I intended to replace the fan belt tensioner along with a new idler pulley.. Found a 12MM hex key locally and managed to get it loose. (It was wicked tight) putting it back together, it did not ever seem to tighten down so thinking that the threads are done in the block.

Suggestions, from those more experienced, or from those who have traveled this path before? Is this a place for a helicoil, or will using a tap to clean up the threads be sufficient (hard to believe, but asking anyway)

While I am at it, I am going to replace the Vacuum pump as well as it is original. Expecting some challenge with those bolts as well, as one has all ready been repaired by me last summer to try and cure the oil leaking from the pump.

Thanks,

If the threads are pulled out of the block ( like a coil spring ) , there isn't any material to clean up with a tap.

Remove the tensioner and make sure the threads are OK or NG. You might be able to use a longer bolt and engage threads farther down but be sure the bolt tightens against the tensioner and just does not bottom out. The risk here is the reso of the threads will pull out leaving the bolt loose.

Helicoils work but you must drill the hole square to the surface , tap square to the surface and install the coil in the first thread ( don't stretch it and get it started a thread down. ) run the top a thread below the surface but not so deep that the bottom coil closes up.

Practice on a piece of metal. You can even reuse the insert if you don't break off the tab and the hole goes all the way through.( just wind it through ) Another hint, if you don't have an insert tool, use a tire valve core remover.

The 12mm hex key threads thru the front plate. A longer bolt will not help. If the thread is NOT completely buggered up then I would just use teflon tape to thread the bolt. Or knowing me, I would use JB weld. JB weld is better. The tensoner pivots on the bolt with a bearing and will very very unlikely work the bolt loose. It will come out easily even with JB weld so all is well. I would not re-tap or put in a heli-coil, it will do more harm than good.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Fortunately the threads did not come out like a coil spring... but it sure did not feel like it was bottomed out ether.

So to clean up the threads in the front cover, and put JB weld on the 12MM bolt as if it was thread sealant? That would be a wonderfully elegant solution. I agree that getting the hole drilled and tapped square would be a real challenge. would running a tap through the hole, before the JB weld be a good idea, or just time and $ with little accomplished?

If you depend on the car the fix is to replace the timing cover, there are updated later versions out there, but an undamaged original should be good enough.

Teflon tape and JB Weld are pretty sketchy unless you like waiting to hear the sound of the tensioner failure and the damage that will occur when stuff breaks loose and the serpentine belt starts wrapping around everything.

If the tensioner is not square and perpendicular to the timing cover the belt is always trying to run off the pulley. Same thing happens when the tensioner bearing fails, the slop allows the pulley to then tilt out of the plane of the other driven pulleys.

New member with 1 post?? Welcome to the forum.

It is best to replace the front timing cover in an ideal world. But we are not living in an ideal world. Do you know how much work it takes to replace the front plate for such a simple issue? The bolt will NOT come loose, a faulty bearing will give you irritating noise. That is all. OTOH, it is not my car, the OP has to weigh the pros and cons. I would do it in a heart beat with JB weld if it were my car.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

I don't know how to do multiple quotes in my response as I have seen others do... But, I am benefiting from the discussion and further information.

yes it is not an ideal world. I may well use JB weld for the moment... AND having learned about an up dated front cover, where would I find one of those??

At the present moment the car has 45,000 on it, and no rust, so I am willing to put some effort and $ into it.. Yet It was also was wrecked, which may be why the belt was making noise.. guessing the front cover may be tweaked, but that is just conjecture. It is not a daily driver, but I do use it for road trips on W/ends and special occasions so it needs to be dependable when in use.

Yeah, I'm hopeful that the tensioner was not quite square enough. It was not at all noticeable that it was out of alignment, but the pivot bearing was kinda rough, so it is getting replaced.

Would finding a tap to chase the threads at on the pivot bolt be of benefit? I've had luck doing so with exhaust manifold studs that were galled by a bad nut, but never done anything with aluminum.

I just got back into the Benz life with an 87 diesel wagon that had the tensioner fail wrapped around the crank, took out the fan, its clutch, the shroud, the radiator, a fist size hole in the belly pan, the TDC pointer, crankshaft seal and a hole poked into the timing cover. The tensioner pulley was broken off the bearing thats pressed into it, I think that happened after things started coming apart.

The guy who I got it from told me he had problems with belts failing to soon, he replaced the belt a couple times but didn't know why that was happening, the last time he didn't catch it soon enough.

The engine has 45,000 miles on it? Oh my, I'd be keeping that one for sure!

If the bolt can't be torqued to spec I don't think that running a tap thru whats left of the aluminum threads will be very helpful except maybe to prevent cross threading.

Did you swap the large washer over to the new tensioner? It's critical to proper operation of the tensioner...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Yes...

I'm starting to think that a new front timing cover is a long term solution, being that an ounce of prevention is worth a pound of cure...

How involved is such a job? I've done a handful of engine swaps, and rebuilt an old dodge flathead 6, but never been inside a mercedes, and I am sure there is plenty more complexity than I've ever seen before.

... timing cover is on top of oil pan and under the head... lots of work...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

It is not something that bolts to the front of the block? but is part of what the head and oil pan clamp on to?

This does not sound like a fun repair!

If the threads didn't come like a spring, they are still in there.

A longer bolt works if there are threads beyond the original bolt engagement distance. Sometimes additional threads can be had by using a bottoming tap and careful bolt length selection, sometimes you end up shimming / cutting a bolt to work.

Teflon tape isn't going to work long term, epoxy would work but the hole needs to be completely free of oil. Locktite makes / made an epoxy system that uses release agent that is applied to the bolt.

I wonder if part of the bolt is broken off in the hole, if it went from super tight to can't tighten up, something drastic happened.