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Piston rings keeps breaking

Old 12-22-2007, 06:30 PM
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Iflyit
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Default Piston rings keeps breaking

I am trying to put a new liner/piston/ and ring in my mvvs .26 gas engine. I have broken 2 rings and cannot figure out how to get it installed. I tried from the top of the liner with a home made ring compressor and from the bottom. Any tricks to getting the ring in without breaking it and if it needs to go in from the top or the bottom?
Old 12-23-2007, 01:07 AM
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DarZeelon
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Default RE: Piston rings keeps breaking

I-fly,


First, trial-fit the ring alone in the sleeve, to verify it can go in.

There is absolutely no need for a ring compressor to do that - just manipulate it gently with your fingers and once it is in the sleeve, push it down using the piston, until it is where it will be at the top of the stroke, about 1.5 cm below the sleeve flange.

Now, make sure there is a small gap between the two ends of the ring. For full size engines, the size of the gap is .15 mm, or .006" for this piston diameter (use a feeler gage to measure).
For a two-stroke model engine it can be slightly larger.

If there is no gap the ring will not fit. If there is too large a gap, compression leakage will result; less power and harder starting.


The ring is pinned.

Search the groove in the piston to find the locating pin. This pin must be in the ring gap, so the ring will be allowed to 'sink' all the way into its groove (without protruding from it).
The purpose of this pin is to ascertain the ring ends are never caught in the intake and the exhaust port. The ring must not rotate.

When assembled, the ring gap must coincide with a clean, straight part of the sleeve, where it can never 'find' a port, while going up and down...

When fitting the sleeve into the crankcase, it must be aligned with its final position, so you would not need to rotate it as it is slid down.


The chamfer in the bottom of the sleeve serves as a makeshift 'ring compressor', as it is slid down over the piston (which is already fitted onto the con-rod and crank-pin).


The whole assembly goes in, of course, from the bottom of the sleeve...
Old 12-23-2007, 01:15 AM
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Iflyit
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Default RE: Piston rings keeps breaking

Thanks, I did not notice it was pinned so probably broke it while trying to compress it in place. I know the ring fit as it was on the piston and in the sleeve when I bought it, I thought the piston needed to be removed to put it in the engine. I assume the piston and sleeve assembly can be put together and installed in the case and the rod will slip on the crank?

Old 12-23-2007, 01:48 AM
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DarZeelon
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Default RE: Piston rings keeps breaking

I-fly,


Most assembly issues have only one possible solution.
To save yourself possible grief in the future; whenever you are not completely sure about how something is to be assembled, just write to the support team here and you'll get a proper answer.

The piston-ring for this 35 cc MVVS engine is not very expensive, but writing to ask is cheaper still...


Using your real name here, by the way, makes it even easier to ask...
Old 12-23-2007, 08:28 AM
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pe reivers
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Default RE: Piston rings keeps breaking

It sometimes is hard to get the ring in well. However, MVVS has made a provision for that. The lower section of the liner is thinner than the rest.
I proceed like this:
Locate the retaining pin, and place the ring on the piston (pull out with thumb nails, and press in with index fingers at the 10 and 2 oƧlock positions at the same time. This makes the ring bend open as round as possible.
Now press the ring closed, with ring notch engaging the retaining pin. Slip the liner over the compressed ring. until the ring just clears the scavenging ports cut-outs.
turn the puston in it's final location, while still at the lowest possible position in the liner.
Now insert the liner-piston assy in the crankcase and fit the conrod over the crankpin. Go gently here.
Push down the liner as far as it will go, Heat the case, and finally push the liner all the way home. (witout rotating!!!!!)
With the piston in top dead center, and the case still hot, you can make final liner position adjstments.

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