Piston Diagnosis

[scrubs]

I recently finished a rebuild on my CRM250AR

I had Stan Stephens here in the UK do some tuning work on the top end when he rebored it.
Put it back together went one size up on the main jet and did a plug chop all seems fine.


Last week I just got on to the motorway (about 2 miles) was riding at about quarter revs at 70MPH
letting the throttle off briefly now and then - and all of a sudden lost all power cruised into hard shoulder -
no compression whatsoever. Took the barrel off and I have a 1" thick score down the exaust port outlet
side of the piston. the rings have welded flush with the piston on that side too. the rest of piston, crown
and barrel appear unscathed and ok apart from a tiny speck of splattered metal like a small bead weld
from the melted rings probably.


What could cause this?

[fastman69]

the piston ring caught the exhaust port

[kxrider]

yea i would have to guess someone didnt relief cut tha exhaust bridge port or if u have a power valve (if it has one) that extends into ur cyl that wasnt accounted for when they bored it

[fastman69]

or the ring could have just twisted around, happen in my zilla twice.

[scrubs]

Cheers Lads,


I just took a look at the barrel, the power valve doesent seem to pertrude
into the barrel at any point when I turn it - it does however have some
remnants of white heat scorch marks on it.

The exhaust port does seem to be slightly scuffed at the edge.
but I'm unsure of whether this is from the rings catching or after
the rings fused and don't know how to tell if this is what caused it
as I'm not really sure how it should look.

If it is the exhaust port that is causing it what do i need
to do to rectify it? round the edges? it's nikasil plated.

I'd be surprised if stan messed up the tuning as he is a pretty respected tuner in Europe and has more experience than anyone else in the game over here.
but you never know..

[fastman69]

it could of just been a bad piston, or the install of the piston was wrong.