Air leak around spark plug

[Rhino363]

Did a leak down on my RD 400c because my right plug was showing a lean condition and found its leaking around my plug like the crush washer isnt sealing. I tried another plug and its doing the same thing. Anyone else run into this before and are there any ways to seal this ?
Thanks

[Barrie]

check for cracks around the plug hole is my first thought.

[Rhino363]

I took the crush washer off and made a gasket out of gasket material and it seals with the paper gasket material

[Dvsrd]

Sounds like some very light spot facing or other machining would fix it.

[1976RD400C]

I took the crush washer off and made a gasket out of gasket material and it seals with the paper gasket material

  Crack check, as mentioned is a good idea, I've seen that on aluminum head car engines. A machine shop could fix that pretty quick if it is not cracked or buy a used head. You could also  try installing 2 new crush washers on that plug and tighten the plug up a bit more than normal.

[SUPERTUNE]

Try torquing to 17 ft-lbs with a new plug and see if it stops the leak. If not, a spotface machining on the seat is needed.
Like this...
Chuck

[Rhino363]

I can not get it seal with ngk crush washers but I took a crush washer off of a autolite plug I had in the garage its a little different style and it seals with it

[quocle603]

nice! I was gonna suggest trying a copper gasket (after annealing it).

[Diablo007]

Is there a special tool that can be used to spot face rather than taking the heads to machine shop?  A quick search turned up nothing, but I'm guessing the process would be similar to squaring a pen blank, or even facing (re-crowning is the proper term, doh) a rifle or pistol muzzle.   Basically a cutter with a guide that passes through the plug hole.

[Yamaha 179]

Yamaha had a kit for the individual that used Timeserts that replaced the spark plug threads.  In fact they sold 6mm and 8mm kits also. I have the entire series in the shop and have used all of them over the years.  Haven't tried to buy the inserts from the local shop bud did find them on the internet.  If anyone wants more info, post the request here and I'll get it for you.  They are very simple to use and better, in my opinion, than heli coils. 
Lyn Garland

[Diablo007]

So...spot facing would increase the protrusion of the spark plug tip into the cylinder.  I'm guessing I would need to ask the machinist how much material was removed, and I would need to find the appropriate spark plug gasket.  Would just replacing the gasket with a solid copper gasket be worth trying?

[quocle603]

Doesn't hurt to try that option. I've been using annealed coppers washers to seal up certain areas that I am getting leaks and it's been great. It will hold up better than the paper gasket material on the spark plug.

[pidjones]

I believe that there are tests to look for hairline surface cracks. If it is just a surface irregularity, a mandrel could be made to permit dressing up the sealing face with very little material removal.