Does anyone have a modern 250cc head insert they could measure

Discussion in 'Trials' started by PMK, Apr 28, 2019.

  1. PMK

    PMK Been here awhile

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    Wanting to update the combustion chamber on my OSSA MAR. I have machined heads / combustion chambers on other non trials bikes in the past. Simply, trying to get an approximate area of the squish band area vs bore diameter and see if it comes close to what is typically done for other modern two strokes.

    No intentions to increase compression ratio or build crazy power, simply want better and smoother engine response on modern fuels or Avgas.
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  2. PMK

    PMK Been here awhile

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    Anyone...
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  3. MATTY

    MATTY BORDER RAIDER

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    Can not help you with Bore squish ratios, but can tell you why i never went down this route with a bike i have recently been improving.
    Engine a 2o3cc minsk in an RTX T212 did not have enough integrity in the cylinder head to stand reworking for an insert.
    I can not say i am familiar with the ossa Mar so your idea might work, but my thoughts on the Whole insert idea were mixed , on the one hand i could see the improvements on normal fuels, ( We Get 95 RON normal unleaded here) and had real potential to increase power. Yet on the other it seemed a lot to do to just get cheap pump fuel to work when just switching to ultimate or something in the 98 range improved how this engine ran with the porting and ignition i had.
    So i think you need to make sure of the heads construction and decide if its man enough to take the profile of an insert as stock or make up another head. I just want this bike of mine running and performing, once its there its there i don’t care how i arrive at the end result. You can do a lot of playing with fuels and octane boosters to arrive at optimum performance, for the time it would take to sort inserts out that was my conclusion. But you may be of a different opinion and wish you all the best whatever you decide to do.
    I did a few home brewed mods to the Minsk engine a flywheel weight longer con rod with different piston barrel (JUG IN USA ) barrel spacer raised exhaust port Reed Valve block and clutch mods for a super light clutch as well as carb swaps and hours rejeting and adjusting modifying ignition. Its all good fun but is it sanity i ask myself,? Well NO! i could have worked more hours the hours i put into this and got a way better bike for my input, but that is not why i did it.
    And probably just the same reasons why you are contemplating what you are with your MAR.
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  4. PMK

    PMK Been here awhile

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    I had considered what you mention, buying an insert and machining the oem MAR finned head to accept the modern head.

    However, this post is more relative to simply seeing the squish band width and squish band ratio, to validate remachining the oem head into a better, more optimized for modern fuels, combustion chamber.

    Typical basis for modern race bikes is a ratio of 50%. I could just shoot from the hip, go with 50% and adjust the squish clearance to typical specs. Just wanted to see how the squish band ratio compares.

    Yes, fuel is a huge factor. Downside being, the old MAR setup certainly prefers race gas at $12 per gallon. Avgas runs ok but is lazy in regards to throttle response. I did try 90 octane unleaded / non ethanol rec gas and that did not work well.

    A few weeks ago I was able to buy a used oem cylinder head. Essentially, if the modifications do not help, it can go right back to stock.
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  5. lineaway

    lineaway Long timer

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    I could send you an insert off my EVO. if you like.
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  6. PMK

    PMK Been here awhile

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    Thanks, but even easier than that. I will post a photo showing what I need in a photo of the EVO head. From the photos I can compare and decide what actions to accomplish.
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  7. PMK

    PMK Been here awhile

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    1521E251-29E0-4EA6-85B8-7A8328890F4E.jpeg
    If you have a metric tape measure or similar, a photo of your head insert with a means for me to see the dimensions would be sweet.

    This is a photo of the MAR head, with a rigid metric scale positioned to make it easy to read the squish band width and combustion chamber dome dimension.
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  8. lineaway

    lineaway Long timer

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    Like this.

    Attached Files:

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  9. PMK

    PMK Been here awhile

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    Yes, exactly, thank you.

    Curious, what year and model EVO?

    Comparing the two it is obvious the squish band width and ratio is quite different. Now the true effort begins.
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  10. PMK

    PMK Been here awhile

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    The MAR head has a 17mm wide band, where as the EVO has about an 11mm wide band.

    MAR is 72mm bore vs the Beta at 72.5mm.

    Quick math and such, the Beta numbers, even crudely measured come in at typical modern squish band ratio of 50%. The OSSA, is very old school at 75%.

    Now to decide what gets done.

    For anyone following or understanding this, yes, I realize squish clearance is important also. That will be factored in before, during and after the changes are made.

    Thanks Lineaway
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  11. lineaway

    lineaway Long timer

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    16 Factory 250 std insert.
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  12. MATTY

    MATTY BORDER RAIDER

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    My son has been playing with a 100cc Youths ATV about 18 months now, he did some compression ratio increasing and a different piston and he did a crude squish band re shape in my Colchester triumph 71/2 lathe ( marketed clausing 15 in usa) it was not much different to this just was chucked up properly and the piston clamped to the tool post after centring up. and he literally did what this guy is doing here only slightly more properly. My son watches this chap and he is quite innovative with his ideas, it might be you can get some inspiration and perhaps add something you see to your modification plans. I know i have enjoyed getting this RTX T212 performing better, it is now a very flexible responsive torquey motor and its not vastly different in tune in fact i even went back to the stock carb after trying several others.
    2 stroke stuffing he is a mad Scandinavian shed builder who gets some interesting stuff done in his own inevitable way. 50cc 20hp is not done without getting something right.
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  13. laser17

    laser17 Long timer

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    No safety glasses? He has the (not to) part down pat.
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  14. PMK

    PMK Been here awhile

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    I have always used the lathe and a mandrel, but the technique in the video works for him.

    Did the math and some simple checks. Need to cc the volume, check existing squish, then decide how to proceed.
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  15. PMK

    PMK Been here awhile

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    Was waiting for a return phone call about these vintage OSSA head designs and modifications. In short, your photo, my math and more essentially matched what I verified in the phone call.

    Thank you for taking the photo.

    I did stumble upon a previous Motobene topic here regarding modifications to cylinder heads. Good technical read, but not full of basic details. All good info though. Sadly, the primary base of knowledge does not seem to post here anymore and has not for several years.
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  16. Brewtus

    Brewtus Buffoonery, Inc.

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