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Txt 250 New Rings Or Piston As Well?


fxs
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Hi

have a 2008 TXT pro 250cc, it has about 200 hours use from new, the bike is running nice now but I have noticed that it has lost some compression, when new, you could not start the engine going downhill, unless you were in 4th gear, now it will start even in first gear, so it is clear that has lost some compression.

The question is, shall I get just a new set of rings, or get the first replacement piston as well?

,

Cheers from Chile

Francisco

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If its running sweet leave it alone.

If you really want to find out what the condition is I suggest you purchase / beg /steal / burrow a bore gauge and a compression tester.

Your method of compression testing by bump starting down a hill is meaningless.

I would suspect there is absolutely nothing wrong with your rings after 200 hrs. I'd be more concerned about the gearbox from all that bump starting!

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I think EVEN MR "JSE" could tell me if I was wrong, but RINGS are a wear item.

I wouldn't hesitate to at least re-ring the existing setup, that is if there isnt a piston rattle (not to be confused of course with CLUTCH BASKET rattle...

At least while you have to remove the cylinder, you could inspect. Deep gouges, stuff like that, you would need help via people that do this sort of thing, more than just once or twice in a few years.

Edited by sting32
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I think EVEN MR "JSE" could tell me if I was wrong, but RINGS are a wear item.

I wouldn't hesitate to at least re-ring the existing setup, that is if there isnt a piston rattle (not to be confused of course with CLUTCH BASKET rattle...

At least while you have to remove the cylinder, you could inspect. Deep gouges, stuff like that, you would need help via people that do this sort of thing, more than just once or twice in a few years.

Yea, Mitch, you're wrong! Just kidding!!!! I agree, rings are a wear item although in a well-maintained Trials bike, they last an amazing long time. We have no idea of the level of care this bike had so we can only guess, a process race tuners like me REALLY hate to do. What we can do is rather than act on guesses, it would be a good idea to actually measure the relative ring wear by checking the ring end gap. Figure than a set of factory installed rings has a ring end gap of .1mm (.0039") per inch of cylinder bore (kinda standard for most 2T Trials bikes), measure the actual R.E.G. of this engine and compare. This would tell us if we needed to install rings. I would think that maybe a 20-25% increase in REG would be a good time to put new rings in.

As for the piston, I'd measure the skirt clearance and the installed skirt clearances I've seen are around .0015 to .002" for a new engine. The cylinder lining and pistons use a ceramic material, Silicon Carbide, the cylinder in a Nickel matrix and the piston in an Aluminum matrix. Pistons generally wear at a slightly greater rate than the cylinder linings, so you'll find pistons will need replacing before the cylinder lining.

Jon

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If it still does what you want it to do with no unusual noises or a need for more throttle than what you had before.......................

Just ride it until you really have a problem.

Or if you are bored during the off season, tear it down and measure and replace. but if you tear it down you may as well rebuild to make tearing it apart worthwhile.

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Hi

Thanks a lot to everybody that contributed with an answer, this forum is invaluable for me 'cause here in Chile even the rep for GG, doesn't have a clue,

bassically I´ll order the rings only, change them and take it from there,

Thanks again

Cheers

Francisco Straub

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JSE, that is per inch in DIAMETER of bore, am I right?

Measure the OLD rings' ring gaps at top and mid stroke Right JSE? Near the top should be like a "reference" of how big the bore was when new, then mid way down, will be bigger slightly, and the gap should be (slightly) bigger... EXAMPLE on a 300 the piston is 3 inches in diameter, I believe, so you should have .3 milimeters (0.0117 inches) before the rings had any wear... IF I DID THE MATH CORRECTLY? Trying to figure what 25% more would be.. I guess if ring gap on this 300 was around or bigger than .0146, then replace rings. .375 mm.

Well fxs,

You decided to do what I said, even after these other guys chimed in, which shouldn't be a problem! But, be sure to order the BASE (of cylinder) gaskets, and head orings while you order... What usually happens is, you take cylinder off, see what color gaskets you have. then order the same ones!!! (color designates thickness) from factory they were supposed to install the minimum thickness for that particular engine/parts combination after all was machined, and mated. 2 same bikes same year can and might have different combinations of base gaskets...

Make sure you check ring gap of new rings! ****EDIT****

JSE (Gas Gas technical guru) updated me a few posts below...

When measuring ring gap, you need to set the ring at the original bore diameter (such as at the top of the Cylinder, above where the installed ring tops out).

I don't know how big the 250 piston is though, in Diameter. so be sure to use JSE's post to do your MATH.

If you are CAREFUL, you might not need either of them, but most times if you do mangle the base gaskets. Learning to install the head, with the orings that seem ever so "slightly" bigger around than the groove is around the cylinder, you could pinch one if NOT CAREFUL. So, they're cheap, and when you want one or both, you have to wait on the mailman again, and that just sucks, lol.

Edited by sting32
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HE SAID he believes it has lost (considerable by his description) COMPRESSION!

Dang.

b40rt: He also said its running "nice"

ding dang dong

I like you guys, that's funny...:)

What you probably don't know about Francisco is that he uses his Trials bike for climbing mountain trails in

the Andes, up to 13,000 ft altitude so the ring wear in his bike is probably a little more than what we experience

(more sustained loads). A fresh set of rings is probably a good idea in his case.

When measuring ring gap, you need to set the ring at the original bore diameter (such as at the top of the cylinder

above where the installed ring tops out). For used rings, this will give a

more accurate measure of specific wear. For new rings, this is important in that, if you set ring end gap in the middle of the cylinder,

the end gap will be too tight when the piston drops to BDC, as the bore is tapered.

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Thanks JSE< I will edit the post so if they stop reading right there, they wont have wrong info.

No problem. Francisco and I have had a previous conversation as to high-altitude jetting so I knew

what he would be using the bike for. Compression is very important at high altitudes which is why

thought he would have a better outcome with a ring change to keep the bike working effeciently

under the operating conditions he would encounter.

Those of us that have been to the Ute Cup in Colorado know how cool the views are, but can you

imagine riding up in the Andes? Wow!

Jon

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