PLEASE HELP !! wiseco CR250 forged piston in newly nikasiled cylinder

[savage sausage]

Ok guys I'm after a bit of help or advice before my full engine rebuild

bikes a 98 ec250 six days
bigend bearing we nt so have stripped the engine and the cases are being ported and cleaned up and the crank is away to have a new rod pressed in it.

my question is after speaking to jim cook he advised me that a CR250 piston will fit my cylinder but MIGHT need a little honing.

this is what jim wrote to me in a PM

The first thing you need to do is to remove your cylinder and slide the CR piston into the bore (without rings) and measure your clearance on one side with a blade type feeler gauge. If you have at least .015", you are fine. I would simply use a Scotch Brite pad and WD40 to clean and scratch the bore a bit.
If you need more clearance than you presently have, then hone a few seconds and do another measurement. Repeat this cycle until you do have enough clearance for the piston to not lock up as it expands

Ok being honest im not familiar with this so I'm a little apprehensive about screwing it up on my freshly nikasiled cylinder.

I managed to score a BNIB wiseco forged CR250 piston for 60 pounds which was a result .....however doing some research a 'forged' piston expands more than a std cast alloy correct.

PK1128/$208.86/66.40mm STD Displacement: 249cc Stroke: 72.00mm Notes:Pro-Lite, Nikasil Bore

what type of clearance should I be looking for as I beleive jim's answer was for a std cr250 cast alloy piston.

man this 98 250 six days is a proper museum piece and maybe some of you have seen what I've done to it since I bought it 'project el diablo' .....being honest hardly no riding and just replacing worn out parts i've probably spent in the region of 1900+pounds including the bike cost at this time

so getting rid is NOT an option plus my sis in canada has offered to pay for the bottom end rebuild so i really dont want to put it all back together all shiny and new for it to seize not long after because I was in a hurry to basically make a 17 soon to be 18 year old ride .....rideable.

im desperate to ride it an with a lot of bad luck i've began to lose faith however this old dog aint ready to die yet.....im 40 years old and im gonna ride it like a teenager when it's back together.

thanks

[Zman]

I put a cr250 piston in a well worn cylinder that I had diamond honed. The cylinder was bigger than stock but I don't recall the actual dimensions. I would be afraid the new plated cylinder may be tight with the CR piston.

[gasgasman]

Usually, the plating is done to match the piston.

Try taking the cylinder and piston to an automotive machine shop and have them measure the cylinder bore and piston diameter.
That would give the best clearance measurement.

[savage sausage]

thanks for the replies

ok so I emailed the company in england I used to replate my cylinder at the time of order I requested the plating to be able to fit a 66.40mm piston

here's the response

Hi Simon,

I have emailed you your invoice separately. I can confirm that your
cylinder was finished to 66.40mm.

Regards
Claire Howell
for Langcourt Ltd

so is it just a of putting it in or is there still work to be done

[hamilton]

You need to check your piston to wall clearance and ring gap.
You can find the spec in the GG service manuals.
Better to be sure its within spec.

[Moto7man]

Originally Posted by hamilton

You need to check your piston to wall clearance and ring gap.
You can find the spec in the GG service manuals.
Better to be sure its within spec.

Exactly, verifying the correct ring endgap is crucial. A while ago, I installed a wossner piston in my 300 and I set the ring endgap at .4mm. I don't know what it is on your model 250, so make sure you check. I also made a leakdown tester to check the cylinder for air leaks after assembly. See photos on the thread below starting at post number #7,

http://www.gasgasrider.org/forum/showthread.php?t=19892

[Brian VT]

He is wondering if specs. for a forged piston may be different.

[RBrider]

Originally Posted by savage sausage

my question is after speaking to jim cook he advised me that a CR250 piston will fit my cylinder but MIGHT need a little honing.

this is what jim wrote to me in a PM

The first thing you need to do is to remove your cylinder and slide the CR piston into the bore (without rings) and measure your clearance on one side with a blade type feeler gauge. If you have at least .015", you are fine. I would simply use a Scotch Brite pad and WD40 to clean and scratch the bore a bit.
If you need more clearance than you presently have, then hone a few seconds and do another measurement. Repeat this cycle until you do have enough clearance for the piston to not lock up as it expands

Ok being honest im not familiar with this so I'm a little apprehensive about screwing it up on my freshly nikasiled cylinder.

I managed to score a BNIB wiseco forged CR250 piston for 60 pounds which was a result .....however doing some research a 'forged' piston expands more than a std cast alloy correct.

PK1128/$208.86/66.40mm STD Displacement: 249cc Stroke: 72.00mm Notes:Pro-Lite, Nikasil Bore

what type of clearance should I be looking for as I beleive jim's answer was for a std cr250 cast alloy piston.






thanks

Refering to Jim's comment specifying a minimum of .015 should read .0015, (or, 1 and one half thousands)

I just installed a new "B" Wossner piston kit in my '01 XC 200 and went thru this.

The online Gas Gas shop manual specifies a .002 clearance for the stock piston (Or, .051 mm if using metric system). But, the manual says up to .004 is acceptable (.102 mm metric)

So do like jim says and insert the piston, without the rings, right side up and see if you can slide the .002 feeler gauge blade between the bottom of the piston and the cylinder wall. If it won't fit, then try the .0015 that Jim recommended. If that'll fit then you should be OK.

When I measured my piston clearance, after checking the gap near the bottom of the cyl, I turned the piston upside down and reinserted it in the cyl and check the clearance there, as well.

If ,002 clearance to too loose, then try a .003 blade, if that feels good, you're still in spec.

Personally, I wouldn't run with just .0015, I'd hone it out and shoot for .002, but .003 would be OK, too.

On the '01 200 I just did, using the Wossner piston, I ended up with .002 on the bottom of the cyl (the area below the ports) and .003 clearance above the ports due to normal wear, using the method out lined above. It all worked out well, bike runs great.

Don't be afraid to check the clearances. It'll only cost you the price of a set of feeler gauges. You can always get a shop to verify your findings if you're still not comfortable with everything.

And they'll probably only charge you a few bucks to double check your work and /or slighly hone the cyl , if neccesary.

The worse thing you could at this stage is do nothing and hope for the best.

Go ahead and check it out and report your findings back here.

RB

[Moto7man]

Here is a thread with some useful information, be sure to check out post 18 as well.

http://www.gasgasrider.org/forum/sho...piston+end+gap

[savage sausage]

thanks guys for the replies i'l get to it

and report back

but what brian vt said is correct, will those gaps need to be bigger for a forged piston which expands more than a stock cast alloy one.