2 stroke ring gap

[Pincushion]

When doing a 2 stroke top end with a cast piston what do you feel is a sufficient ring gap? I know wear limits are usually near .26" or thereabouts, but that is the wear limit. Why dont manuals list a safe ring gap to start with?

Bike in question is a 293cc 2 stroke Husky motor. What would your starting ring gap be with a cast piston?

[KJ790]

The type of piston has no effect on what the proper ring gap should be. Ring gap is necessary since the ring itself expands as it gets hot, thus the gap closes up as the ring gets warmer. Too little ring gap and the ends touch before the ring is fully expanded and the ring is forced into the cylinder wall, thus seizing your engine. The larger the bore, the more ring gap is necessary. I have never seen a manual that did not say what the minimum and maximum ring gap is for that engine. What is the bore on the engine?

[shortburb]

What I dont understand, when you buy a wiseco piston it says you need to have .004/inch of bore

Ill use my 125 as the example

54mm is a little over 2 inch bore so I should need .008 according to wiseco. BUT I put the brand new ring in the cylinder and measure end gap and get .025.... BRAND NEW RING!

I check my yamaha manual and it says .020-.028 for standard ring gap.

What the hell is wiseco saying when they ask for .004/inch of bore when the ring they gave me is bigger than THEIR spec?

[Pincushion]

Why does Wiseco recommend smaller than OEM gaps? Example... a stock yz250 w/ a 66.35mm bore (2.61") would call for a .010 gap for a Wiseco piston, but the manual states between .16 and .22 for OEM. Quite a bit looser.

My Husky WR300 manual may state the ring gap recommendations, but I cant find them. I cant find anything in that crazy manual.

The cylinder in question is 71.94mm/2.83in. Using Wiseco's formula (.004 for every inch of bore) it would call for a .011 gap. Seems awful tight to me.

Thanks!

[shortburb]

My question exactly...

[shortburb]

http://www.wiseco.com/PDFs/Manuals/RingEndGap.pdf

[KJ790]

Wiseco's formula does seem a little tight, but there are a couple variable's that go into it. The last wiseco 2-stroke piston I bought came with a piece of paper that said what ring gap to run, and it was almost the same as what the owner's manual for the bike said said (and much larger than their formula suggests). I know most 250 two strokes suggest somewhere between 0.5-0.6mm end gap, so a 290 would most likely be a hair more than that. Are you sure the manual doesn't say?

[Towman910]

rings and pistons are different shapes and made of different alloys and shaped and machined by different processes so they expand differently. Wiseco and Pro-X call for tight cylinder tolerance (when cold) because they are forged alloy and expand less than a cast (oem, vertex, etc), piston once the engine reaches operating temp.

Piston rings, however, are usually made of steel or cast iron coated with chromium or nitride, which expands more than aluminum. Also, the geometry and physics involved in closing a ring gap vs expanding a whole piston to fill a cylinder are completely different. A piston would have to expand a measureable percentage of its original volume to become .008" larger in diameter. A ring is long and thin, and so most of the measureable expansion is going towards making it longer. It has to expand a similar percentage of its original length to close a .029" gap in length. The idea is, once the ring heats up and expands, the ends are just about touching, sealing the cylinder properly.

[ztangoKLX]

Husqvarna WR 250 ring end gap specification is 0.6mm, maximum.

[shortburb]

I think I was mixibg up inches and MM FWIW....sorry for adding confusions....

[36MotoMarc]

Yes you guys are certainly confusing mm and thousands measurements. Wiseco's recommendation of .004/inch of bore is certainly safe. I consider it too large, actually. I go with .0035 in general, and .003/inch on my personal stuff, no problems. Honda, on a CRF450, 96mm bore (just shy of 4") recommends .010 to .012 ringend gap. worn-out is .018. At .004/inch, you would barely seat the rings, and they would be out of spec.

Go on the tight side. You'll be fine.

[KJ790]

Yes you guys are certainly confusing mm and thousands measurements. Wiseco's recommendation of .004/inch of bore is certainly safe. I consider it too large, actually. I go with .0035 in general, and .003/inch on my personal stuff, no problems. Honda, on a CRF450, 96mm bore (just shy of 4") recommends .010 to .012 ringend gap. worn-out is .018. At .004/inch, you would barely seat the rings, and they would be out of spec.

Go on the tight side. You'll be fine.

I'm a Yamaha guy, so I am not sure about other brands. I know the YZ250 has a 2.6142" bore, so by wiseco's formula that would require a 0.0104" end gap. The Yamaha owner's manual says end gap should be 0.0197" minimum, which is almost twice as much as wiseco calls for. The wear limit according to Yamaha is 0.0276". I know for the YZ125 I just rebuilt that wiseco's .004/inch ratio is again way smaller than Yamaha recommends, and way smaller than the sticker on the wiseco ring box said to run (which matched Yamaha's spec).

[wallybean]

Brandon,

My 2000 shop manual for my WR 360 that also covers the WR 250 says that the ring gap should not exceed .0236". They don't give a minimum clearance that is readable but they do say that wear should not exceed .006" which extrapolates to .0176" as an initial gap for the WR360.

Sounds about right for your 300 too.

Just what it says.

Walt

[Pincushion]

Thanks guys. I've scoured the documentation I have for the 300 and I've scoured the Husky service manual for an 08' WR250 and cant find it. It might be in there, but the manual is printed in 4 different languages and all 4 languages are printed on the same page.... hard to decipher.

Thanks Walt.

I like the sound of .018" and that's the magic number I had in my head too.

Appreciate all the input folks!

Good reading.

[The Captain]

I would go with the piston manufacturer recommendation on ring end gap. OEM piston and rings? Go with the service manual.

Wiseco? CP? JE? Go with their recommendation.

I wouldn't go on "the tight side" as stated above. Though your engine won't seize by any means, you'll scuff the jug walls and not get a good seat.

[36MotoMarc]

No, you won't. The ring ends won't touch I've done hundreds of race engines. What you will get at .018 is pleanty of blowby, lower kicking compression, and a shorter usefull lifespan of the topend. If you've got .0035 to .oo4 per inch of bore for god's sake don't go filing your rings for more gap. there's no chance for "scuffing the cylinder walls" or a poor seat. Quite the opposite. The higher cylinder pressures will give a better ring seat.

But, do what you feel cozy with! Running a large ringend gap on your motor, won't bother my motor one bit!

[Pincushion]

If you've got .0035 to .oo4 per inch of bore for god's sake don't go filing your rings for more gap. there's no chance for "scuffing the cylinder walls" or a poor seat. Quite the opposite. The higher cylinder pressures will give a better ring seat.

As it sits right now it is right at .004 per inch of bore (2.83 x .004 = 0.011"). The rings measured @ .011" when I measure them near the bottom of the jug.

I'm not opposed to running it this tight as I know service manuals are not written for optimal power output, but for margin of error for all the backyard mechanics. Of course I do have concerns about the ring ends butting together which leads to the magic question.... how much expansion will the rings exhibit when up to operating temp ?

[36MotoMarc]

I PROMISE you'll be fine. I've run them at .003/inch for years on 2t and 4t (when you can actually get rings to be that tight) with not a one failure.

Check out this thread Kelstr is pretty well respected in the Honda crowd, runs a race team, etc. He wants to run 450's at .008 ringend!

https://www.thumpertalk.com/forum/showthread.php?t=248372&highlight=ring+gap

He's probably right but I'm afraid to go that gung-ho, but yours is fine DO NOT FILE IT!!!!

If it squeeks on the first ride ( it WON'T!!) you can flame me and tell everyone I don't know diddly squat! You'll thank me in the end that you didn't file away precious thousands and pre-wear out your expensive new top-end!! Others have done the testing for you......FEAR NOT!

[KJ790]

As it sits right now it is right at .004 per inch of bore (2.83 x .004 = 0.011"). The rings measured @ .011" when I measure them near the bottom of the jug.

I'm not opposed to running it this tight as I know service manuals are not written for optimal power output, but for margin of error for all the backyard mechanics. Of course I do have concerns about the ring ends butting together which leads to the magic question.... how much expansion will the rings exhibit when up to operating temp ?

Well, You can look at it mathematically. The linear coefficient of expansion for an iron ring is around 6.8 microinch/(inch*deg. F). So if you have a bore of 2.83" and a end gap of 0.011" then the ends will touch when the ring temperature is elevated 182 degrees F. So if you measured your ring gap at 70 deg. F, the ends will touch at 252 deg. F. Some forms of iron have larger expansion rates, so it would be even lower in those cases. Seems to me a ring would run a bit hotter than that.

[36MotoMarc]

Well, You can look at it mathematically. The linear coefficient of expansion for an iron ring is around 6.8 microinch/(inch*deg. F). So if you have a bore of 2.83" and a end gap of 0.011" then the ends will touch when the ring temperature is elevated 182 degrees F. So if you measured your ring gap at 70 deg. F, the ends will touch at 252 deg. F. Some forms of iron have larger expansion rates, so it would be even lower in those cases. Seems to me a ring would run a bit hotter than that.

Pretty cool info! I would think there are more variables, though. And I'm guessing that that info is from automotive sources? Which have mach thicker rings and more total material to expand. As well as the actual expansion of the alloy that the ring is made of. In practice, there's no way the ringends will touch at .013 (or .011 for that matter).

Also that equation appears to be for a ring in a constant hole size. Let's not forget that the bore grows a bit, also, offsetting your math equation.

I'm not sure and just speculating but I would think the ring would run in the 300-400 range?? Do you know what is typical?

It's nice to see hard factual numbers as a reference!

Edited July 14, 2010 by 36MotoMarc