JPH08189351A - Double exhaust pipe for engine - Google Patents

Abstract

PURPOSE: To prevent gas leakage from an inner pipe to an outer pipe side, and prevent breakage of the inner pipe due to thermal expansion difference between the inner pipe and the outer pipe at an installation part of nipples etc., on a double exhaust pipe. CONSTITUTION: Provided are an outer pipe 1, an inner pipe 2 made of material thinner than that of the outer pipe 1 and inserted into the outer pipe 1 with a clearance 3, and a installation member 20 installed on installation ports 4, 7 formed on pipe walls of the outer pipe 1 and the inner pipe 2 from the outside of the outer pipe 1. The inner pipe 2 serves as a passage for exhaust gas. In such a double exhaust pipe for an engine, a cylindrical wall 5 is projected to the side of the outer pipe 1 from a periphery of the installation port 4 of the inner pipe 2. The cylindrical wall 5 is provided with a displacement elimination part 5 which eliminates relative displacement in respect to a member inserted into the cylindrical wall 5. The installation member 20 is inserted into the cylindrical wall 5 with a state in close contact with the displacement elimination part 5, and airtightly connected around the installation port 7 of the outer pipe 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a double exhaust pipe for an engine in which a pipe wall is provided with a mounting member such as a nipple for mounting a sensor.

[0002]

2. Description of the Related Art In automobiles, in order to reduce the amount of heat radiated from the exhaust pipe into the engine room, the exhaust pipe is constructed of a double pipe. Also, in order to early activate the exhaust gas purifying catalyst that is provided downstream when the engine is cold started, the inner pipe of the double exhaust pipe is made thin, and the heat capacity of the portion in contact with the exhaust gas is reduced to reduce the exhaust gas. It has been attempted to improve the temperature rising characteristics of the.

In such a double exhaust pipe, when exhaust gas leaks into the gap between the inner pipe and the outer pipe, the amount of heat escapes to the gap,
As a result, the temperature of the exhaust gas in the inner pipe decreases, so it is important to take a structure that does not leak the exhaust gas to the gap between the inner pipe and the outer pipe as much as possible.

Further, when the temperature of the exhaust pipe is high, such as during high-load running, a difference in thermal expansion occurs between the outer pipe and the inner pipe, and therefore it is necessary to have a structure capable of absorbing the difference. is there.

From the above point of view, conventionally, a nipple (mounting member) for mounting an oxygen sensor or the like on the wall of the double exhaust pipe.
In the case of attaching, a structure as shown in, for example, FIGS.

FIG. 9 shows a conventional structure (first conventional example) described in JP-A-6-10662.

In this double exhaust pipe, the outer pipe 1 and the inner pipe 2 are brought into close contact with each other at the nipple mounting portion, and mounting holes 7 and 4 are provided in that portion, respectively. The nipple 20 has a large head 2
1, an insertion portion 22 having a small diameter, and a through hole 23 are provided, and the outer tube 1
The insertion portion 22 of the nipple 20 is passed through the attachment hole 7 of the outer pipe 1 from the outer side of the nipple 20 to be joined only to the inner pipe 2, and the head 21 of the nipple 20 and the inner pipe 2 form the attachment hole 7 The peripheral portion is tightly held in a state where the mounting hole 7 is allowed to move in the radial direction.

The structure shown in FIG. 10 (second conventional example)
Then, the nipple 20 is welded and fixed to the outer pipe 1, and the nipple 2
The insertion portion 22 of No. 0 is closely fitted to the mounting hole 4 of the inner pipe 2.

In addition, the structure shown in FIGS.
In the fifth conventional example), the portion where the nipple 20 is attached is
The outer tube 1 is partially formed as a single tube structure, and the nipple 20 is joined to the outer tube 1 at that portion.

In this case, as shown in FIG. 11, when the nipple 20 is mounted close to the upstream side flange 11 of the exhaust pipe, the inner pipe 2 cannot be extended to the flange 11, so the downstream side of the mounting portion of the nipple 20. Then, the end portion 2e of the inner pipe 2 is joined to the inner circumference of the outer pipe 1, and the gap 3 between the inner pipe 2 on the downstream side and the outer pipe 1 is formed.
The exhaust gas is prevented from entering.

Further, as shown in FIG. 12, the nipple 20
When installing in the vicinity of the downstream flange of the exhaust pipe,
Since the inner pipe 2 cannot be extended to the flange 11, the nipple 2
The inner tube 2 is held by the outer tube 1 via the spacer 12 on the upstream side of the mounting portion of No. 0.

Further, as shown in FIG. 13, the nipple 20
When installing in the middle of the upstream side and the downstream side of the exhaust pipe, the inner pipe 2 is divided into the upstream side and the downstream side, and the ends 2e of the upstream and downstream inner pipes are joined to the outer pipe 1. ing.

[0013]

By the way, in the structure shown in FIG. 9, the outer tube 1 is only tightly held between the head 21 of the nipple 20 and the inner tube 2, that is, the outer tube 1 and the nipple 21. Since the and are not joined in an airtight manner, if exhaust gas enters the gap 3 between the inner pipe 2 and the outer pipe 1, the exhaust gas will pass outside through the slight gap of the tight sandwiching part. It may leak.

Therefore, it is necessary to join the inner pipe 2 and the outer pipe 1 at another place, but this causes an adverse effect in that the difference in thermal expansion between the inner pipe 2 and the outer pipe 1 is absorbed. Further, it is practically difficult to join the nipple 20 only to the inner pipe 2 depending on the mounting location.

In the structure shown in FIG. 10, the exhaust gas may not leak to the outside of the outer pipe 1, but the inner hole 2 is thermally expanded and the mounting hole 4 contracts and the positions of the mounting hole 4 and the nipple 20 are reduced. Due to the displacement, an unreasonable force is applied to the periphery of the mounting hole 4, and repeated stress may cause damage to the inner tube 2. Further, by repeating thermal expansion and thermal contraction, cracks or gaps are generated due to stress concentration, and exhaust gas leaks from the gaps to the gap 3 between the inner pipe 2 and the outer pipe 1, which may deteriorate the temperature rise characteristic of the exhaust gas. There is also a nature.

In the structure shown in FIGS. 11 to 13,
There is no possibility of exhaust gas leaking to the outside or damage to the inner pipe 2, but the mounting position of the nipple 20 is limited to a place where a single pipe is easily formed, and heat is released at the place where the single pipe is formed. The temperature characteristics tend to deteriorate.

Further, as shown in FIGS. 11 and 13, when mounting the nipple 20 on the upstream side or the intermediate portion of the exhaust pipe,
Since the inner pipe 2 and the outer pipe 1 must be joined at a position apart from the flange 11, it is difficult to manufacture, and if the joining is not perfect, the exhaust gas into the gap 3 between the inner pipe 2 and the outer pipe 1 There is a possibility that leakage may occur, or the difference in thermal expansion between the inner tube 2 and the outer tube 1 cannot be sufficiently absorbed and the inner tube 2 may be damaged.

In consideration of the above circumstances, the present invention can reliably prevent the exhaust gas from leaking to the outside, suppress the exhaust gas leak to the gap between the inner pipe and the outer pipe, and raise the temperature of the exhaust gas. A double exhaust pipe that can improve the characteristics, prevent damage to the inner pipe due to the difference in thermal expansion between the inner pipe and the outer pipe, have no limitation on the mounting position of the mounting member, and are easy to manufacture. The purpose is to provide.

[0019]

According to a first aspect of the present invention, an outer tube, an inner tube that is thinner than the outer tube and is inserted into the outer tube with a gap, and the outer tube from the outside of the outer tube. And a mounting member mounted in a mounting hole provided in the wall of the inner pipe,
In a double exhaust pipe of an engine in which the inside of the inner pipe serves as a passage for exhaust gas, a cylinder wall protruding toward the outer pipe is formed at a peripheral edge of a mounting hole of the inner pipe, and the cylinder wall is formed in the cylinder wall. A displacement absorbing portion that absorbs a relative displacement with the inserted member is provided, and the mounting member is inserted into the inside of the cylindrical wall in a state in which the displacement absorbing portion is in close contact with the displacement absorbing portion, and the mounting member is attached to the outer pipe. It is characterized in that it is airtightly joined to the periphery of the mounting hole.

A second aspect of the present invention is the double exhaust pipe of the engine according to the first aspect, wherein the displacement absorbing portion is formed by curving a part or all of the cylindrical wall inwardly convexly. It is characterized by comprising an annular curved portion.

According to a third aspect of the present invention, there is provided the dual exhaust pipe for an engine according to the second aspect, wherein the outer exhaust pipe extends continuously from the minimum diameter portion of the annular curved portion toward the outer end of the cylinder wall. It is characterized in that a tapered portion that spreads outward is formed.

According to a fourth aspect of the present invention, there is provided the double exhaust pipe for an engine according to the third aspect, wherein the tip of the tapered portion is projected to the outside from a mounting hole of the outer pipe.

[0023]

According to the invention of claim 1, since the mounting member is airtightly joined to the outer pipe, the leakage of exhaust gas from the mounting position of the mounting member is completely prevented. In addition, when relative displacement occurs between the mounting member and the inner pipe due to thermal expansion of the outer pipe and inner pipe, the displacement absorbing part absorbs it, so that the inner pipe is not unduly stressed and mounted. There is no gap between the member and the cylindrical wall of the inner pipe.

According to the second aspect of the invention, when a relative displacement occurs between the mounting member and the inner pipe due to a difference in thermal expansion between the outer pipe and the inner pipe, the relative displacement is absorbed by the deformation of the annular curved portion. To be done. Further, even if the ring-shaped curved portion is deformed in such a manner, the ring-shaped curved portion is kept in close contact with the mounting member, and there is no gap between the mounting member and the ring-shaped curved portion. Gas leakage into the tube gap is suppressed.

According to the third aspect of the present invention, since the tip end of the annular curved portion provided on the cylinder wall has a taper portion, when the mounting member is inserted into the cylinder wall, the taper portion functions as a guide.

In the invention of claim 4, since the tip of the taper portion is projected out of the mounting hole of the outer pipe, the position of the cylinder wall from the outside of the outer pipe, that is, the position of the mounting hole of the inner pipe can be easily confirmed. The mounting member can be easily inserted into the mounting hole of the inner pipe.

[0027]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a vertical sectional view showing the overall construction of the double exhaust pipe of the first embodiment, and FIG. 2 is a sectional view taken along the line II--II of FIG.
3 is an enlarged view of section III in FIG. 1, FIG. 4 is an enlarged view of section IV in FIG. 1, and FIG. 5 is a diagram showing a modification of section IV.

As shown in FIG. 1, in this double exhaust pipe,
An inner pipe 2 formed to be thinner (for example, 0.5 mm) than the outer pipe 1 is concentrically inserted with a gap 3 inside an outer pipe 1 formed to be a strong member. .
Further, flanges 11 and 11 are welded to both ends of the outer tube 1.

At the upstream end of the exhaust gas flow direction F, the inner pipe 2 is closely fixed to the flange 11 as shown in FIG. 4, or is held by the collar 10 as shown in FIG.
The exhaust gas does not leak into the gap 3 between the inner pipe 2 and the outer pipe 1.

At the downstream end in the flow direction F of the exhaust gas, the thermal expansion difference due to the temperature difference between the inner pipe 2 and the outer pipe 1 is absorbed, so that the inner pipe 2 is made of a resilient metal wire made of heat resistant metal. The inner tube 2 is held by the outer tube 1 via a spacer 12 so that the inner tube 2 can move relative to the outer tube 1.

A nipple (mounting member) 20 for mounting an oxygen sensor or the like is attached to the pipe wall in the longitudinal direction of the double exhaust pipe having such a structure. At the mounting portion of the nipple 20, as shown in FIGS.
And mounting holes 4 and 7 are provided in the tube wall of the outer tube 1, respectively.

On the periphery of the mounting hole 4 on the inner pipe 2 side, there is integrally formed a cylindrical wall-shaped flare portion 5 projecting to the outer pipe 1 side. The flare portion 5 is formed by bending a tubular peripheral wall inwardly so that the middle portion in the height direction has a minimum diameter, and is configured as an annular curved portion over the entire circumference, and is inserted inside. Function as a displacement absorbing portion that absorbs relative displacement in the radial direction with the member. The tip 5a of this flare portion 5
May or may not be in contact with the inner surface of the outer tube 1 as shown in FIG.

The flare portion 5 is circular in section AA in FIG. 3, and has a minimum diameter of the insertion portion 22 of the nipple 20.
Is formed to be slightly smaller than the outer diameter of. The flare portion 5 can be easily formed by press forming, bulge forming using hydraulic pressure, or the like.

The mounting hole 7 of the outer tube 1 is formed larger than the minimum diameter of the flare portion 5. The nipple 20 is mounted from the outside of the outer tube 1, and the insertion portion 22 is attached to the mounting hole 7 of the outer tube 1.
And is inserted into the flare portion 5 of the inner pipe 2 in a close contact state. The nipple 20 is hermetically welded to the outer tube 1 with the lower surface of the head 21 being in close contact with the outer tube 1.

Next, the operation will be described.

First, in manufacturing this double exhaust pipe, the nipple 2 is made with the inner pipe 2 and the outer pipe 1 being integrated.
The insertion portion 22 of 0 is inserted into the mounting hole 7 of the outer tube 1. Then, the tip of the insertion portion 22 of the nipple 20 is press-fitted into the flare portion 5 of the inner tube 2, and the outer peripheral surface of the insertion portion 22 and the inner peripheral surface of the flare portion 5 are in close contact with each other without a gap. When the nipple 20 is inserted, the head 21 is welded to the outer tube 1 to complete the production.

In this way, the nipple 20 is simply inserted into the mounting hole 7 of the outer tube 1 and welded to the outer tube 1.
Since the attachment of 0 can be completed and structurally only the flare portion 5 needs to be formed, it can be easily manufactured. Further, since it is not necessary to make the position at which the nipple 20 is attached into a single tube, the attachment position is not limited, and there is no troublesome processing in accordance therewith.

Next, the temperature raising performance of the exhaust gas when the engine is cold started will be examined.

Except immediately after the engine is stopped, the temperature of the double exhaust pipe at the time of starting the engine is approximately the same as the outside air temperature.
After the engine is started, in the double exhaust pipe, the inner pipe 2 is first warmed by the exhaust gas flowing in the inner pipe 2. As a result, the wall temperature of the inner pipe 2 increases, but the temperature of the exhaust gas decreases by the amount of heat applied to the inner pipe 2.

In this double exhaust pipe, since the heat capacity is reduced by reducing the wall thickness of the inner pipe 2, the wall temperature of the inner pipe 2 can be raised at an early stage. Therefore, the amount of heat transferred from the exhaust gas to the inner pipe 2 is suppressed to a low level, the temperature decrease of the exhaust gas is suppressed, and as a result, the temperature of the exhaust gas at the inlet of the catalyst installed downstream of the exhaust pipe can be increased. Therefore, the catalyst can be activated early.

At this time, if a part of the exhaust gas leaks from the mounting portion of the nipple 20 into the gap 3 between the inner pipe 2 and the outer pipe 1, a part of the heat quantity of the exhaust gas in the inner pipe 2 becomes a gap. However, the exhaust gas temperature in the inner pipe 2 is lowered, and the double pipe effect of the thinned inner pipe 2 is impaired. In the trachea, nipple 2
Since 0 and the flare portion 5 of the inner pipe 2 are in close contact with each other, the exhaust gas in the inner pipe 2 rarely leaks to the gap 3 and the deterioration of the temperature rise characteristic of the exhaust gas due to the leak is suppressed. Also,
Since the nipple 20 and the outer pipe 1 are welded and fixed to each other, there is no fear that the exhaust gas leaks to the outside.

Next, when the exhaust gas or the exhaust pipe is sufficiently warmed when driving in urban areas, the inner pipe 2 and the outer pipe 1 are separated from each other due to the heat capacity difference and the temperature difference due to the difference in the wall thickness of the inner pipe 2 and the outer pipe 1. A difference in thermal expansion occurs between the two.

That is, since the outer tube 1 is not directly exposed to the exhaust gas, has a low surface temperature, and has a large heat capacity due to its thick wall, the thermal expansion is smaller than that of the inner tube 2. On the other hand, the inner pipe 2, which is directly exposed to high-temperature exhaust gas and has a small heat capacity due to its thin wall, has a high temperature and undergoes large thermal expansion.

Due to the difference in thermal expansion between the inner pipe 2 and the outer pipe 1, the inner pipe 2 and the outer pipe 1 are relatively movable in the exhaust gas outlet portion as a whole of the double exhaust pipe as shown in FIG. As a result, the inner pipe 2 can be absorbed without being damaged.

At the mounting portion of the nipple 20, the expansion of the inner tube 2 is concentrated in the mounting hole 4 which is present as a notch, but since the flare portion 5 is provided in the mounting hole 4, this flare portion 5 is deformed. , Can absorb the elongation.

Further, due to the thermal expansion of the inner tube 2, a shift (relative displacement) occurs between the position of the flare portion 5 and the position of the nipple 20, and this shift also causes the flare portion 5 having a displacement absorbing function.
It can be absorbed by deforming. Therefore, the thermal stress generated in the mounting hole 4 of the inner tube 2 is reduced by the flare portion 5
It is possible to absorb all of them and prevent damage to the inner tube 2 in advance.

When the inner pipe 2 is greatly expanded by heat, a gap is created between the nipple 20 and the flare portion 5 (on the side where the expansion is large), and the exhaust gas has a gap 3 between the inner pipe 2 and the outer pipe 1.
However, even if the exhaust gas leaks into the gap 3, since the exhaust gas should be above the activation temperature of the downstream catalyst when the inner pipe 2 is greatly thermally expanded, There is no fear of affecting the performance of the catalyst.

Further, when the engine is stopped and the entire double exhaust pipe is cooled, the inner pipe 2 contracts, so that even if there is a gap between the nipple 20 and the flare portion 5, that gap is originally caused. The exhaust gas after the engine restarts from the gap between the flare portion 5 and the nipple 20.
There is no leakage into the gap 3 between the inner pipe 2 and the outer pipe 1.

Next, another embodiment of the present invention will be described.

FIG. 6 shows the essential parts of the second embodiment of the present invention.
In this embodiment, the flared portion 5 is formed with a taper portion 5b which continuously extends outward on the tip side of the smallest diameter portion,
The tip 5a of the tapered portion 5b is slightly projected outward from the mounting hole 7 of the outer tube 1 and is housed in an annular recess 25 provided on the bottom surface of the head 21 of the nipple 20B. The annular recess 25 has such a depth that the tip 5a does not contact the inner surface even when the temperature of the exhaust gas is high and the tip 5a of the flare portion 5 extends to the nipple 20 side.

According to this embodiment, since the tip 5a of the flare portion 5 projects outside the outer tube 1, the nipple 2
0 can be easily inserted into the flare portion 5. In particular, since it is sufficient to insert the tip of the nipple 20 into the tapered portion 5b, the insertion can be performed very easily and the manufacturing can be further facilitated.

If a nipple 20C having a tapered tapered insertion portion 22C as shown in FIG. 7 is used, it can be inserted more easily.

Further, as shown in the third embodiment of FIG. 8, on the inner pipe 2 around the flare portion 5, the flare portion 5 is surrounded, or the flare portion 5 is arranged in front and rear (upstream and downstream sides). If the curved portion 9 is provided so as to be sandwiched between the two, the influence of thermal expansion of the entire inner tube 2 on the flare portion 5 can be reduced by the deformation of the curved portion 9, so that the above-described effect can be more reliably exhibited. be able to.

In the above embodiment, the mounting hole 4 of the inner pipe 2 is
The flare portion 5 is formed by curving the tubular wall provided on the peripheral edge of the whole as a whole, but the curved portion may be provided in a part in the height direction of the tubular wall.

Further, instead of bending the cylinder wall,
A displacement absorbing portion having another structure may be provided.

In the above embodiment, the mounting member is
Although the case of the nipple 20 for mounting the oxygen sensor and the like is shown, a mounting member other than the nipple 20 may be mounted. For example, a mounting member for mounting an exhaust gas recirculation device (EGR) or the like may be provided.

[0058]

As described above, according to the first aspect of the invention, since the outer pipe and the mounting member are airtightly joined, it is possible to completely block the leakage of exhaust gas to the outside.
Further, the difference in thermal expansion between the inner pipe and the outer pipe is absorbed by the displacement absorbing portion provided on the cylinder wall, so that no undue stress is generated in the inner pipe and damage to the inner pipe can be prevented in advance.

Further, since the displacement absorbing portion absorbs the relative displacement between the inner pipe and the mounting member, a gap is not formed between the inner pipe and the mounting member, so that the exhaust gas is exhausted from inside the inner pipe to the outer pipe. It does not leak into the gap with the pipe, and the decrease in exhaust gas temperature due to it is suppressed, and as a result, the temperature rising characteristic of exhaust gas can be improved.

Further, since it is not necessary to provide a part where a single pipe is partially formed, the mounting position of the mounting member is not limited, and furthermore, a cylindrical wall is provided in the mounting hole of the inner pipe, and the cylindrical wall is displaced. Since the structure is only provided with the absorbing portion, no difficult processing such as welding from the inner side of the inner pipe is completely unnecessary, and the manufacturing is extremely easy.

According to the second aspect of the present invention, the displacement absorbing portion for absorbing the relative displacement between the inner pipe and the mounting member is constituted by the annular curved portion formed by bending the cylindrical wall inwardly convexly. The displacement absorbing portion can be easily formed by press molding, bulging or the like, and the processing can be facilitated.

According to the third aspect of the present invention, since the tapered portion provided on the cylindrical wall serves as a guide when inserting the mounting member, the mounting member can be easily inserted into the mounting hole of the inner pipe, and further manufacturing is possible. Can be facilitated.

According to the invention of claim 4, since the tip of the taper portion is projected to the outside of the attachment hole of the outer pipe, the position of the attachment hole of the inner pipe can be easily confirmed from the outside of the outer pipe. ,
It becomes easy to insert the mounting member by the mounting hole of the inner pipe.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing the overall configuration of a double exhaust pipe according to a first embodiment of the present invention.

FIG. 2 is a sectional view taken along the line II-II of FIG.

FIG. 3 is an enlarged view of a part III in FIG.

FIG. 4 is an enlarged view of section IV in FIG.

5 is a cross-sectional view showing a modified example of the same portion as FIG.

FIG. 6 is a cross-sectional view of essential parts of a second embodiment of the present invention.

FIG. 7 is a cross-sectional view of another shape of the nipple used in the embodiment of the present invention.

FIG. 8 is a cross-sectional view of essential parts of a third embodiment of the present invention.

FIG. 9 is a sectional view of a main part of a first conventional example.

FIG. 10 is a cross-sectional view of a main part of a second conventional example.

FIG. 11 is a sectional view of an essential part of a third conventional example.

FIG. 12 is a cross-sectional view of an essential part of a fourth conventional example.

FIG. 13 is a sectional view of a main part of a fifth conventional collar.

[Explanation of symbols]

 1 Outer Tube 2 Inner Tube 3 Gap 4 Mounting Hole 5 Flare Part (Cylinder Wall, Displacement Absorbing Part, Annular Curved Part) 7 Mounting Hole 20, 20B, 20C Nipple (Mounting Member)

Claims (4)

[Claims] 1. An outer pipe, an inner pipe that is thinner than the outer pipe and is inserted into the outer pipe with a gap, and a mounting hole provided on the wall of the outer pipe and the inner pipe from the outside of the outer pipe. And a mounting member attached to, in a double exhaust pipe of an engine having a passage for exhaust gas in the inner pipe, a cylinder wall protruding toward the outer pipe side is formed at a peripheral edge of a mounting hole of the inner pipe, The cylinder wall is provided with a displacement absorbing portion that absorbs relative displacement with a member inserted into the cylinder wall, and the mounting member is inserted into the inside of the cylinder wall while being in close contact with the displacement absorbing portion. A dual exhaust pipe for an engine, wherein the attachment member is airtightly joined to the periphery of the attachment hole of the outer pipe. 2. The double exhaust pipe of the engine according to claim 1, wherein the displacement absorbing portion is formed by an annular curved portion formed by curving a part or all of the cylindrical wall inwardly convexly. A double exhaust pipe of the engine, which is characterized by that. 3. The double exhaust pipe of the engine according to claim 2, wherein a tapered portion that continuously expands to an outer widened portion on the tip end side of the cylindrical wall with respect to the minimum diameter portion of the annular curved portion. The double exhaust pipe of the engine characterized by forming the. 4. The double exhaust pipe for an engine according to claim 3, wherein the tip of the tapered portion is projected to the outside from a mounting hole of the outer pipe.