CR250 1981 to 1983 Piston Dimension require please

[oldyzman]

Hello Gents, I have just pulled my recently purchased bike apart and there is something wrong from a previous rebuild. It seems that there is a component wrongly sized and the piston does not go right to the top of the bore infact misses by about 5mm. There is a lack of markings on the piston and conrod. Could someone please measure the piston pin bore size and also the distance from the top of the Pin bore to the top of the piston as this will indicate to me where the issue lies for me and i will be able to the put it right. I only need the measurements within about 1/2mm.
Thing is, would you believe this bike has been running like this... The top revrange seems Good but the low and midrange has had a large flat spot and whatever i did with jetting just never made it better.
Thanks in advance for any help
Brett

[Snowy 76]

Not sure of the dimentions, but I had the same issue, only in reverse, split 84 cr250 to repair gearbox, on re-assembly found crack in piston, fitted another piston I had but it protruded about 5mm out of barrel? ended up being piston for 81 to 83 CR 250,  Think you have a 84 piston?

[oldyzman]

Snowey thanks for that, you may be right I have just measured this piston, it is 66.4mm Dia which is the std size fo the 84 piston.. Any help to verify from others woud also be appreciated, even if someone can measure the overall height of just the cylinder on an 83 model as i am also wondering if this may be the wrong cylinder for the bike. once again measurements within about  a mm woud be sufficient..
Brett

[Snowy 76]

Will measure tomorrow 83 piston against cracked 84 piston {have that on my desk as a pen holder}  pin to piston crown.

[Lozza]

Just put a 5mm longer rod in

[oldyzman]

That would be really helpful snowey,
Lozza, i reckon the piston is the wrong part here and i really dont wish to split the case as the piston is ready for renew anyway.
Brett

[Hoony]

just a thought.

with the CR500's (85 & 86'models) the rods are 5 mm shorter that the models that followed in 87' onwards. i

 am not sure of the 84 or 83'. wonder if the 83' has a longer rod and an 85/86' was sourced instead. i have heard the 83 rods are had to get?

the 480's and 500's share the same bore size @ 89mm standard
stroke for 480 is 76mm and 500 is 79mm.

LATE EDIT......... I NOW REALISE THIS THREAD IS ABOUT 250'S AND I WENT OFF FULL COCKED.

[oldyzman]

All very interesting, the rod has KA4 on it which may seem to be correct...
Brett

[Hoony]

Shit sorry oldy, i miss read you post and started rattling off about 480's and 500's. i NOW realise your are discussing the 250's.


my mistake.

[oldyzman]

No worries Hooney

[Snowy 76]

Sorry Oldy, didn`t get down to the workshop today, will check those Pistons tommorrow

[oldyzman]

That would be great snowey i will keep an on on here...
Cheers Brett

[Snowy 76]

G`day Brett, Yes I think that's your problem, measured 3mm diff between 84 and earlier piston { from piston pin to top, not including crown} the piston I have here is .5 over if you need one. Mark.

[oldyzman]

Snowey, thanks for that mate. Can you please give me the dimension of the '83 cause i need to confirm before i order a new piston. the bore size seems to be 66.4 but i'm only measuring with vernieiers. So i dont think the your 66.5 will do it as there is a bit of pitting on the bore also.
Cheers Brett

[Snowy 76]

All my Specs are at Work ,  Not sure of bore sizes ETC  but the Piston I have is .5 over /second over size.