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Thread: Air cooled 250r squish band clearance?

  1. #1
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    Air cooled 250r squish band clearance?

    Hey everyone, Iím hoping someone can tell me the squish band clearance for my 82 250r. I also have a sunburst DG head that Iím gonna put on and would like a squish band recommendation for that as well if different. Iíve looked through the manual and internet with no success. Thanks
    1982 Honda 250r ATC
    DG pipe
    DG silencer
    Boyesen carbon fiber reeds
    K&N filter
    Carlisle tires
    Tusk enduro lighting kit (100% street legal)

  2. #2
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    This is a question that can get dozens of different answers even from 2 stroke experts, and can ultimately just leave a person dazed and confused.

    I have read nearly everything I could find on 2 stroke squish bands and combustion chamber designs, and have a fair amount of first hand experience with squish band clearance on both 2 strokes and 4 strokes, but I am not a 2 stroke expert. From my experience, I can tell you that the squish clearance is nearly irrelevant on 2 stroke engines with stock, or nearly stock displacement, and stock, or nearly stock compression.

    The angle, and width of the squish band and the blend angle (transition from the squish band to the combustion bowl) are critical, but all of these are most important on engines modified for high performance.

    Poorly designed 2 stroke combustion chambers, like many were back in the day, can cause some detonation on some models. If you have detonation, you can often cure it with higher octane fuel. If the combustion chamber design, and not the compression ratio, is causing the detonation, then correcting the design will reduce or eliminate detonation when using the same octane fuel you were before.

    Using higher octane fuel to reduce detonation in an engine that detonates due to a poor head design works, but it is more of a band aid then a cure because it is not addressing the cause of the issue, it is simply curing the symptoms. If the cause was corrected such as improving the combustion chamber design etc, the symptoms would not be as severe and the higher octane fuel may not be needed,

    In your particular case, you can check the volume of both heads. Some of those aftermarket heads had a smaller combustion chamber to increase compression and I'm pretty sure the DG head is one of those although I think DG may have made 2 versions, and the 82 250r is a low compression engine, so the boost in compression is a significant benefit to that particular engine, although, it may cause some detonation but I doubt it. Hopefully there is someone here that has put the same head you have, on to an 82 can give you more info.

    Also, if you have more than around 160 psi of compression, it will likely need higher octane than you can get at the pump. If you have around 140 psi and you have detonation and are using at least 91 octane, then the octane is not the problem and the timing or jetting or combustion chamber design is etc. Also, in general, it is preferable to use a copper head gasket if available as it will improve heat transfer over the paper composite ones.

  3. #3
    Join Date
    Nov 2018
    Location
    Tucson, Arizona
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    I understand what you’re saying and I appreciate it. It has all the vintage racer type add ons minus any machine work so I’d say it lightly modified. I’m about to put it back together after a complete rebuild and I’m just not satisfied with reading on the web, “typical squish band for a 250 type 2 stroke.” I can’t even find a solid factory squish band measurement to go off of. I can’t just accept bolting it back together with a oem head/base gasket and feeling confident that since nothing has been machined, then everything is fine. There’s that 10% voodoo witch doctor factor in every engine that magically changes things inside that screw things up, (my scapegoat for falling complacent in a moment of enjoyment while building an engine and overlooking something.)
    1982 Honda 250r ATC
    DG pipe
    DG silencer
    Boyesen carbon fiber reeds
    K&N filter
    Carlisle tires
    Tusk enduro lighting kit (100% street legal)

  4. #4
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    Quote Originally Posted by MisFit138 View Post
    I understand what youíre saying and I appreciate it. It has all the vintage racer type add ons minus any machine work so Iíd say it lightly modified. Iím about to put it back together after a complete rebuild and Iím just not satisfied with reading on the web, ďtypical squish band for a 250 type 2 stroke.Ē I canít even find a solid factory squish band measurement to go off of. I canít just accept bolting it back together with a oem head/base gasket and feeling confident that since nothing has been machined, then everything is fine. Thereís that 10% voodoo witch doctor factor in every engine that magically changes things inside that screw things up, (my scapegoat for falling complacent in a moment of enjoyment while building an engine and overlooking something.)
    Well about a bazillion people have simply bolted the DG heads to the early 250r's and never had any real problem from what I have seen other than possibly some slight detonation which is cured with higher octane, but you will probably get some more posts from others that may have some more useful info for you.

    As far as I know, there is no factory squish band spec for any 2 stroke that is available to the public, plus, due to production tolerances, a production spec could vary by around 010" etc. The production spec will always be at least whatever the thickness of the stock head gasket is, and the squish band on most of these older heads is recessed into the head by at least .010" and often much more. The factory squish clearance on my 1980 YZ465 is around .074", and some people have in fact simply added a second head gasket to these bikes to lower the compression so they can start them, and that makes the clearance around .114", lol, and they claim they run just fine that way.

    The stock clearance on my 1980 YZ250 is around .060" but the head looks like it was milled a little and the clearance is now around .045 and it pinged more than a Pachinko machine on 91 octane, so I mix 25% VP Street Blaze (which is 10% alcohol) with Shell 91 which also has some alcohol and that stopped the pinging stopped the majority of the pinging.

    There is such a thing as overthinking a problem or being over concerned about something and the squish clearance is one of those things as far as stock and near stock engines go.

  5. #5
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    If you want to know what a safe minimum squish clearance is on a 2 stroke, then around .036" is it, but again, squish angle and blend radius etc are also important

    Below is just a little info on the subject which you may have already seen, but another article discusses the speed of the flame front towards the center of the dome and how the squish band width and angle and blend angle might affect it, but I will have to look through my pile of notes to find one of the articles I have regarding it.

    http://www.klemmvintage.com/91oct.htm

    http://www.gasgasrider.org/html/measuring_squish.html

    http://www.southwestairsports.com/pp...ish/squish.htm

  6. #6
    Join Date
    Nov 2018
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    Tucson, Arizona
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    I agree I’m overthinking it. I just want as much knowledge going into this cus I’m much newer to 2 strokes then 4. With car engines i try to stay as surgical as possible, since I feel it’s the difference of getting 250k vs over 300k miles out of it and you’ll really only know once you get up there. Here’s a pic of the DG head. It’s got some pitting already by the prior owner and I’ve been told it’s fine as is. I’m wondering if I should machine the pitted surface and deck it or just leave it. This is why I’m searching for all this info because I plan on doing the milling in my shop.
    1982 Honda 250r ATC
    DG pipe
    DG silencer
    Boyesen carbon fiber reeds
    K&N filter
    Carlisle tires
    Tusk enduro lighting kit (100% street legal)

  7. #7
    Join Date
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    Click image for larger version. 

Name:	C5009D5B-0554-4AF9-8D4C-E22A424A877A.jpeg 
Views:	19 
Size:	1.86 MB 
ID:	258241Click image for larger version. 

Name:	C5009D5B-0554-4AF9-8D4C-E22A424A877A.jpeg 
Views:	19 
Size:	1.86 MB 
ID:	258241
    1982 Honda 250r ATC
    DG pipe
    DG silencer
    Boyesen carbon fiber reeds
    K&N filter
    Carlisle tires
    Tusk enduro lighting kit (100% street legal)

  8. #8
    Join Date
    Nov 2018
    Location
    Tucson, Arizona
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    Click image for larger version. 

Name:	image.jpg 
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Size:	2.24 MB 
ID:	258242y Here’s another
    1982 Honda 250r ATC
    DG pipe
    DG silencer
    Boyesen carbon fiber reeds
    K&N filter
    Carlisle tires
    Tusk enduro lighting kit (100% street legal)

  9. #9
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    Unfortunately I am the only person on the site that can't view photos in most formats because the moderators have blocked me from seeing them, lol, so I for one can't offer you any suggestions for the pitting unless you can post your photo in a format that does not require a link to be clicked on, but hopefully someone else here will see the photo and be able to offer some good suggestions, however, unless there is a bazillion fairly large craters in it, it will be fine providing the surface around the craters is smooth so there isn't any "shards/peaks" of metal sticking up around them, because these can sometimes get hot enough to ignite the fuel mixture before the ignition does which could lead to an "unfavorable" outcome.

    132 words and only 1 period.

  10. #10
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    Nov 2018
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    Yeah there’s no sharp points, it’s just a lightly textured surface on the exhaust side. I’ll probably just run some 800 grit on it to shine it up and clean out the carbon.

    I welcome a run on sentence
    1982 Honda 250r ATC
    DG pipe
    DG silencer
    Boyesen carbon fiber reeds
    K&N filter
    Carlisle tires
    Tusk enduro lighting kit (100% street legal)

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