Heli-Coil Repair--One or All???? - PeachParts Mercedes-Benz Forum

sublettd · #1 03-18-2009, 11:07 PM

In the course of putting the heads back on after a complete engine rebuild, we pulled the threads out of one of the head bolt holes. My mechanic ordered the plate and heli-coil kit from Baum tools and we heli-coiled the ONE bolt hole. However, when reviewing the engine manual--which is a must for the rebuild--the manual says if you have to heli-coil one hole that you MUST heli-coil all in that bank.

Well, my mechanic and I discussed whether heli-coiling all the bolts was actually necessary. Neither of us could understand why the MUST was stated. So, I decided that I was willing to burn a $55.00 head gasket in an effort to see if all the other bolts would torque without pulling. Fourtunately, all the other bolts went in an torqued to the 60nm (43 ft lbs).

Both heads are now back on and we should wrap up the rebuild in three or four days. However, I'm curious if anyone has any insight into why Mercedes would say you MUSt heli-coil all the bolts in that bank, or what potentail pitfalls might be encountered by not doing so. Particularly since we opted to only do the one that stripped out.

Also, the MB manual says "bank." My mechanic and I differ on what "bank" means. He says it is the entire side and I'm thinking a bank is just the row in which the bolt sits. He's probably right, but I have a hard time understanding why MB would make this a must do if just one bolt thread strips out.

FYI, it took probably 1.5 hours just to heli-coil the one hole. Granted we went slow, because it was only the second time John had ever done this. Also, you use only hand tools. ..cut, back it out, oil, cut, etc. Interestingly, when John was backing out the drill bit (provided with the kit) after getting down to the right depth, it snapped in two pieces even with the head! You should have seen the look of disbelief on both of our faces. We were stunned that just backing it out by hand would cause it to snap. Fortunately, it was where we could fish it out without doing any damage.

JonL · #2 03-18-2009, 11:35 PM

My guess is that MB is concerned that if the threads have failed in one hole, it is an indication that the strength of the casting has deteriorated, from heat or from stress cycles on the threads. I'm guessing that they feel that if one hole has failed, the others are marginal and may fail in short order. I'm with your mechanic that "bank" means all the holes related to one cylinder head.

Good luck!

Oh, the broken drill... sure seems like the drill was bending while the drilling was going on. That'll cause a drill to break quickly, just like what happened.

compress ignite · #3 03-19-2009, 12:12 AM

If you have to go in again(God forbid)...

http://www.timesert.com/

Once they're in,they're in forever.

sublettd · #4 03-19-2009, 10:16 AM

Makes sense that the bulk of the concern would be from a durability standpoint. However, it could also be from a time and money standpoint. Though my mechanic said that the local dealership would be unlikely to do the repair. Instead, they would either order a new block or install a new engine. The likelihood of another block being available for the '85 500SL would be pretty slim.

Also, regarding the Time Serts. They do make for a nice repair and there is no question about their capability to hold up. It's an interesting process.

The plate used to drill the holes has two guides. One is for cutting out the old threads and the other is to tap the hole to install the heli-coil. It's pretty close to impossible to bend the drill bit when doing either process, especially when going slowly and not applying much pressure. Breaking that bit was very surprising.