Piston Rings

[steveenduro]

Hi.

Just ordered a set of rings for a 250 txt pro 2002.

The ones on the bike have 2 "normal" rings plus a top ring with a lip.

I've just looked at the parts list for a 2003 pro, and it just lists a pair of rings.

Do I have a non standard piston/rings or is the top lipped ring not part of the piston ring assembly?

I hope to receive the parts tomorrow and re-assemble, so any replies appreciated.

Thanks

Steve

[jse]

Hi.

Just ordered a set of rings for a 250 txt pro 2002.

The ones on the bike have 2 "normal" rings plus a top ring with a lip.

I've just looked at the parts list for a 2003 pro, and it just lists a pair of rings.

Do I have a non standard piston/rings or is the top lipped ring not part of the piston ring assembly?

I hope to receive the parts tomorrow and re-assemble, so any replies appreciated.

Thanks

Steve

I just heard of someone mentioning three rings on the 250 piston. 250's are really rare here in the U.S. (in spite of the fact they are great bikes) and the 280 and 300's have two "regular" rings and I haven't been in a 250 as yet. Some of the pre-Pro's used "Dykes" top rings which are "L" shaped in profile and you can tell if your piston takes a top Dykes ring by the way the very top ring land is machined in more to make allowance for the thickness the outside of the ring. If you, in fact, have two regular type rings a couple of things to remember is to install the ring with the writing on the top and measure ring end gap by using the piston to push the free ring about 15mm down the bore and use a feeler gauge to check the gap. Ring end gap should be .1mm per inch of cylinder bore diameter.

Jon

[steveenduro]

Thanks.

Yes I have 2 regular rings plus a top L shaped "dyke" ring.

I have received one dyke ring and a regular ring. These fit in the bore with a 0.2mm end gap. I guess that the clearance is enough?

I'm getting hold of the 3rd ring and will re-assemble.

Steve

[jse]

Thanks.

Yes I have 2 regular rings plus a top L shaped "dyke" ring.

I have received one dyke ring and a regular ring. These fit in the bore with a 0.2mm end gap. I guess that the clearance is enough?

I'm getting hold of the 3rd ring and will re-assemble.

Steve

Steve,

As your bore is about 2.86", I'd probably opt for a little more clearance, maybe a little less than .3mm.

As can be the case, my eyes glazed over a little when I quickly proofed my post on how to measure ring end gap (we have two Nitromethane-powered 10 year old twin Grandkids visiting for a couple of weeks) and I left off a decimal point. The free ring should be down about .15mm from the top of the bore when measuring gap, or you can use the bottom most part of the bore where the circular lining is continous (no ports). The idea is that you want to measure the gap at a point in the cylinder where it will be the closest, which will be an area that will have the least wear or "taper", the very top or very bottom. It's not such a critical thing in a Nikasil lined cylinder as the taper is usual minimal (pistons wear faster than cylinders, normally) but you want to be as accurate as possible when taking any engine measurements.

The idea behind using a "Dykes" top ring is that exhaust pressure acts to push the ring out against the cylinder wall, helping sealing. They are usually too heavy and have too much drag for high-speed engines like roadracers, but work really well in under 10K RPM two-strokes.

Jon

[steveenduro]

Thanks Jon.

I'll ease a touch off the rings with a file.

Any ideas of the max. end gap spec. on these motors?

My top ring was ok at 0.3 but mid and bottom were 0.5mm and 0.4mm.

I'm changing them anyway, but it would be good to know for future.

Steve

[jse]

Thanks Jon.

I'll ease a touch off the rings with a file.

Any ideas of the max. end gap spec. on these motors?

My top ring was ok at 0.3 but mid and bottom were 0.5mm and 0.4mm.

I'm changing them anyway, but it would be good to know for future.

Steve

Steve,

I don't know what the "max" serviceable life would be in terms of end gap but I would guess that at 2 or 3 times starting end gap (.6mm-.024" or .9mm-.035" for instance) the ring would have lost a fair amount of wall contact tension and you would be giving away some low RPM power. This is just a guess and I've seen some engines with a surprizingly wide end gap run fine and others with a fairly tight gap lose some compression. Trials engines are a funny animal and sometimes seat-of-the-pants analysis works better than a caliper.

I like to use a thin, fine grit stone rather than a file to dress the end gap, it takes off less at a swipe (easier to not go overboard) and leaves a better finish.

If you want to do it the scientific way, after proper break-in, take a compression reading (plug out and grounded and throttle WFO) and record that number. When in checking it later, you might want to look at replacing rings when compression drops off 15% of that number if you want to keep the engine in top notch shape. Most of us go a little past that point till our seat-of-the-pants gauge tells us somethings off, but that's how you would maintain a Pro's engine.

Cheers.

Jon