250-500cc Top end advice 09 wr250.

Hi,
I'm starting to think about replacing the piston as its getting close to 100 hours with half of that racing. No loss in performance , still pulls like a train.But I have a few questions;

Is the wiseco piston listed for this bike(part# 761m06640) a twin ring piston and will the oem small end bearing be compatible. On my kdx200 wiseco included the bearing in the kit but its not included for the husky.

What gasket sets do you recommend , any to avoid?

All this talk of squish has me concerned, what should it measure, do I adjust with base gaskets , are different thickness gaskets provided? Do I try and measure existing gasket?Will this effect the power valve ?

On the kdx I dismantled the KIPS power valve and cleaned everything. I don't hear about this with the Husky, looking at the workshop manual I see there are a lot of components . So do you normally dismantle or can it be cleaned in situ ?


This site has been invaluable over the last year that I have owned my Husky. Thanks everyone.
Paul
New Zealand

You may be well served to run compression numbers to quantify your engine health. Typically you will observe the number getting smaller as things wear, trend this number and you will gain an idea as to when to open the top end. Generally a set of rings will renew the engine for a season or so. Seems that back in the day I would generally get two sets of rings before needing a piston. You would also do well to test your squish number before you dismantle your bike. A simple gauge you can make is to use a spark plug shell with the porcelain removed and fit with some kind of adjustable rod. The reference point is typically the top of the spark plug threads on the head and you wish to know the distance to the top of the installed piston. Be adjusting the rod to touch the piston and then locking this adjustment you may easily measure your particular number. In my case I'm about 0.010" too much from specifications so my rebuild may be considered "de-tuned" a bit. Were I a professional racer this would be an issue. Since I'm slow and fat now, for me this is a non-issue. I've 150 psi compression so I'm good until that number falls to the 115 psi range (75%). This is simply my opinion, works well for me. In my experience you will find your base gasket will go to about 50~55% of thickness when torqued to specs. YMMV.

That piston should be replaced due to hours. Esp if you track race it.

WISECO/Pro-X pin use OE spec bearings as they are made to be used with the stock con rod.

Gaskets: use the best you can afford.

Squish: ignore if just doing a top end freshen. Check the ring gap instead. That's WAY more important.

PV- if you use a good oil there should be no coke on the PV. Mild varnish is easy to remove with Purple Power soap and a super soft tooth brush/q-tip w/o teardown. Coke means you should take it apart to clean.

The WR250 and 300 can have good compression numbers and still need a piston. The intake side of the piston will wear on the reed port edges and once it's grooved enough it will break the skirt. I pull the reeds and the pipe every 6 months and have a look. Mine is still oem, still looks great and probably close to 200 hrs with 205 psi cranking compression. I ride mostly tight single track so it doesn't get a lot of WFO. This winter, even if it looks good, my 09 wr250 is getting a new piston,

Thanks for the advice guys.Piston ordered!
Mitaka,athena,namura and cometic gasket sets are all listed on ebay. Any reason I shouldn't use Athena?
I bought the bike new old stock one year ago so original piston.
Expecting pv to be reasonably clean as have it jetted pretty good (jd Mikuni) and running full synthetic(shell advance ultra2) at 40:1.
Spend a fair amount of time in sand so hasn't been babied though.

if its like all the other 2 strokes its 2 o-rings and some base gaskets to set the squish
Athena is nothing special that i know of
the separate o-rings and gaskets were less on my 360