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Help Yamaha SRX250 Overlooked head problem

Discussion in 'Yamaha 250cc Singles' started by GeorgeT, Aug 27, 2021.

  1. GeorgeT

    GeorgeT Active Member

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    After getting the bike's cold start issues sorted a much larger issue has surfaced. What I and the previous owner thought was a leaking rocker cover gasket has turned out to be much worse, a probably cracked head. On close inspection, the oil leak is not coming from the gasket but between the gasket and the top cooling fin on the left front of the motor just above the left header pipe. If anyone knows where I can access a replacement head (including valves, camshafts, etc if possible) I would much appreciate it. I have located a wrecked bike with a full motor but it will be difficult to get because of the current NSW lock-down rules. The wrecked bike option may be my only one given the rarity of bits for this model.
     
  2. jmw76

    jmw76 Well-Known Member

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    I think I would start by taking the head off and determining what is really going on.
    You may actually be able to repair the head, if that is actually the issue.
     
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  3. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Can you put up a pic of exactly where you think it is leaking? Normally they ONLY crack between the valves into the spark plug hole... and that just creates a loss of compression when the engine is hot.
     
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  4. GeorgeT

    GeorgeT Active Member

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    Yes, I can do that tomorrow. It does not seem to have gone as far as the spark plug area. While I have you. Is it possible to use a TT250 head. The gasket is the same while the bore to stroke ratio is different, i.e., the bore diameter is 2mm smaller for the SRX, i.e., the SRX is 73mm while the TT is 75mm? Another question. If the journals are worn on a replacement head can my bearings be fitted satisfactorily or are the cams dependent on good journals? In fact, are there indeed any bearings at all? Sounds like a silly question but I have not worked on an SRX head.
     
  5. GeorgeT

    GeorgeT Active Member

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    It seems it may be as simple as an oil-pressure check screw that I have overlooked. I will check that tomorrow. HeadLeakArea.jpg
     
  6. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    The basic head casting is the same for XT, TT 250 and 350.
    The TT, XT series have a decompression actuator to allow easy kick starting and the SRX uses a seal to block that off.
    If the seal isn’t 100% then you could get a leak.


    You can certainly use the TT250 twin cam head on the SRX.
    There are no bearings in the head for the cams, the bearing caps are matched so you can’t use caps from one head on another……easily.

    Any cracks will be in the combustion chamber. You would not get a crack where you indicated.
    I am trying to remember if the SRX head has a plug where the TT350 has an external oil line. This would be on the the inlet side on the the side of the engine.
    I suspect yours has a leak in the middle above the exhaust ports.
    Easy fixed.
    I will take a pic of mine in the morning.


    Sent from my iPhone using Tapatalk Pro
     
  7. GeorgeT

    GeorgeT Active Member

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    Thanks for that great information. I'm assuming then that the TT350 is also a compatible head for the SRX250 albeit the bore is a little increased giving a potential increased cubic capacity and slightly lower compression ratio? The thing is it does mean that more options are available for refurbishment. The presence of a decompression actuator would be of no real consequence. That is like when I had a BMW R100/7, I swapped, of necessity, the gearbox from an R90/6 that had a kick start when there was no way a person like me, 65kg soaking wet at the time, could kick start an R100 BM.
     
  8. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    The 350 head isnt an option as there is no internal oil feed from the barrel.
    Apart from that you wouldnt be able to use the head gasket and the compression would suffer considerably.
    If you look at the diagram below from Megazip items 3, 4 and 23 are the most logical place for the leak.3 and 4 are left overs from the decompression system (you take that out from a TT head) and 23 plugs the hole.
    There is no oil pressure check point around there that I can remember. The lobes are splash lubricated and the journals are fed from the internal drillings in the head via feed from the barrel.
    SRX250 head.png
     
  9. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    This is the oil feed system... Off to have my 2nd Jab... will take pics of my SRX head later
    upload_2021-8-28_7-56-42.png
     
  10. GeorgeT

    GeorgeT Active Member

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    Where items 3 and 4 go is also plugged on mine. There is a vertically screwed hex bolt above item 23 that probably held the decompression unit in place on the TT? The oil-pressure test point is on the inlet side. see attached image. HeadOilPoints.jpg
     
  11. GeorgeT

    GeorgeT Active Member

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    If it is either the 3, 4, or 23 points causing the leak it will mean removing either the head or exhaust header pipes to fix I think? I did something similar many years ago on a VW carby that had a manually choked carby converted to automatic choking. It was sucking air into where the manual choking mechanism would have been. I Araldited a $2 dollar coin in there, worked a treat.
     
  12. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Items 21 and 22 as well could be the leak point.
    What you say is an Oil pressure check point is actually where the oil feed for the TT350 goes in ( I will do a pic of mine shortly) and is also there to enable them to drill for the oil feed to the journals. Mine doesnt go all the way through so I am surprised yours has a plug in it...
    I dont think you need to remove the head to sort it out... I would check items 21, 22 (should be a copper washer to seal it)
    Ammended
    Item 3, 4 would be where the lever for the decompression system bolts on and "should" be a blind hole if memory serves correct.
    21 and 22 are easy to get at... the plug for the larger hole is a little more difficult but should be easy enough.
     
    Last edited: Aug 28, 2021
  13. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Ok... pic from my partially completed SRX350 3WP :D

    Thsi is showing the blanking plug and what would have been the locating bolt for the decompression pivot on the 350 head. Exactly the same on the SRX head.
    IMG_6048.jpeg

    This is the oil feed to the cam bearings. the SRX has the oil feed internally through the barrel. This can also be used for air bleed and to ensure you have oil going to the head. Best not to take the sealing bolt all the way out if you are going to run the bike though.
    IMG_6049.jpeg


    View from the top showing the locating bolt. there is a copper washer under that bolt.
    The boss in the foreground is where the cable mount bracket for the decompression lever was bolted... it is a blind hole... well it should be.
    IMG_6050.jpeg

    wider view showing all of the front of the head... note the boss on the left.. this is a cast boss which is opposite to the oil feed... it isnt plugged (even though it looks like it) that is just the casting marks.
    IMG_6051.jpeg

    Last one... this shows the spare threaded hole where the decompression cable mount was on the 350 head. Again... this is a blind hole but if you are concerned you can use a short bolt with an o ring on it to seal it off.
    IMG_6052.jpeg

    All in all I think. your most likely leak point will be the blanking plug... you can buy them new. You will most likely find that somebody previously has removed it for some reason and pushed it back in... and probably damaged the rubber out coating. It is a rubber coated steel plug so if the rubber is compromised... it can leak as that area is where oil pools to spash lubricate the cam lobes...
    Hope it all helps...
    Dont worry about trying to find a new head... I am sure yours will be fine, just fix the leak.
     
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  14. GeorgeT

    GeorgeT Active Member

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    I have fixed the leak, temporarily at least. It was the screw and washer 21 & 22. I found that to be not very tight at all. I undid it, cleaned it up then put it back in coated with aeronautical Loctite gasket sealant. I attempted to torque it but felt the thread seemed too flimsy so got it as tight as I was game to and it sealed but I think I should get a new copper washer then refit it more permanently. The bolt seemed to be not exactly the right bolt as it had been cut short and filed on the end. It also had a Phillps-head option for tightening which seemed inappropriate for a screw at this location?
     

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