Threads on cylinder head stripped - Suggestions please
 

Vehicle : '86 300E M103 Engine

I was replacing the valve guide seals and when I removed one of the four bolts (item 13 in the attached image) that holds the one of the camshaft bearing caps (item 12 in the attached image), I noticed that the threads were stripped. All I did was unscrew the bolt and found that the thread was stripped.

Note: In the manual(job 05-2190), it says to replace the camshaft bearing cap bolts (item 13) with longer bolts to avoid threads stripping - so I guess that this is a known issue

What are my options. Can I just leave it to let just 3 bolts hold the bearing cap in place, or with the head in place, can I "carefully" drill and tap and install a helicoil. Or, do I have to take the head out to a machine shop and have them do it.

Any and all suggestion are appreciated.

TIA.

I'd hate to try drilling by hand because you only have one chance to do it. A machine shop would do a superior job of drilling and tapping for a helicoil.

How many miles on the head? Has the head ever been off before? It may not be such a terrible expense if the head is due for reconditioning anyhow to have a machine shop do the repair.

One more thought - how much longer of a bolt can go in there? Job 05-2190 suggests that you may have some good thread further down the hole to bite into for a patch repair. Though I wouldn't expect a lot of strength from just a few intact threads.

What's the worst that can happen if the patch repair wouldn't hold and the assembly started working loose?... incomplete valve opening?

Check the price on a new cam shaft retainer cap. If its cheep or available at a junk yard. Buy one and use it as a drill guide for your helicoil drilling. You can then drill out the cap more and use it to pilot the helicoil tap if you are real nervous about it. Discard when done.

John Roncallo

The head has 103000 miles on it and it has never been off before.

Based on the manual, I think there may be about 5mm of addtional thread left, if I decide to go with a longer bolt.

Thanks.

Sounds like a good idea. I'll give it a shot.

Thanks.

Good tip by John!

Instead of going by what the manual says about additional thread left, thread in a much longer bolt (or the original bolt without the retainer cap), see where it bottoms out, and make careful measurements. This'll tell you where you can trim the longer bolt just before it bottoms out, to take full advantage of the remaining threads.

If the first 10-15 mm are striped, you dont have a hope in hell with just 5 mm.

John Roncallo

hs 300e
At this point I would not go with the longer bolt. The most vibration will be at the top of bolt thread. The best choice would of course be to take the head to a machine shop which will cost some big bucks plus your own labor to remove the head. Don't try to run on 3 bolts, you'll end up stripping more. That's the bad news.

Some good news:

The helicoil fix will work on this part, (I did this fix 18,000 mi. ago).
After you tap the hole be sure to clean out any metal chips and oil, then
add locktite (per directions) then the helicoil. Torque to specs to be sure
the repair will last.

I did this repair on my 300e alot of miles back, she's on her way to hit
400,000 miles (I'm going to bring her past 500).

I'm not an expert, just a person that has been fixing and running my
cars for a long time, I have a 20 year old Benz that will travel dependably
anywhere as long as there is good premium gas. :)

A much better repair than a HeliCoil is a KeenSert. Quite a bit more expensive, but last forever.

I wouldn't hesitate to do the job myself. When you drill it out, you can put RTV on the drill bit to pick up the slivers of metal. The bit won't be much bigger then the hole you already have so it would be very hard to get off line. Piece of cake.

the rule about threads and bolts and nuts is that if the nut and the bolt are the same material, an equal length of threads to the diameter of the bolt will develop the full strength of the bolt. ie 1/2" of threads in length will carry a 1/2" bolt. if the materials are not the same strength, as in aluminum, if the strength of the threaded material were, say, 1/3 as much as the bolt then a 1/2" bolt would need 1 1/2" of threads in the weaker material.

so the person who said definately dont try to get by on the remaining theads in the hole is right on.

if it were mine i would take it to my favorite machinist and have him do the deed. i estimate it would cost 50 to 75 at most. labor.

tom w

Update:
 

Without removing the head, I very carefully helicoiled the two holes where the treads were stipped and all is well now.

Thanks for all the inputs.

An after thought for those who wish to repair anything with damaged threads from spark plug hole to whatever Helicoil is one of the worst things I have ever used for any thread repair.
Use TimeSerts made by Wurth Germany yes they cost more and you need the kit to use them however they are very easy to install and it will not pull out like the helicoil will. I have repaired machines where there is high temperatures of 450 to 550 degrees mixed with high pressures of 3000 psi and more with TimeSerts and we disassemble theses things all the time with no failures to the thread repair. The web page for Wurth is .......
< www.wurthusa.com > most of their stuff is over priced however the quality is there.