KR-1S.co.uk Forum

Hi again, would you recommend I refit these or do they look too worn, not an expert on these type pistons and bores, but as in my other thread, all looks good with no scores or nip-ups, head is undamaged, and approx 15,000 miles.

Don't know if thats correct, and unknown history of the bike but all looked standard, (these are the later ring-peg modified pistons too)







Pic of the head

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it the past given the effort vs expense of replacing the top end...
I have always replaced the small end bearings, pins, rings and pistons at the same time. Now that original pistons arent at hand need to reconsider pistons every second ringing.

I got new ring sets from Cradleys last month - I am not sure what stocks are left (being you can't get the OEM pistons) - genuine head gasket and base gaskets are still available.

Remember it not just the piston and ring that wears out
You may find you might scratch a new pin fitting into an old piston - not sure what other people recommend here - certainly if you do go for new bearing get a new pin as well.
If you do fit new pins (doing bearings), you could, on the safe side (protect and) chill the pin, warm the piston to get the loosest/biggest fit (dont just push them together old with new at room temperature) at time of assembly

Can't really tell from the photos what ridge surface is left on the piston (theres still appears to be some teflon on the skirt?) so probably okay. I bet most people never measure the piston and bore (clearance) or ring - so thats up to your discretion on how worn they are - given you dont have first hand (previous) assembly history on the motor. You wouldnt take the motor apart measure the ring gap find it in within tolerance and put it back together with the old parts again - pointless

Refittig the barrel, very loosely and rotate crank through several strokes, then stroke the crank midway and fasten down - you're trying to get the cylinder bore the most concentric with the piston here. MY OPINION is you will only achieve half the accuracy if you follow some peoples advice on both barrels and head as one unit. You are aligning multiple parts to ONE plane of reference - NOT trying to conform multiple planes/axis to one reference (a warp in the head will create cylinder axes that are non-parallel to the rods and non perpendicular to the crank/smallend) and then try to bend them back into line by bolting them to the crankcase?? - a bit like digging a tunnel from two separate ends they never will meet exactly - therefore is not completely as straight as it could have been starting at one end going straight through.

You should polish the combustion chamber surface with wet and dry - or even just Autosol and patience - to get a mirror finish !!
Consider your riding style and what oil you are using and if youre only pootling around a hotter plug - (the piston crown seems quite clean but the chambers not so well )
A two stroke roadie that gets treated too gently (laboured and never reved out) wont run clean (liquid oil buildup in pipes and eventually pistons) - if for no other reason bomb the tin-boxes with giant plumes of KR smoke

Minimum course of action (cheapest)... new rings (old pistons), and genuine gaskets

Good advice KR1R
I will add to it based on my experience,
If you have put more than 3 to 4000 miles on the current pistons then personally i would fit a new set,if youve only done 2000 or so miles then fit new rings.
I know there not the old style problematic pistons but ive still known the ring pegs to come out if left too long,there isnt really anyway of measuring this or predicting when/if it will happen so keep to a mileage schedule and dont deviate from it.
a word on piston pins/gudgeon pins,
They should be a slide fit (by hand) if there not the most likely cause is a slight burr on the inside of the piston boss,just put a chamfer round this area and 9 times out of 10 they will slide in.
OE pistons are normally ok but the latest Mitaka seem to have this problem (its an easy fix).
is the motor standard and running stock jetting?
The heads point to either too rich or more likely for a road bike too cold a plug,i normally use B9EGV for road and 10's for track,you just dont spend that amount of time at high revs/high load too keep the 10's clean on the road.
What oil are you running?not much carbon buildup on the crowns but evidence of varnishing on the ring lands (when the oil breaks down under heavy load) or it could possibly be blow by from worn rings (hard to tell from pic).
there are some fully synthetics that dont seem up to the demands the KR puts on it,Comp 2 plus and Motul 710 are 2 ive noticed that cause alot of wear.
My favourites,
Motul 800 (off road version works fine on the road)
silkolene Pro 2
Belray H1R
putoline Rs959
They all work fine in the pump.
it might be worth doing a dry build and checking the barrels are the same length by bolting them down without the head and putting a staright edge across them,and also worth checking the squish is between 0.7 to 0.8mm.
HTH
atb
Dave

Thanks for the interesting replies, don't know any history about the bike unfortunately, was someones off here at some point, had apparently been laid up for 10 years.

My thinking was, if everything was in tolerance, which it seems to be, (thoughts on whether it's had a topend rebuild??) is to rebuild it, run it around for a while, and see if the crank/seals etc are all good, then stripdown if needed and do full rebuild, including new pistons/rings etc etc

As I know nothing about the engine didn't want to throw some genuine pistons at it, and hope for the best? Stupid idea or not?

Also correct Dave running 10ES plugs, oil unknown too.

My thinking was, if everything was in tolerance, which it seems to be, (thoughts on whether it's had a topend rebuild??) is to rebuild it, run it around for a while, and see if the crank/seals etc are all good, then stripdown if needed and do full rebuild, including new pistons/rings etc etc

Thats pretty much what I would do.

Definately new gaskets, little end bearings/pins & clips for the running around for a bit & if like you say SP, it needs new crank seals/rebuild etc................ then spend top dollar.

You never know, you might get lucky & have an engine that will last or it might just go bang.........

If nothing else, it'll give you the opportunity to assess what else may be wrong with the engine, gearbox, water/oil pumps etc. I know it sounds a bit cavalier but when ever I have bought a bike of unknown history that's what I have done/do

I sit here waiting to be slaughtered

i can see the point to some extent,but the main things that go wrong are piston ring pegs working loose then destroying everything in its path,as said you CANT measure or know when it will happen it just does.
Most engine parts are very reliable on the KR's,the cranks last well,gearboxes dont have any design faults (providing there run on decent oil and its changed regularly).
Crank seals are normally ok ,there not like the old 70's early 80's strokers where the seals went brittle if left for 6 months, .
you maybe lucky and have no problems but then again a locked up engine isnt a pleasent experience.
cheers
Dave

but then again a locked up engine isnt a pleasent experience

To true & I still ride with my hand over the clutch

Hand over clutch,its the only way