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Re-threading Cylinder Head

Meatybeaty

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New to this site, just got a CBX750 1984 , cylinder head has 2 helicoils already , what do you guy's think to re-threading the sparkplug hole and replacing the plug with 4 NGK7822 - BPR6ES same length as existing but a 14mm thread
 
That would likely be a mistake for several reasons. And why on earth do you want such a hot plug? A '6' plug is a car plug , most bikes it is way too hot for.
 
Hi thanks for the reply, the plug I mentioned was more for size , I can always change heat , as it has the same reach as the standard plug and there's no interference with the valves, I wondered whether this might work rather than a new head, just wondered if anyone had done it
 
Not wise due to too much metal removal in a 4 valve chamber. The metal removal will affect the plug heat dissipation too. The seat for plug gasket has to be recut as it is too small for bigger plug. Maybe the head has not enough relief to get a bigger socket in as well. Bigger plug tightens more so easier to crack too. The job with all 4 needs to be done on a machine to get all 4 holes straight, doing it by hand is a mistake.

Main thing is the plug is picked with the proper amount of metal around it to dissipate heat correctly, that metal not there will both weaken the head to crack easier and not make plug get rid of heat correctly too. You think there is no interference with the valves but they have to be kept away a certain amount from plug hole or cracking due to thinness of metal cross-sectioning then becomes an issue even if it looks OK. You need thicker there than just an eyeball says.
 
Not wise due to too much metal removal in a 4 valve chamber. The metal removal will affect the plug heat dissipation too. The seat for plug gasket has to be recut as it is too small for bigger plug. Maybe the head has not enough relief to get a bigger socket in as well. Bigger plug tightens more so easier to crack too. The job with all 4 needs to be done on a machine to get all 4 holes straight, doing it by hand is a mistake.

Main thing is the plug is picked with the proper amount of metal around it to dissipate heat correctly, that metal not there will both weaken the head to crack easier and not make plug get rid of heat correctly too. You think there is no interference with the valves but they have to be kept away a certain amount from plug hole or cracking due to thinness of metal cross-sectioning then becomes an issue even if it looks OK. You need thicker there than just an eyeball says.

Good counsel , will go for a new (second Hand ) head, like a lot of other bikes this age parts are hard to get, but I want to keep this one ,done a lot of work on it, and to fit the right parts is the sensible way to go,Thanks for you Advice. Bill
 
Not wise due to too much metal removal in a 4 valve chamber. The metal removal will affect the plug heat dissipation too. The seat for plug gasket has to be recut as it is too small for bigger plug. Maybe the head has not enough relief to get a bigger socket in as well. Bigger plug tightens more so easier to crack too. The job with all 4 needs to be done on a machine to get all 4 holes straight, doing it by hand is a mistake.

Main thing is the plug is picked with the proper amount of metal around it to dissipate heat correctly, that metal not there will both weaken the head to crack easier and not make plug get rid of heat correctly too. You think there is no interference with the valves but they have to be kept away a certain amount from plug hole or cracking due to thinness of metal cross-sectioning then becomes an issue even if it looks OK. You need thicker there than just an eyeball says.

Got a replacement head, looking more closely at the original head , as you said about the cracking, here are all four spark holes , and this is with helicoils in, makes me really think twice about using Helicoils or Inserts
 

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