My Mercedes is still having burning smoke and smelling oil in the back of the rear engine area. I checked the valve cover bolts and four of them were loose, two on the each side in the rear engine area. Tightened two bolts on the valve cover on the rear engine area on each side of the exhaust manifold and the intake manifold area.

However, another two of the bolts (one of each side of the intake side and the other bolt on the exhaust side) were still not able to be tighten. I feared the inside thread holes on top of the clyinder head inside threads are stripped again. I thought the issue was resolved by putting in helicoils. When I removed the two bolts that were not tightened, I saw helicoills still attached on the end of each bolt below the bolt cover/sleeve. See the attached photo on this post, thread #1. The 2 blue circles on the inside valve cover indicates the locations of the stripped threads on the top of the clyinder head.

It seems the helicoils are not doing its job. If I go ahead and retread again and using helicoils at a greater size hole, I believe it will crack the head since the top of the clyinder head thread hole is rather small on its width size.I have three options:

1. Take off the valve cover and using JR Weld and fill in the two holes using a syringe and dowel to push in the mixture to make it solid and tight. After 24 hours to dry, use a threaded drill bit and drill inside the hole. The trick is the drill have to be very straight going down on the new thread hole so the new rebuild thread will work on the valve cover bolts.

2. optimusprime on this post, thread #9 wrote "Clean the holes out make sure they are clean and without any oil ,or the repair wont work .Fill the holes with plastic metal .Push the bolt in and let it set for a day .Then unscrew the bolt , Then ,clean the top of the hole flush .These are very low torque and if you over do it they will strip out in normal circumstances . I think 9nm to lock up . Put some hylomar blue on the gasket as well as you refit it. Also keep it all clean, or bits might get in to the oil"

Is optimusprime referring to Bondo® Plastic Metal, 5 ounce, Plastic Metal seals?


3. The other option is I am trying to avoid and that is to replace the head on top of the engine block with another head that have no issues on the inside threaded holes.

As for the three choices above, it seems that number 2 is the better choice. Any comments/feedback/advice is greatly appreciated.