled on a bottom end. I might go back later and see if I can line things up better.


Originally posted by Langbolt
Now if only we had a YZ cylinder to see how much overlap (distance from ring pin to edge of port) the Yamaha engineers allowed for.

:devil:

After I get my 250R together and worn in, my '02 YZ250 is well overdue for a top end job.

[Langbolt]

Great RustyATV!

Keep us posted.....I'm sure there's a lot of people that have wondered the YZ piston adaptation to a TRX for many years

Thanks

:devil:

[C-LEIGH RACING]

Putting different piston in a TRX250R engine could be kinda endless long as they have the right size wrist pin hole.

What about using a Yamaha TriZ 250 piston ??, or a Polaris trail boss Fugu engine 250cc 74mm piston, for the next size over once your past the last 73.50mm size in the big bore sleeved 295 TRX cylinders.
Neil

[HondaATC500X]

I had an LRD cylinder that was setup for the 83-84 YZ250 piston. Which is the same as the Tri-Z. The wristpin spacings are slightly different from the 250R, and you gotta use the thin spacer plate like C-LEIGH said. I didn't have any problems with ring pin locations, and the sucker ran good.

[250R_SV]

Hi folks, Im using a 250r suzuki piston 70.00 in my trx, it started a s a option because I didt have a honda in hand ans had a race in a few days, and it worked up, changed dramatically, more pull in the engine. has a lot of compression kick back sometimes if not hitted right. I have a 88 rod and oem dome .36mm carb. althounght sometimes its hard to start . I dont know if I need a bigger carb or a different dome or a spacer or something, any recommendations??

[regg187]

I recall back in the old days, that builders also used to move the ring locater pin around to fit their needs, while it would add 1 more step, it could save you a motor!

[rustyATV]

I'm actually trying to get my hands on 250 two stroke dirt bike pistons to examine them. Unfortunately, so far, nobody wants to give them up.

[factoryX]

I could ask for some junk pistons from the local moto shop...

[C-LEIGH RACING]

Which ones you looking rusty, Yamaha, Honda & what years.
Neil

[rustyATV]

So far I've only posted on a local dirt bike forum for any and all pistons, but got zero responses. I have to remember that most of those guys don't do their own work.

Calling shops: I might try that as well.

[Honda 250r 001]

Great information in this post. I just have one more question. Im going to try to put a yz 250 piston into my duncan racing pc2000 cylinder. 265. My one question is the area that the wrist pin bearing rides narrow enough to keep the bearing pushed inside the rod? Or will the wrist pin bearing slide back and forth too much?
Thanks
Will

[C-LEIGH RACING]

It will stay in place.
Those old YZ pistons been used in TRX engines for years & no problem with the top end bearings.

Now, as far as using a YZ piston in that Duncan PC2000 cylinder, you need to get the measurements from the piston that is in the cylinder right now, or either post what the numbers are on the top of it so we'll know which piston it is.

Theres small differences between the YZ & the TRX piston, which will result in how much the cylinder is spaced up or down.
Neil

[Honda 250r 001]

The piston that was in the cylinder when i bought it was just a regular 86 trx piston. 68.5 size. So my plan was to get a 02 YZ 250 piston, 68.5 and play around with different base gasket thicknesses to get the squish and bdc piston ports to line up.

[C-LEIGH RACING]

Ok, so if that Duncan PC2000 cylinder is machined just like an OEM TRX cylinder, from the base up to the deck, then a 2.5mm spacer plate would be needed, if you have the new model long rod crank.

I cant remember, but did the PC2000 cylinders, use a base spacer plate, or did they come machined so that no spacer plate would be needed with the long rod crank.
Neil

[Honda 250r 001]

Yes you are correct. I keep thinking I have a 86 crank in mine.
I dont believe these cylinders were machined to have the spacer built into them.
Thanks
Will

[C-LEIGH RACING]

Ok, so take a measurement of the spacer that is being used & you'll probably find it is 3mm thick, so to space the cylinder to the right height.

YZ piston, the wrist pin hole is just a hair higher up in the piston than in a TRX piston, so the spacer needs to be thinner so the piston top will come up flat with the deck.
Neil

[Honda 250r 001]

Originally posted by C-LEIGH RACING
Ok, so take a measurement of the spacer that is being used & you'll probably find it is 3mm thick, so to space the cylinder to the right height.

YZ piston, the wrist pin hole is just a hair higher up in the piston than in a TRX piston, so the spacer needs to be thinner so the piston top will come up flat with the deck.
Neil

What year YZ 250 piston are you talking about using?

[C-LEIGH RACING]

Most that have used the YZ piston will say 86, but 1983 to 87 uses the same piston.
Neil

[Honda 250r 001]

I was talking about using a 02-10 YZ 250 piston. I orderd one in 68.5 for my duncan cylinder to give it a try. Has anyone else tried the new YZ piston on a 250r?


OKAY. Got the YZ 250 piston. Boy the rings are close to popping out into the ports but I think it will work. When I put my pistons together with the wrist pin, I found that they were the same height from the wrist pin up, with the YZ 250 piston being just slightly shorter below the wrist pin.
One thing I noticed is the YZ piston is MUCH thicker around the skirt area, which I believe should be a good thing. Almost double the thickness at the bottom of the skirt on the intake side.
Once I get the cylinder bolted on and run it for a while Ill post back up.