2 pistons in 1 week. WHY?!?

[tshort2007]

Just had my piston go pop on me tonight. The 2nd piston in 6 days. Both have gone in the same way. It seems the rings have caught the exhaust port on the downstoke. Its a 2 stroke 125cc single cylinder.

Why does it keep doing this?

Also, since i had the bike, i get a metallic rattling from the engine, like a bolt in a tin can, even when just kicking over, but all is quiet wih the piston off.

Do you think i may have the wrong size piston?

Cheers
Tom

[tutton]

Sounds like crank bearings are gone, that would also explain the play in the conrod that could well have caused the piston pivoting and catching the rings just like that...
Time to check play on conrod?
Metallic is crank bearings
washing machine sound is main bearings..
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Past: '05 Peugeot XPS '99 Aprillia RS125 '94 FZR600R
Current: '02 ZX636 A1P '51 CCM R30

[Timmeh]

Take a photo of your barrel, so we can see the ports, maybe we can spot something you've missed.


____________________
GSXR400 x2 | '94 RVF400 | '93 TZR125 4DL (again)
20:22:30 BLUEX5: i would love to be forced to undergo a**l plugging with different sizes
20:48:18 Temeluchus: comp you hunk of smouldering homos3x you

[tutton]

I had presumed he had it rebored before a new piston right?
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Past: '05 Peugeot XPS '99 Aprillia RS125 '94 FZR600R
Current: '02 ZX636 A1P '51 CCM R30

[tshort2007]

Have checked bottom end bearings, well, best i can. The flywheel turns smoothly with no grittyness, and no lateral movement. The conrod has no up and down movement when the crank is stationary, and no twisting. There is a slight amount of side to side movement (as in left to right), with about 1mm movement at the small end side of the rod, but i belive this is ok?

I put an elastic cord through the small end, and tied it to the frame (to keep the conrod vertical), put the bike in 3rd, raised the back wheel, and span it like f**k. Not a single rattle from the engine. If i did it with the barrel and piston on, i would get a scraping sound, and a "knock" approx every rotation.

IMO i think the barral has been rebored, and myself and the previous owner put standard pistons in.

How feasable is this?

[baldy]

Check to see if any of the ports have burrs on them. They should be slightly chamfered.

Did you check the ring end gaps when installed in the piston?

[tshort2007]

[Timmeh]

Thanks fella

Looks like the exhaust port has some damage to its edge, you'll have to get that smoothed out else the rings will catch on them.

Like Baldy says are you sure the ring gaps were in the right place? They need to sit in the space between the ports.

All too often we see people wrecking their barrels because they've put the ring gap so it runs over the edge of a port.
____________________
GSXR400 x2 | '94 RVF400 | '93 TZR125 4DL (again)
20:22:30 BLUEX5: i would love to be forced to undergo a**l plugging with different sizes
20:48:18 Temeluchus: comp you hunk of smouldering homos3x you

[baldy]

There should be little ring locating dowels in the piston ring grooves to stop the ring gaps going over the ports. If the piston is an aftermarket one, they may be in the wrong place. I would check before i ordered any more pistons.

If i was you i wouldn't get that re bored, i would just give it a quick hone and run it till it pops, but i'm just a proper tight arse.

[tshort2007]

[baldy]

The picture looks like the little pin is facing the exhaust port, so that would be why the rings are snapping. Did you put the piston in the right way with the arrow facing the exhaust port?

[tshort2007]

The first piston that went had the ring gaps at the exhaust port, and no arrow on the piston. The piston i put in had an arrow, but below the arrow were the locating pins, so i put the arrow away from the exhaust, which puts the ring gap away from the exhaust port, and its STILL caught on the exhaust port.

[Rockhopper]

I would say that the compression reading you are getting is way too low, should be more like 130 psi.

[bmwsport]

That sucks dude.. did you measure the cylinder/check for ovality before fitting the piston? Also did you calculate your compression after?

[Theoutsider]

It looks to me like you`ve fitted the piston with the arrow heading the carburettor and not the exhaust.Everything is essential.

[alains]

i agree with you , normaly the rings close intake side (facing back)
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the worst day fishing is better than the best day working

[tshort2007]

The engine isnt a reed valve, its a rotary valve, so the piston doesnt have the 2 holes for the inlet. If i install the piston with the arrow facing the exhaust, then my ring gaps will be in the exhaust port.

Thats why i belive the first piston went, so when i installed the 2nd piston, i put the ring gaps to the opposite side, and it still damaged at the exhaust port. it seems that either my piston was too small, and moved/twisted in the barral, catching the large exhaust port, or the first piston burred the exhaust port, damaging the second piston. I'm sending it off for a rebore and piston to match today, so i wil ask the mechanics which way round it should go.

[Pete.]

Picture 2227 shows a silver band above the exhaust port. Looks like a raised section where the ring has connected with it - you'll have to have that honed down flat to stop it catching the ring land and trapping the ring. That's probably your rattling, you can see corresponding scrape-marks on the lip of the piston crown.

Picture 2230 shows a faint silver line in the barrel just between the 2nd and 3rd transfer port counting clockwise. This is where the pin should normally be, the other pin should be same place between the 3rd and 4th port, usually. Certainly, no ring-end shold be over any port opening.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Kickstart]

Hi

Possible someone has rebored it and then used a stock size piston. I would expect it to rattle badly if this was done. Would also explain the dire compression readings (is it an AR125, as if so the Autodata book gives compression as 7.2~11.3 bar, so 104~164psi).

I would expect the ring gaps to be on the opposite side to the exhaust port.

What is possble is that there was still some debris in the engine from the previous blow up, and this debris caused the 2nd failure.

Personally I would get a rebore and new piston (of the right size).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[tshort2007]

Ok, i've checked the bottom end for debris, all clean and running smooth. Bearings still all working!

Took barrel in today for a rebore. just ordered 2mm oversize piston, new small end bearing, new gudgeon pin, new circlips, new gaskets and a rebore, all for £88, and its not a cheap chinese piston, its a quality japanese one, and i'm assured its supurb quality.

should have it all back for tuesday, so i will let you know how it goes!

Cheers for your help.
tom

[Kickstart]

Hi

2mm oversize? That suonds like max oversize, which I would only go for if really desperate (if you need another rebore then at best you will be left trying to get a new liner fitted).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[tshort2007]

Thats what they recommend. my barrel has ovaled, now measuring 50.2x51.6. Its being rebored to 52x52, the nearest size up. I know it maxes the barrel, but its the only thing they can do