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Thread: Engine Head spark plug thread damage- repair options

  1. #1
    Rusted srikany's Avatar
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    Default Engine Head spark plug thread damage- repair options

    I have a STD 350 Cast iron here in Mumbai that has done 3600km till date and used with utmost care and properly run-in. Recently I have noticed that the spark plug threads have been damaged . I was able to put the plug back in and the bike runs fine. However, I have been told that I will have to get this repaired by having a bush inserted.

    Here are my queries: -

    1) usually how much does it cost to get this done here in Mumbai

    2) what all things will have to be changed once the head is opened. ( considering that the bike has beeen used only for 3600kms)

    3) Is there any mechanic that you would recommend here in Mumbai for getting this done.

    4) Will this bush be safe in the long run or do I need to replace the entire head assembly.

    Guys I would need your inputs for the same, as I am seriously looking at getting this done at the earliest.

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    MotoGrapher Sunny's Avatar
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    Rusted jd666's Avatar
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    I have a STD 350 Cast iron here in Mumbai that has done 3600km till date and used with utmost care and properly run-in. Recently I have noticed that the spark plug threads have been damaged . I was able to put the plug back in and the bike runs fine. However, I have been told that I will have to get this repaired by having a bush inserted.

    Here are my queries: -

    1) usually how much does it cost to get this done here in Mumbai

    - no idea about mumbai rates, but should not be more than 200-300 rs. But get it done by a good mechanic.

    - If theres no leakage, let it run as far as possible, and keep sparkplug changing to a minimum.

    2) what all things will have to be changed once the head is opened. ( considering that the bike has beeen used only for 3600kms)

    - Nothing actually. Only head gasket will have to be changed, and you will have to make sure that the tappet rods are in place, and dont fall to the sides of the cams, else the timing cover will have to be opened to position them again.

    3) Is there any mechanic that you would recommend here in Mumbai for getting this done.

    4) Will this bush be safe in the long run or do I need to replace the entire head assembly.


    - I suggest let it run as far as possible, im sure it can take a few more spark plug changes. If you really want a robust solution, its a new head ( around 1000-1200, but i dont know current prices ).

    Bushing can also be done.

    Guys I would need your inputs for the same, as I am seriously looking at getting this done at the earliest.


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  4. #4
    Rusted srikany's Avatar
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    Default Thanks for the reply

    Quote Originally Posted by jd666 View Post
    I have a STD 350 Cast iron here in Mumbai that has done 3600km till date and used with utmost care and properly run-in. Recently I have noticed that the spark plug threads have been damaged . I was able to put the plug back in and the bike runs fine. However, I have been told that I will have to get this repaired by having a bush inserted.

    Here are my queries: -

    1) usually how much does it cost to get this done here in Mumbai

    - no idea about mumbai rates, but should not be more than 200-300 rs. But get it done by a good mechanic.

    - If theres no leakage, let it run as far as possible, and keep sparkplug changing to a minimum.

    2) what all things will have to be changed once the head is opened. ( considering that the bike has beeen used only for 3600kms)

    - Nothing actually. Only head gasket will have to be changed, and you will have to make sure that the tappet rods are in place, and dont fall to the sides of the cams, else the timing cover will have to be opened to position them again.

    3) Is there any mechanic that you would recommend here in Mumbai for getting this done.

    4) Will this bush be safe in the long run or do I need to replace the entire head assembly.


    - I suggest let it run as far as possible, im sure it can take a few more spark plug changes. If you really want a robust solution, its a new head ( around 1000-1200, but i dont know current prices ).

    Bushing can also be done.

    Guys I would need your inputs for the same, as I am seriously looking at getting this done at the earliest.
    ================

    Thank you for the prompt and detailed reply. I checked with a mechanic called Nagesh from churchgate here in Mumbai, and he said that only the bush will be put in and it will be as good as a new head in terms of reliability.

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    The bush can be inserted for the spark plug threading.. I have done this on my old cd100.. If the fitting os done correctly, the bush works as good as an original fit....

    But get this done from a mechanic who is an expert in such jobs... Else you might end up creating more problems than those you will solve
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    I would suggest having this fixed asap. You wouldn’t want the spark plug coming loose in the middle of nowhere. Even if it doesn’t come out fully, it can result in the loss of compression and charge(fuel-air) that could make your engine run lean and even exhaust valve failure in the long run!!
    Im not sure what this bush is but one of my RD's had a cylindrical 'bush' put in. They basically drilled a big hole where the spark plug is and inserted a bush/sleeve. That was a big mistake as it started giving way under high compression (just a measly 140 psi when cold).
    I would suggest getting a Helicoil implant. Google for more info. They basically re-tap the spark plug hole to a bigger size and put in a helicoil. Its a spring like thingie. This seldom leaks or gets loose. Just remember not to take out/force out the plug when the head is hot!
    I had a Helicoil put into another RD’s head yesterday and it cost me a measly 70 bucks from a very reputed lathe!

    JD666, good points you have mentioned there but I would like to make a slight correction. Loosen the pushrods from the side cover before fixing the head. The thickness of each head gasket is different. Once the head is tightened into place, adjust the pushrods. I made the mistake of not loosening and then tightening the pushrods. I ended up with a bent rod! Im sure Srikany’s mech will do that.

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    Rusted srikany's Avatar
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    Default Thank you all

    Hi there,

    Got the Bike serviced yesterday and also the cylinder head fixed as well. thanks for all your replies and suggestions.

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    @james - yes, the push rods will have to be re adjusted once the head is put back. The issue is if the pushrods fall into the side of the cams, the timing cover will have to be opened to position them properly.


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    As somone earlier told u need to solve this problem quickly else the weakened threads may give in at the most unexpected time & worse the plug may fly out from the head like a projectile & cause injury.

    The bush (socket) solution is cheap, tried & tested but it needs to be done properly at a good lathe.

    Actually they put a mildsteel socket by oversizing the plug hole & jam it by hitting its edges on the inside ( like a mushroom) & apply araldite to seal air leaks. the plug then tightens into the threads of the socket. the solution is reliable in my experience. no problem.

    but u should always avoid opening the plug when engine is hot & avoid overtightening in general.
    Last edited by srpsinu; 12-06-2010 at 08:00 PM.

  10. #10
    Rusted srikany's Avatar
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    Default Yes got the cylinder head repaired

    Quote Originally Posted by srpsinu View Post
    As somone earlier told u need to solve this problem quickly else the weakened threads may give in at the most unexpected time & worse the plug may fly out from the head like a projectile & cause injury.

    The bush (socket) solution is cheap, tried & tested but it needs to be done properly at a good lathe.

    Actually they put a mildsteel socket by oversizing the plug hole & jam it by hitting its edges on the inside ( like a mushroom) & apply araldite to seal air leaks. the plug then tightens into the threads of the socket. the solution is reliable in my experience. no problem.

    but u should always avoid opening the plug when engine is hot & avoid overtightening in general.
    ========

    Hi,

    I got the head repaired with the bush method along with my general servicing. I happened to see a head that had been fixed this way of another bike and your description exactly matches the way they had fixed the bush.

    I have driven with speeds upto 80kmph and travelled almost 100 kms after this fix and there is no visible or noticeable air leak anywhere from the spark plug. I hope the job was well done and wont cause any problems in future.

    The head gasket used is an original one and there is no oil seepage or leakage as well

    thanks for your reply
    Last edited by srikany; 12-07-2010 at 11:26 AM.

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