compression

ather7on · Derestricted Danger Joined: 21 Oct 2012 Karma :

hi, i have an aprilia rs 125 year 2000 and been having probs with it dying while riding, ive done a compression test and it came back at around 25 psi, ive taken off the cylinder head and the gaskets seem knacked so ill replace them, but the thing that disturbs me is that there seems to be a little bit play around the piston, is this normal? should i ponder having a new piston and re-bore? any help appreciated, thanks.

WannaBeDude · World Chat Champion Joined: 05 Jul 2011 Karma :

STOP riding her, she needs work BEFORE she blows. Idea

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mentalboy · World Chat Champion Joined: 05 May 2012 Karma :

Depends on what you mean by a little play. You will get some movement in the bore when the piston's cold otherwise it'd stick when everything gets warmed up and expands. Best bet is to lift off barrel and check it for any untoward scarring. Whilst you're in there check that piston rings still move freely, otherwise though loss of compression could easily be down to the mullered gasket.
If you've got a manual find out what the gap should be when the rings are in the bore and measure it, this will give a fairly good idea if you've got to much play going on.

EDIT: As Covdude says don't ride her, it's bloody hard work on the legs when you've got the head off!!! Laughing

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WannaBeDude · World Chat Champion Joined: 05 Jul 2011 Karma :

Yeah, I leave specifics to likes of above, but these bikes need to run spot on really, they just fuck you off if you chance your arm with them, hence all the bullshit storys ...

Afaik - the Rs125 being arguably the fastest road 125, needs the upmost care - Thumbs Up

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ather7on · Derestricted Danger Joined: 21 Oct 2012 Karma :

mentalboy · World Chat Champion Joined: 05 May 2012 Karma :

If it all looks clean in there then you should be ok. Replace head gasket and wouldn't hurt with a new plug.
Thermostat gasket may be able to gop up sealant but new would be easier if you think the current one may leak (either way it wouldn't account for compression problems)
Head bolts aren't usually torqued down very tight and would be best using recommended bolt down sequence (see your manual, usually you can follow a numbering sequence stamped on the head beside each bolt but you may have to torque all then re-torque as specified) and a torque wrench to avoid possibility of warping head.
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