Exhaust Clamp Preload

[bmwstephen]

hey guys- trying to chase down some occasional rattles in the exhaust system.

The bentely manual references something about preloading the muffler seam clamps by shifting the lower portion 15mm but was vague about it..."To prevent exhaust system rattles and vibration, preload the rear clamping brackets in the forward direction"



If I am looking at this old UUC diagram correctly, it means sliding the clamp (so that the center of the hanger, shifts 15mm towards the front of the car)?



Edit: Why, this step is important is explained in the video below. Your exhaust system naturally goes through thermal expansion and contraction. If any part made from more thermal conductive material such as stainless steel or titanium, there needs to be some flex engineered into mounting locations:

Exhaust Pipe Thermal Expansion and Vibrations (NVH) - YouTube

[M3C]

Let us know how did it go?

[Braymond141]

It means slide the clamp forward on the canister edge and then tighten it down there. So start at the neutral 90 degree position (looking at the side) and then move it forward and tighten. Ignore whatever the rubber hanger is doing in the process, though if it is failing, replace it and make sure the clamping arm position isn't clocked against the rubber bushing (like it is in your spaced out fix for your muffler).

[bmwstephen]

It means slide the clamp forward on the canister edge and then tighten it down there. So start at the neutral 90 degree position (looking at the side) and then move it forward and tighten. Ignore whatever the rubber hanger is doing in the process, though if it is failing, replace it and make sure the clamping arm position isn't clocked against the rubber bushing (like it is in your spaced out fix for your muffler).

slide the clamp forward to the front of the car? Hangers are new which is why I want to make sure I am doing it right

[Braymond141]

slide the clamp forward to the front of the car? Hangers are new which is why I want to make sure I am doing it right

Unless you drive backwards, yes.

[M3C]

Slide each side on opposite direction.

[bmwstephen]

It means slide the clamp forward on the canister edge and then tighten it down there. So start at the neutral 90 degree position (looking at the side) and then move it forward and tighten. Ignore whatever the rubber hanger is doing in the process, though if it is failing, replace it and make sure the clamping arm position isn't clocked against the rubber bushing (like it is in your spaced out fix for your muffler).

Okay so I focused on sliding the clamp and tightening it down. The orientation of the center rubber and bushing effectively gets pulled to the other side when doing that so I guess that's what they meant by preload. I presume once the exhaust system heats up and there by expanding, the center rubber and bushing will now go back to its natural position.

Slide each side on opposite direction.

I also tried your method Mark where the other clamp would be preloaded in the opposite direction. However, the clearance to preload would be less (only about 5mm) since the default orientation of the rubber hanger and center bushing was already pointing towards the rear of the car; pushing the bushing up against the side of the rubber hanger. I would have to flip one hanger in the opposite orientation (where the front facing part would now be rear facing and vice versa), just to get the 15mm of preloaded. But in doing so the muffler gets pulled closer to the rear diffuser opening which could result less clearance on one side of the against the rear diffuser.

[M3C]

The idea is to avoid preloading both hanger sides on same direction, this would generate pushing or pulling force to the exhaust system and when at operating temp it will stretch or bend the exhaust.
By preloading on opposite directions it will keep the exhaust neutral but also prevent any excessive movement front/back and left/right.
If diffuser clearance is uneven start again but by preloading the other side first and see how that results.
You wanna post pictures it might also help.

[bmwstephen]

The idea is to avoid preloading both hanger sides on same direction, this would generate pushing or pulling force to the exhaust system and when at operating temp it will stretch or bend the exhaust.
By preloading on opposite directions it will keep the exhaust neutral but also prevent any excessive movement front/back and left/right.
If diffuser clearance is uneven start again but by preloading the other side first and see how that results.
You wanna post pictures it might also help.

noted, I might have to just flip one side's hanger orientation in the other direction to accomplish that preload in the opposite direction.

On a separate note Mark, was the idea to do both sides in opposing directions was that your idea or the official BMW repair method? The bentley only references a single direction (towards the front of the car)

[Floxer]

what's going on here

you preload the hanger to the front so that when the exhaust expands due to heat, the hanger is in the middle. Doing one side to the front and one to the back is a bad idea. If the pipes expand, the one preloaded to the back will be touching the bracket even more, the whole idea of this procedure is to prevent that happening.

[bmwstephen]

It means slide the clamp forward on the canister edge and then tighten it down there. So start at the neutral 90 degree position (looking at the side) and then move it forward and tighten. Ignore whatever the rubber hanger is doing in the process, though if it is failing, replace it and make sure the clamping arm position isn't clocked against the rubber bushing (like it is in your spaced out fix for your muffler).

Brett- on a side note, have you had experience getting preload clamp position on the Dinan mufflers for e36? I recall, the way the support rod that is supposed to be clamped down does not have a whole lot of surface area for the preload position. If you see in this silly OG video when he did the install (between 18:16 to 18:45 clip), the hanger and clamp in its natural position is already at where the clamp area begins

DINAN Exhaust Install on my E36 M3 - YouTube

[Braymond141]

Just put the clamps on. I didn’t even know preloading was a thing and have never done it or had an issue. My only concern is trueness of the clamp and tip position within the diffuser. Anything else is exhausting to keep rehashing.

[M3C]

Picture is a thousand words.
So it appears there is limited sliding length on both sides and you might not get same results as with stock muffler.
Take your time and try your best, remember by proper preloading (this at least applies to stock muffler) you will also adjust the diffuser gap to be even on both sides.
About the other topic, I will send you dimensions later.

post #10 above...so what is the point of preloading the front so that when the exhaust expands due to heat, the hanger is in the middle?
By preloading the front in this case it will generate excessive pulling force to the exhaust system and when it gets hot it will contribute on deforming the original shape, same if preloading only the rear -it would cause pushing force.
Why anyone would do this to the exhaust, it should only be neutral just hanging.

By preloading properly one side towards front and the opposite side towards back it will lock the muffler movement and prevent any pulling/pushing forces but also it will prevent any left/right excessive movements/vibrations.

I had hard time having to replace rear rubber hangers every now and then, after applying this method about 5 years ago no more need to replace, it is the same as the day I did this.

[Floxer]

By preloading the front in this case it will generate excessive pulling force to the exhaust system and when it gets hot it will contribute on deforming the original shape

The pulling force of the clamps or rubber deform the exhaust system in your theory? You must be joking.

[M3C]

The pulling force of the clamps or rubber deform the exhaust system in your theory? You must be joking.

Yes, you already confirmed the same... "preload the hanger to the front so that when the exhaust expands due to heat, the hanger is in the middle".
Just a reminder, this hanger has the U shaped strong metal reinforcement so when it is mounted is fixed and if preloaded it will generate directional force.
Again I am not insisting you (or anyone else) has to follow this method, at the end to each his own.

[Floxer]

this hanger has the U shaped strong metal reinforcement

I don't know what hanger you mean with "this hanger". I'm talking about OEM and there's nothing about the OEM hanger or clamp that bends the exhaust.

Yes, you already confirmed the same... "preload the hanger to the front so that when the exhaust expands due to heat, the hanger is in the middle".

I don't see how that confirms your theory. I said the opposite. The exhaust expands. Forcing the clamp back. Not the clamp changing the shape of the exhaust system.

[RBNetEngr]

Amazingly long debate about exhaust hangers!

I know that I experimented with exhaust hanger positioning, but I was less concerned with whether the hanger was hanging perfectly vertically when the exhaust system was hot. I adjusted them so that the exhaust tips remained as close to centered in the lower bumper cutout, whether the exhaust was hot or cold. When I initially installed the catback exhaust I noticed that the exhaust tips would move to one side of the opening as it heated up, so I managed to position the hangers to minimize the side-to-side shift as it heated and cooled.

The Bentley E36 manual just has this brief bit of info:




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