86 TRX250r Cylinder, Head, Gasket Problems
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Thread: 86 TRX250r Cylinder, Head, Gasket Problems

  1. #1
    Junior Member SoCal2Stroke's Avatar
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    86 TRX250r Cylinder, Head, Gasket Problems

    First of all thank you guys for having this awesome forum. I've been riding two strokes (banshees and yz250s) for almost 20 years. When I was young I always wanted a 250r and finally made that happen in December. It was an all stock 86 with the exception of an FMF exhaust system. After riding it a few times I decided to try and give it a little power. This is where my problem came in.

    I bought a used Paul Turner Hyperdyne cylinder, and Powerhead. The cylinder is bored to 69.5mm with matching Wiseco piston. When I bought the used cylinder/head it came with a spacer plate I did not use in my install. I was told these parts were off a 1985 250r three wheeler and all came off a running bike as a complete set. I checked that the original piston was the same height as the 69.5mm one and that the wrist pin location was at the same location, which it was. When I put the top end back together and kicked it over it leaked terribly (Don't hate me, I tried to reuse the original head gasket). I ordered a new cosmetic head gasket kit but when I went to install it I noticed it was much thinner than the original (at the time I didn't know there were different ones, I have since read that there is a 0.5mm and a 1.5mm and the old 69.5mm gasket was the 1.5mm thick). To make a long story short the thin gasket it too thin and would allow the piston to hit the head if I put it on. The problem seems to be that the piston comes over the deck ~0.5mm. Herein lies my question, is this normal and the thick gasket allows the piston to operate like this or do I need to use the spacer (~1/4" thick). I have rebuilt numerous yz250 and banshee motors and they all seem to have the piston ~1/8-1/16" below the deck at TDC, should my 250r be the same? I'm trying to avoid having to put all the stock stuff back on to figure this all out and was hoping some of you will know. I was about to order a thick Wiseco gasket last night and then started reading they were junk. Do you guys think a thick OEM gasket cut to 69.5mm ordered from BDT would solve my problems or does the piston coming over the deck lead you to think I have bigger issues with the cylinder?

    Thanks in advance for your help!
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    Last edited by SoCal2Stroke; 05-17-2017 at 06:11 PM.

  2. #2
    Senior Member american opel's Avatar
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    well with some of the larger pistons you need to run a 2mm spacer,or you need to have the head cut of the piston.even after that you need to check squish.

  3. #3
    Junior Member SoCal2Stroke's Avatar
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    Quote Originally Posted by american opel View Post
    well with some of the larger pistons you need to run a 2mm spacer,or you need to have the head cut of the piston.even after that you need to check squish.
    Thank you for the reply. Do you know if there is a way to know whether or not I need to run a spacer? I did not put the spacer that came with the cylinder (which is larger than 2mm, I don't have it with me so I'm not exactly sure how thick it is) in because the pistons were the same height and wrist pin locations the same. Also the port timing, to the best I can tell, is unchanged in the Turner cylinder from stock so I assumed no spacer. In regards to squish, can I do a compression check to see if its adequate or do I need to get the solder out and start measuring?

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  5. #4
    Senior Member acecarlos's Avatar
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    86 TRX250r Cylinder, Head, Gasket Problems

    You can't just install a spacer plate, 2 MM, 3/32", 3/16" or 1/4".....it all depends on how the cylinder was set up. I have seen PT Hyperdyne cylinders setup all different ways.

    You need to check port timing, you also need to check the head and dome to see what squish step is machined. Then decide if your running Race Fuel or Pump Gas, then you can begin to properly setup and determine the spacer plate needed, if any and its thickness.

    Do not buy the BDT 69.50 MM 3 piece OEM head gasket until you have determined what you have, and what you need....I would rather loose a Head Gasket sale than assist in destroying your engine.

    Remember, you can always call BDT and we or I can assist you in properly setting your engine-cylinder, no need to purchase anything; here to help.

    Carlos


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    american opel and Shawn11211 like this.

  6. #5
    Junior Member SoCal2Stroke's Avatar
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    Quote Originally Posted by acecarlos View Post
    You can't just install a spacer plate, 2 MM, 3/32", 3/16" or 1/4".....it all depends on how the cylinder was set up. I have seen PT Hyperdyne cylinders setup all different ways.

    You need to check port timing, you also need to check the head and dome to see what squish step is machined. Then decide if your running Race Fuel or Pump Gas, then you can begin to properly setup and determine the spacer plate needed, if any and its thickness.

    Do not buy the BDT 69.50 MM 3 piece OEM head gasket until you have determined what you have, and what you need....I would rather loose a Head Gasket sale than assist in destroying your engine.

    Remember, you can always call BDT and we or I can assist you in properly setting your engine-cylinder, no need to purchase anything; here to help.

    Carlos


    Sent from my iPhone using Tapatalk
    Thank you for the generous offer Carlos. The bike is stored at my Dad's house and I will be there in a couple weeks to visit and work on the bike. I will give you guys a call and try and get to the bottom of what this cylinder needs to run correctly. I will be sure to buy all the required parts from you guys, all of your help on this forum has certainly earned my business.

  7. #6
    Senior Member acecarlos's Avatar
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    Call when and as needed, best to determine what is wrong, and go in the right direction.

    Carlos


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