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cuddies say YEE 925

Milling Head

24 posts in this topic

I'm having a pinging problem, and I have heard/read/been told that the squish clearance is way off on these bikes (talking about 02 CR250 here), and that milling the head down is a common fix. Anyways, I went ahead and did a squish test a while back, and the thing really was WACK!

So, would gettin the head milled down be good idea? Checked jetting and the motor is fresh with about 3 hours on it. Blows hard and is a tight motor. How much should be milled off? .01?

No leaks in the motor either, all seals are good.

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To refresh the collective memory, what is the cranking compression, your elevation and the squish clearance?

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Define "WACK"

Is that the squish area is too small and or the clearence is to tight?

Milling the head will only add to your already pinging problem, as therefore making the squish area even smaller.

How long have you had this pinging problem? Maybe the new piston (I'm reading between the lines because you tell us that it is a fresh topend) you installed has a higher dome that the last.

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Milling the head will only add to your already pinging problem, as therefore making the squish area even smaller.
Not necessarily. In his book "Two-Stroke Tuners Handbook", Gordon Jennings describes the reasoning behind the squish band head used in most modern 2-stroke MX bike engines. According to Jennings, during normal "ideal" combustion, the flamefront proceeds from the spark plug to the outer reaches of the combustion chamber. As the flamefront proceeds outward, the temperature and pressure of the un-burned mixture increase due to increasing combustion chamber pressure as well as radiation from the burning mixture. The increased pressure and temperature in the un-burned mixture causes the flamefront to rapidly accelerate (move faster) as it moves outward. If the temperature of the un-burned mixture is raised to it's ignition point, it detonates, or, explodes rapidly. Generally, the further the flamefront has to travel, the greater the probabilty of detonation.

The squish band chamber was devised to minimize the chance of detonation by concentrating the majority of the air fuel mixture in a small pocket near the spark plug, thereby minimize the distance the flamefront must travel during combustion. The remaining air fuel mixture (outside the small pocket) is trapped in the small gap (squish clearance) between the cylinder head squish band and the piston. The small squish clearance keeps the remaining mixture cool to avoid detonation two ways; the small gap shields remaining mixture from combustion radiation and it provides nearby metal surfaces (piston, cylinder, head) to rapidly conduct heat away from the remaining fuel mixture. The downside is that the remaining mixture in the squish band gap never combusts and, therefore, never contributes to power production.

To be effective in shielding and cooling the remaining mixture (thereby minimizing the chance of detonation), the squish clearance should be small. Fortunately, a small squish clearance also minimizes the un-burned mixture which should help power production. Jennings states that the squish clearance should be no more than 0.060" and should actually be as small as possible while still avoiding mechanical contact between the piston and head. Fresh and healthy bottom end parts should allow a squish clearance as small as 0.040" or even less in a CR250.

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Sorry for not including all the information...

I put a fresh top end in about a month ago, and I only have about 3-4 hours on it. I did a squish clearance and it came out to be .076 both times I measured....and I know that is not what it's supposed to be. With milling the head down, I am removing material from the head, thence making it "shorter"..which will bring the combustion chamber down, making the squish bang smaller...which is what I need.

Like I mentioned, the motor is all fresh, and tight. All clearances were within spec. I have not done a leakdown test, but did a compression test after motor was broken in, and blew 190~PSI on a warm motor, WOT.

Pinging has been a problem since the day i bought it, but switched to race gas (110) about 8 months ago. I did the rebuild hoping it would go away (old top end had way too many hours on it), and figured it just needed to be tightened up. Guess not. I was also told could be a seal, so I had all seals check and rechecked, all are good and not leaking. No air leaks anywhere, and jetting was put in touch for my area...which leaves me to the last step, the squish clearance.

FWIW, I'm only +800FT altitude with a typical humidity of +/- 23%.

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.076 for the stock clearance seems high, mine measured at .060".

A better way than milling the head would be to lower the cylinder .010" by using a gasket for an '05 CR, it is a metal one and can be used more than once ('02 CR gasket .020", '05 gasket .010"). You get the added benefit of lowering the port timing to help with the bottom end. Put in a JD jet kit , v-force reeds, pipe and shorty silencer and and the bike will work really well.

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...Milling the head will only add to your already pinging problem, as therefore making the squish area even smaller...

There are two competing factors at play when milling a head. skipn8r did an excellent job describing how a tighter squish band acts to reduce the occurrence of detonation. :thumbsup: The other factor--the one you're likely thinking of--goes the other way. Milling the head without re-chambering will make combustion chamber volume smaller thus increasing compression ratio. And of course increased compression increases the occurrence of deto if all else remains the same.

To keep the cranking compression the same the head would need to me re-chambered after milling to get the combustion chamber volume back to where you started (except now with the tighter squish band).

I'm not sure which of the two competing factors would have the greater influence. If the problem is in fact deto brought on by excessive squish then I'd guess reducing the squish to ideal would reduce the occurrence of deto more than increased compression would increase its occurrence--and the problem might disappear. I guess it just depends on how much you've got to mill off to get the squish right and what that amount will do to the compression ratio.

Seems odd though that it would ping on race gas at 190psi even if the squish was off some. .076" does sound excessive but it doesn't strike me as out of this world outrageous. Pump gas--sure. Race gas? Dunno. :ride:

Cuddies, have you verified the ignition timing with a timing light?

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The bike only pings on 91 octane, not on the 110. I am getting tired of running the 110 because it costs me $7 a gallon and prices are only increasing. Its pumping 190~PSI and pings on 91 octane, but once I put in the the 110, it runs like a champ, no pinging or knocking. THe whole reason I rebuilt the motor was to get rid of this so I don't have to keep paying the money for the race gas.

No, I have not looked at the timing. It has also never been touched (which doesn't mean it couldn't have been messed up, but unlikely).

I did a compression test about 5 months ago when it was all stock, and it was blowing 215~PSI.

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Oh, I see. I misunderstood. Probably no reason to check the timing in that case.

How is it that it pumped 215psi before and your fresh top end is pumping 190psi now? Something changed I assume?

If you have the head milled just make sure the machine shop re-chambers the dome as necessary so you don't wind up with an engine that still pings on pump even though your squish is good.

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Not necessarily. In his book "Two-Stroke Tuners Handbook", Gordon Jennings describes the reasoning behind the squish band head used in most modern 2-stroke MX bike engines. According to Jennings, during normal "ideal" combustion, the flamefront proceeds from the spark plug to the outer reaches of the combustion chamber. As the flamefront proceeds outward, the temperature and pressure of the un-burned mixture increase due to increasing combustion chamber pressure as well as radiation from the burning mixture. The increased pressure and temperature in the un-burned mixture causes the flamefront to rapidly accelerate (move faster) as it moves outward. If the temperature of the un-burned mixture is raised to it's ignition point, it detonates, or, explodes rapidly. Generally, the further the flamefront has to travel, the greater the probabilty of detonation.

The squish band chamber was devised to minimize the chance of detonation by concentrating the majority of the air fuel mixture in a small pocket near the spark plug, thereby minimize the distance the flamefront must travel during combustion. The remaining air fuel mixture (outside the small pocket) is trapped in the small gap (squish clearance) between the cylinder head squish band and the piston. The small squish clearance keeps the remaining mixture cool to avoid detonation two ways; the small gap shields remaining mixture from combustion radiation and it provides nearby metal surfaces (piston, cylinder, head) to rapidly conduct heat away from the remaining fuel mixture. The downside is that the remaining mixture in the squish band gap never combusts and, therefore, never contributes to power production.

To be effective in shielding and cooling the remaining mixture (thereby minimizing the chance of detonation), the squish clearance should be small. Fortunately, a small squish clearance also minimizes the un-burned mixture which should help power production. Jennings states that the squish clearance should be no more than 0.060" and should actually be as small as possible while still avoiding mechanical contact between the piston and head. Fresh and healthy bottom end parts should allow a squish clearance as small as 0.040" or even less in a CR250.

LeadDoggy has alot to learn about 2 strokes.

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A better way than milling the head would be to lower the cylinder .010" by using a gasket for an '05 CR, it is a metal one and can be used more than once ('02 CR gasket .020", '05 gasket .010").
That would be an interesting experiment with very little $$ invested.
If you have the head milled just make sure the machine shop re-chambers the dome as necessary so you don't wind up with an engine that still pings on pump even though your squish is good.
Based on the past posts by Faded, I suspect the problem only with the squish; not the compression. If it were mine, I'd just mill the head and leave the chamber alone.

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I put a fresh top end in about a month ago, and I only have about 3-4 hours on it. I did a squish clearance and it came out to be .076 both times I measured....

....

Like I mentioned, the motor is all fresh, and tight. All clearances were within spec. I have not done a leakdown test, but did a compression test after motor was broken in, and blew 190~PSI on a warm motor, WOT.

Its pumping 190~PSI and pings on 91 octane, but once I put in the the 110, it runs like a champ, no pinging or knocking.

....

I did a compression test about 5 months ago when it was all stock, and it was blowing 215~PSI.

According to the above as well as your previous post, you had 0.067" squish clearance and 215 psi before the top end job. Now, after the top end, you have 0.076" squish clearance and 190 psi. It sounds to me like the old base gasket was 0.010" thinner than the new one. If that's the case, going to a thinner base gasket will probably only get you back to where you were - 215 psi, 0.067" squish and detonation. If you liked the way the bike ran before the rebuild, I'd go back with a thin base gasket (either OEM '05 or Cometic 0.010" thick steel), measure the squish clearance and then have the head milled to get something around 0.050" squish clearance. Like I said above, I wouldn't touch the chamber yet.

Note: If you do use a steel gasket, I'd recommend the Permatex Copper gasket dressing or something similar.

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Note: If you do use a steel gasket, I'd recommend the Permatex Copper gasket dressing or something similar.

The gasket actually comes with a thin urethance coating, but when used a couple of times putting the Permatex copper spray on gasket is a good idea.

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The gasket actually comes with a thin urethance coating, but when used a couple of times putting the Permatex copper spray on gasket is a good idea.
Yeah, it's like the OEM head gasket. I've been told that the rubber-coated steel gaskets sometimes don't seal as well as a fiber gasket at the case seam. Did yours seal up with no problems?

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Of course dropping the barrel also changes your port timing.........and your ports may not fully open with the piston being 0.010 higher. The potentially partially blocked transfer ports (all be it small) could also effect the flow pattern more than any suspect squish problem.

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...Based on the past posts by Faded, I suspect the problem only with the squish; not the compression. If it were mine, I'd just mill the head and leave the chamber alone.

Point taken.

Although that's why I said "re-chamber the dome as necessary". If milling the head by whatever amount needed to get the squish right is going to add 35psi (or whatever), that might leave him at a compression that's not going to work on pump--even if the squish is right. Situation = necessary to recut the dome. But if it's unknown if the increased compression will actually put the engine out of pump gas range (or decided that it won't), situation = unnecessary or unknown if the dome should be cut. Like I mentioned, there ARE two competing factors (squish and trapped compression). I think you're saying that the increased compression will be overshadowed by the corrected squish. And that could very well be true--I don't know.

After rereading my comment, I should have said it differently. What I meant to say is: If you need or want to keep the same compression, you'll need to recut the dome.

Leaving the chamber alone until it's determined whether it needs to be changed is good advice. :thumbsup:

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So fellas, I'm looking for a final number to be around .045 +/- ?
I'd shoot for 0.045" - 0.050", but only if I was certain that the crank/rod/bearings were healthy. If you have any concerns, shoot for 0.050" - 0.055".

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