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Hi folks,

I just finished replacing the timing chain, upper guide rails, and tensioner on my 91 560 SEL. I benefited a lot from the threads here, plus the factory manual and one from Kent Bergsma to boot. All went well and all timing marks line up. Great! Except...

The tensioner bolts are not getting tight. The old tensioner came out easy enough; bolts broke free with what I'd consider a normal amount of force. The new tensioner went in fine and I was able to get the bolts started and in. But they don't seem to be getting very tight - nowhere near the 20NM I saw listed in the manual. Distressingly, they seem to spin past maybe 5NM. And predictably, there's a sizable oil leak from the tensioner when I start the engine. This is the car's third tensioner; it has 170K miles.

I'm very careful whenever I seat bolts in the aluminum bits, so I'm confident I didn't force these beyond what they should be torqued to. So I'm looking for your input and experience! Please help. Ideas: the seat was already worn or stripped from the last replacement and this put it over the edge; I've gotten something backwards and I don't realize it; there's something else I'm missing. Appreciate all input.

Thanks,
Robert D

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Stripped bolt hole. You have no choice but to heli-coil the holes. Happened to me when I rebuilt my engine. I was fortunate that my engine was out of the car and my brother had a full heli-coil tool and metric insert set. Not a terribly tough process but the angle and access to the bolt holes will be tricky.
 

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Yes, stripped holes. You will need to borrow an angled drill to get atthem inthe car. Should be easy to do though. Maybe 30 minutes if you have all the stuff ready. Second option is to go tothe next bolt size but I would not. When you do this I might install studs in instead os using bolts. Usually you don’t strip studs as easily. Let us know
 

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Thanks for the input, @Stutz and @alydon! Your conclusion was my suspicion, but I wanted to check...and I fantasized about having missed some other solution. :) At least the access is pretty good.

It's so ironic: of all the many steps in that long maintenance job, removing and replacing the tensioner was arguably the easiest and seemed the least prone to error. The aluminum obviously had other ideas...

I will circle back as I get this worked out.

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Any opinions or guidance on helicoils vs. time-serts for this? Is one better for the aluminum head than the other? I know time-serts were the preferred choice for Porsche head stud repairs back in the day...

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Stick with helicoils especially for the heads. Timeserts need to be countersunk and that is a pain. I did both and when ai researched the issue, helicoils surprisingly tested as strong. Again once you helicoil it I would put in two studs so the never knicker the threads again.
 

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Great, thanks so much! And roger on switching to studs. Sensible upgrade.

I have a friend who works at a shop here in Seattle that restores and race preps vintage race cars. He says he's done a bunch of these (or similar) and has all the kit. We may attempt it as soon as tomorrow. I'll report back!

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Helicoils are stronger than timeserts, and, as mentioned above, do not require countersinking. Helicoils have a little tang at the bottom of the coil that you may have to snap off after insertion if you are inserting the coil midway down the bolt bore. Also, be sure to put a rag in the tensioner hole to avoid metal chips getting in the block. You'll be fine. Of all the place to have a stripped bolt hole, the tensioner is not bad due to the relatively open access. If only one of the bolt holes is stripped, maybe do both while you are there.

Here is a pic of the exact same tensioner bolt with the helicoil on it. You can see the little tang at the bottom of the coil.
 

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Both heli-coils and timeserts are stronger than the original threads. I won't get into the argument about which is stronger; it's moot. The heli-coils are a lot easier to install. You should be able to get away with out drilling if you use the old bolt to pull all the old threads out.
 

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Thanks again for everyone's replies and input. It was helpful to hear that others have experienced this and to get suggestions on my options.

My friend had the time-sert tools and supplies on hand, so we went that route. From the replies here and some searches I did, it looks like either those or helicoils would be OK for the application. My friend's company rebuilds, maintains, and race preps multi-million dollar vintage race cars; he's a pretty experienced wrench. It took us less than an hour to do the work and it was a good bit easier than I would have thought. Everything sealed up nicely and looks good after driving the car today.

In the attached pic, you can see the aluminum threads that came out with each of the bolts. And again, I applied very little torque when inserting them - so my conclusion is that it's a weak spot. Since the solution was pretty straightforward, I'm glad this happened now instead of letting go (possibly surreptitiously, one bolt at a time) while driving! IMG_20190729_205758143.jpg IMG_20190729_194313390.jpg

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I had three or four bolt holes strip out when I did the top end of my SEC (fortunately none of the head bolts), seems like all it takes is a little sliver of aluminum to gall to the threads and you're done. Related to that, I was trying to salvage some parts from a snowmobile a while back, and the steel heim joints would not come out of the aluminum suspension arms and such. After ruining a couple of those, I discovered I could work them free by going back and forth rather than just trying to brute force them. So, if you come across other bolts that don't come out freely, spray some penetrating oil on it and work it back and forth. Anyway, the ones I had go bad I was able to heli-coil, and I'm religious about coating bolts that go into aluminum with anti-seize. Just one of the downsides of aluminum, but if it's properly fixed, it'll last forever.
 

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If you buy a helicoil kit or a timesert one you get lots of the repair coils with them. My suggestion is you do as many bolt holes as you can. Might save grief later.
 

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@Stutz, thank you. I agree. Now that I see how straightforward it is, I'll definitely be getting a metric kit for future projects.

I guess I was lucky with my intake rebuild project? That went back in pretty easy and all bolts tightened up well.

-RD

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Does the chain tensioner have to be replaced when doing chain guides? With low kilometers I feel bit reluctant to replace chain but as car is almost 30 years old guides should be replaced for peace of mind. If new tensioner is needed when replacing guides how is it if one replaces just the chain (later)?

Are the parts the same for M116 as for M117? I find a bit confusing that some web shops say 1160501811 is compatible with 420 SE/ SEC only until 1987, for 560 SEC until 1989, for 560 SE until 1991, and for 500 SEC until 1991. Is this just because how different models were sold on some markets or is there a technical difference?

Have you noticed any differences in quality between different brands if chain guides?
 

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If you buy a helicoil kit or a timesert one you get lots of the repair coils with them. My suggestion is you do as many bolt holes as you can. Might save grief later.
That's not a bad idea. If I ever do a full rebuild, I'll probably take that approach. Only trouble with doing that with the engine not torn down is keeping the metal shavings out of things. It's surprising how easily some holes strip out. When I had my engine out, I blew every hole out and ran a thread cleaner down it with WD-40, then used a torque wrench and put anti-seize on the bolts. Still, a couple bolts just spun with barely any torque (like 5Nm on a bolt that's supposed to go to 30). Some aftermarket performance parts have heli-coils from the factory – like Edelbrock heads, for one. It'd definitely save you down the road from repairs that should be a few hours and turn into a few days.
 

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Does the chain tensioner have to be replaced when doing chain guides? With low kilometers I feel bit reluctant to replace chain but as car is almost 30 years old guides should be replaced for peace of mind. If new tensioner is needed when replacing guides how is it if one replaces just the chain (later)?

Are the parts the same for M116 as for M117? I find a bit confusing that some web shops say 1160501811 is compatible with 420 SE/ SEC only until 1987, for 560 SEC until 1989, for 560 SE until 1991, and for 500 SEC until 1991. Is this just because how different models were sold on some markets or is there a technical difference?

Have you noticed any differences in quality between different brands if chain guides?
If the tensioner has never been replaced, I'd probably do it as a matter of course. It might be ok if the car was well-kept with regular oil changes, but hard to be sure. Then again, I'd replace the chain, tensioner, and guides on any car that I didn't know they'd been replaced on. That said, it's usually the guides that take down the chain and destroy the engine, so that's the place to start if you can't do it all. With regard to the guides, I'd only use OE unless I was sure the aftermarket was the same supplier. I'm not sure what's out there, but cheap knock-off guide could be as dangerous as a weathered original.
 

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The guides are ridiculously cheap from MB so it's a no brainer. You could do those on their own for a peace of mind very cheap solution.

The tensioner absolutely needs to be MB only, but it's a stupid £££ price so people do cut corners and buy cheap. Such a critical part, some people will get lucky but the potential risk is way to costly for me.

On my SEC I've done guides, tensioner and cam oiler orifices. Well I got my indie to but I got the parts and managed to find a genuine new MB tensioner for about £100. They said the chain wasn't badly stretched despite 175k miles.
Job was around £200 labour I think.

You could just do guides if needed though. Any tensioner weakness that might create a slap, would be much less likely to crack a brittle old guide.

Sorry to digress, OP. Excellent work BTW.
 
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