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Author Topic: SR250 New Piston ring help please?  (Read 62 times)

Offline vortexracing

  • Posts: 44
SR250 New Piston ring help please?
« on: Jan 16, 2018, 15:58:55 »
Just about to pop new rings in and checked the sizes of the old rings, they are mainly gapped at 0.65mm ish. Took the new rings out of the packet and placed them in the bore only to find they overlap 5mm!! is this correct. In my old karting/2 stroke days they rings only needed a small fettle if anything, these appear massive.

I have checked the part number on the new ring box and the bore size (73.5mm) and they match. Surely you shouldn't have to try and trim 5mm off them

if that is the case, how the hell do I do the oil control ring? I'm hoping they are just the wrong rings.

any help would be much appreciated for this numpty   :-[

Thanks in advance

Dave


Untitled by eastham_david, on Flickr

Offline jpmobius

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Re: SR250 New Piston ring help please?
« Reply #1 on: Jan 16, 2018, 16:17:40 »
Those are, of course, not the right rings.  Likely they have been swapped into the wrong box somehow.   You could indeed trim the compression ring(s) to fit properly, and I doubt the small extra squeeze from being the small bore would result in trouble.  The oil scrapers on the other hand, probably won't like that and distort and squeeze the separator.  That's a guess as I have never done it myself.  I'd be getting the correct parts.  The ring manufacturer should supply the correct gap required, otherwise the service manual will have the measurement.  The gap is needed to ensure the ring does not try to seize in the bore when it expands due to temperature increase.  For that reason it is always better to ere on the "too much gap" is better than "not enough gap" perspective of things as the tiny loss of seal from too much gap is far better than the damage that results when the ring binds in the bore.  So when fitting new rings to an existing (used) bore, you want to measure the gap at the smallest diameter within the bore, which invariably is at the top.
« Last Edit: Jan 16, 2018, 16:26:52 by jpmobius »
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline vortexracing

  • Posts: 44
Re: SR250 New Piston ring help please?
« Reply #2 on: Jan 16, 2018, 16:23:28 »
Thought as much Thanks.

I'll see if the Yamaha dealer will take them back as it's been two years since I bought them. I'll pop the old rings back in for now and look to change them next winter.

Cheers for the advice

Dave