AR125 Gone BANG :(

tshort2007 · Scooby Slapper Joined: 07 Jun 2009 Karma :

Finally fixed the problem with my AR running rich and misfiring, the coil pickup was loose!
So decided to take it out for a run
5 miles of gentle riding got the engine nice and warm, so decided to kick it a notch on the dual carriageway.

about 5 seconds later, a thud and a rattling noise occured, resulting in the bike not idleing, but still fine at high RPM (apart from the quiet rattling)

Got back to the garage, and pulled the plug, looked perfect, nice mocha coffee brown colour. Fearing shattered bearings, i proceeded to strip the top end. Took the exhaust off, and out fell a chunk of piston, and a bit of ring. Images attatched (hopefully)

It appears the piston caught the exhaust port on the way down, as the piston has been burred over the bottom ring, just below the missing chunk.

The ar125 engine uses a rotary valve, with the injection port at the side of the block, so the piston has no holes for the inlet port. However, where the split is in the rings has been installed to the wider exhaust port. Is this correct? as the piston has no markings for inlet

What would cause this to happen?
And does my barrel look ok to you?

Thanks for all your help today

ms51ves3 · Super Spammer Joined: 07 Jun 2007 Karma :

If there are no deep scores in the barrel you may get away with just cleaning up the exhaust port with a dremel and putting a chamfer on it.

bmwsport · Nitrous Nuisance Joined: 25 Oct 2008 Karma :

wow at least you did it properly lol, err the barrell doesn't look very good to me, is that a bit of plating missing just below the exhaust port? does the ar have a powervalve? is that ok? Check out the mikata thread in general, couple of us going through the top end rebuild phase at the mo lol

tshort2007 · Scooby Slapper Joined: 07 Jun 2009 Karma :

tshort2007 · Scooby Slapper Joined: 07 Jun 2009 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

tshort2007 · Scooby Slapper Joined: 07 Jun 2009 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Just my own! With the small amount of damage you would be best off reboring it, but could probably get away with a new piston.

All the best

Keith
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tshort2007 · Scooby Slapper Joined: 07 Jun 2009 Karma :

TBH, i bought a cheap 125 to learn on, with the intention of getting a cbr600 later this year, so if the AR will last 1000 miles with just a piston, i'm gonna do it on the cheap!

welshlad25 · Two Stroke Sniffer Joined: 27 May 2009 Karma :

this exact thing happend to my honda mtx m8 .i got told if barrel is not to bad the rub down with very fine wet and dry to take high points of were damaged and fit new piston and rings m8

i did that and fitted new piston and now bikes up and running spot on if u look in show and tell i think theres pics of my piston and looks like yours