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Discussion Starter #1
On my 1994 Mercedes E320 wagon, I replaced the valve cover gasket with a new one. When I put in the 12 bolts on the valve cover and goes on top of the head, all but two of the bolts went through ok and tightened. The other two bolts did not tightened and upon looking at the threads of those two bolts, it appears to be some form of liquid on the bolts threads. The 2 bolt threads themselves are fine, no thread stripping on the bolts. It seems that the 2 inside threads on the top head are stripped and the bolts will not tightened. Here is the location of the two holes on the valve cover that the 2 bolts will not tighten on the head. (blue circle):

Is there some kind of thread repair that can be apply to the two holes on the head and be able to tighten the bolts? I believe there is something like liquid thread that can be apply on the bolts. I appreciate some comments/feedback on this issue. Thanks for reading.
 

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You could buy yourself a m6 x 1.5 Helicoil repair kit. Google it & see. Kits should be on eBay. Just make sure to clean & protect the motor from swarf
 

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Discussion Starter #3
JC220 thanks for responding. I was doing some checking on this issue. I came across this Loctite® Threadlocker Red 271™ Nut and Bolt Locker. Will this work? Also, I remember someone using some white tape on the bolt thread and shows through the groves of the bolt thread. Don't remember the name of this white tape. Also, will the m6 x 1.5 Helicoil repair kit will work on 1 m6 x 50 bolt and 1 m6 x 65 bolt? Both bolts go on inside the top of the valve cover and threaded to the top head, two different lengths.
 

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A helicoil repair kit is the way to go. They are relatively easy to use. Don't mess with adhesives or "thread repair" compounds because the only replacement for metal threads is...metal threads.
 

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Discussion Starter #5
I found Helicoil 5546-6 M6 x 1 Metric Coarse Thread Repair Kit on Amazon. Is that the correct size and length for both m6 x 50 bolt and m6 x 65 bolts?

I can't see to find Helicoil m6 x 1.5 Helicoil repair kit will work on 1 m6 x 50 bolt and 1 m6 x 65 bolt? or does that matter? What would be correct Helicoil thread repair kit based on the bolts size as mentioned above?
 

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Take one of the bolts down to Lowes or Home Depot and use their bolt size template (in the area where they have loose bolts and nuts ) to get the exact pitch and size of the bolt you have. Then, as Lefcoastgeek says, you must get the exact same helicoil kit to do the repair.

Its not hard but you want to make sure that during the drilling and tapping process you don't get aluminum chips in the engine. You can cover the head surface with rags to control the chips. Except for an electric drill, all the tools needed for the thread repair are in the kit.
 

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Discussion Starter #9
helicoil kit

tuttebenne, Thanks for your comments in your latest post. I went to autozone yesterday and pick up the helicoils kit. All of the items in the kit have the correct size and type for the 1994 Mercedes E320 wagon. I also found out that another hole in the female thread on the cylinder head block that was stripped as well. That makes 3 holes that need to be repair. My question is one helicoil per hole is enough? Some people said two helicoils per hole would be better to ensure tighter fit and lessen the chance of leaking oil on the valve cover gasket. I was under the impression one helicoil per hole would do the job. Any comments on this issue?
 

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Use one helicoil per thread. You can use some threadlock on the helicoil itself when fitting into head. Then leave overnight for it to set before fitting the new bolts in.

Threadlock is not needed per say- but reduces the chance of it winding out with the bolt in the future. Once fitted correctly a helicoil is stronger than the original threads since the bolt is now turning inside a hardened steel thread.
 

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Discussion Starter #11
JC220, Thanks for the tip in your last post. I think your idea to use threadlock on the helicoils is a great one!! Hopefully, using threadlocks on the helicoils will prevent damage, stripped threads on both the bolts and cylinder female threads when replacing new valve cover gasket again in the future.
 

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Discussion Starter #12
JC220, just for clarification on the helicoil using threadlocks. Do I apply threadlocks inside the helicoils where the bolts goes through or outside the helicoils where the helicoils are threaded to the female thread hole clyinder head or both inside and outside of the helicoil? Or on the bolt itself as well?
 

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Only use a small amount of threadlock applied to the outside of the helicoil insert before installing. This is to retain the helicoil inside the head.

Use minimal amounts so as the bolt thread area is clean. Then use no threadlock on any of the cam cover bolts.

I highly recommend getting a 1/4 drive low range torque wrench. The torque setting for the cam bolts is something like 10 -12 nm a very hard to do without a small torque wrench. I got a draper one new on fleabay for £20 - I'm sure you can pick one up over there or under 30usd. A worthwhile tool to have when working on aluminium heads.
 

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Discussion Starter #17
I forgot to ask a question earlier. Prior tapping the hole and putting in the helicoils, I will be using titanium coated left handed drill bit. When I am ready to start drilling using a electric drill, do I switch to forward (clockwise) speed or reverse (counterclockwise) speed while I am drilling into the female thread hole on top of the cylinder head?
 

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I forgot to ask a question earlier. Prior tapping the hole and putting in the helicoils, I will be using titanium coated left handed drill bit. When I am ready to start drilling using a electric drill, do I switch to forward (clockwise) speed or reverse (counterclockwise) speed while I am drilling into the female thread hole on top of the cylinder head?
I answered your question on drill bit direction in your other thread. Remember you are drilling out aluminum; it is soft so go slow.

As for applying thead locker, you can but just realize how a helicoil works. You will be winding it into your newly tapped hole and the reason it will go in is because the direction you go in causes the insert to get narrower, allowing it to thread its way in. Once you stop turning the insert it expands against the new threads you tapped in and it essentially is in place. It would take a really odd situation for the threads to come out with the bolt at some point in the future. I have never seen a helicoil insert come out under regular use.
 
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