GB2281600A

GB2281600A - Tappet shim and method of replacing it

Info

Publication number: GB2281600A
Application number: GB9317874A
Authority: GB
Inventors: Roger Derrick Stone; Mark Showell
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1993-08-27
Filing date: 1993-08-27
Publication date: 1995-03-08
Anticipated expiration: 2013-08-27
Also published as: GB9317874D0; GB2281600B

Abstract

Shims 26 for tappets of an ic engine of the type wherein the cam wherein the cam directly engages the shim are of the same diameter as the bores 22 in which the tappets slide, and guide means eg raised side walls of the bores 22 keep the shims in place over the full length of their travel. The side walls of the bores 22 may have cutaways 34 to allow clearance for the cams 30 to rotate. A method of replacing a shim is disclosed wherein a tappet retaining tool 44 engages the tappet, eg by locating in a groove 36 in the tappet skirt, to hold the tappet at its lowest position while the cam is rotated away from the shim which can then be replaced using a shim replacing tool 31. Tappet retaining tool 44 may have a threaded portion 47 and may access the tappet through a threaded bore in valve case 38. The shim may have a curved radially outer peripheral surface and means, eg a groove on its underside, for engaging the shim replacing tool. <IMAGE>

Description

Tappets for Internal Combustion Engines The present invention relates to valve gear for internal combustion engines which generally includes a tappet mounted on the top of the stem of each valve. In order to allow adjustment of the height of the tappet, a shim is usually provided, either between the valve stem and the tappet or on the top surface of the tappet, which can be removed and replaced by another of different thickness.

An example of a tappet with a top located shim is shown in Figure 1. In order for the shim to be retained on the tappet it fits inside a rim which extends around the circumference of the top surface of the tappet. However this has the disadvantage that the shim has to be smaller in diameter than the tappet which places limits on the achievable valve lift characteristics.

Accordingly the present invention provides a valve gear for an internal combustion engine comprising a valve gear casing having a bore therein, valve means including a tappet slidably mounted in the bore with shim means on one end, and cam means having a cam surface arranged to act on the shim means to move the tappet in the bore between two extreme positions, wherein the shim means is of substantially the same diameter as the tappet and guide Figure 3 illustrates the dislodging of a shim from the tappet of Figure 2, and Figure 4 illustrates the removal of a shim from the tappet of Figure 2.

Referring to Figures 2 to 4 an internal combustion engine has, for each of its cylinders, an inlet valve and an exhaust valve. The inlet valve gear, shown on the left in Figs 2 and 3, will be described, that for the exhaust valve, shown on the right, being identical. The valve gear includes a valve having a valve stem 10 with a spring retaining cap 12 on its upper end and a bucket tappet 14 fitting over the cap 12 and having a flat circular end portion 16 with a central projection 18 on its under side which rests on the end of the valve stem 10, and a skirt 20 extending from the outer edge of the end portion 16 down over the spring retaining cap 12. The tappet 14 is situated in a bore 22 in the cylinder head 24 of the engine and the diameter of the tappet skirt 20 15 such that the tappet can slide in the bore 22.A shim 26 rests on the upper end of the tappet 14. It is of the same diameter as the tappet and the bore so that it is accurately located, and its outer edge 28 is curved so The present invention also provides a methoa of replacing the shim on a tappet of an internal combustion engine having cam means arranged to act on the shim to move the tappet, the method comprising the steps of rotating the cam means until the tappet is at or near its furthest point from the centre of rotation of the cam means, engaging the tappet with a retaining tool to hold it stationary, rotating the cam means away from the tappet and replacing the shim means The step of replacing the shim means may include at least one of the steps of dislodging the shim with a shim dislodging tool and removing the shim means with a shim removal tool.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a section through a tappet of the prior art, Figure 2 is a section through a tappet according to the present invention, In order to adjust the tappet by replacing the shim 26 the engine is first turned until the inlet valve is at full lift, ie the tappet 14 is at the position furthest from the centre of rotation of the cam 30. The tappet retaining tool is then screwed downwards through the threaded bore 42 until its end engages the annular groove 36 in the tappet hoiding the tappet stationary. The tappet retaining tool 44 comprises a shaft 46, a portion 47 of which is threaded, with a projection 48 on one end for engaging the groove 36 in the tappet 14 and a knurled handle 50 on the other end.The engine is then turned so that the cam 30 rotates through 90 degrees to the position shown in Figure 3 so that the cam surface is space from the shim 26. A shim dislodging tool 52 is inserted between the edge of the shim 26 and the side of the bore 22 to dislodge the shim 26, and then removed so that the shim removing tool 31 can be used to remove the shim. The shim removing tool has a pair of resilient arms 54, 56 arranged to grip the shim between them, the lower one 56 having its end bent over so that it can fit into the annular groove 29 on the shim. This allows the tool 31 to grip the shim 26 securely so that it can be removed. A new shim of the required thickness is then inserted, the engine is turned so that the cam 30 is in contact with the new shim, and the tappet that it can easily slide axially or rotate in the bore.

The shim 26 has an annular groove 29 on its under side for engaging a shim removing tool 31.

A cam shaft mounted over the valve has a cam 30 arranged to act directly on the shim 26 to move the tappet axially in the bore 22 against the force of a valve spring 32. The side walls of the bore 22 extend upwards towards the cam 30 far enough to keep the shim 26 aligned with the tappet 14 over the full length of travel of the tappet. However the side walls of the bore 22 have cutaways 34 at their upper ends to allow the cam 30 to rotate.

Around the outer surface of the tappet skirt 20 is an annular groove 36, and the valve casing 38, which comprises the cylinder head 24 and an upper casing member 40, has a threaded bore 42 extending through it from the top downwards opening into the tappet bore 22 at a point which is level with the the annular groove 36 when the tappet 14 is at its lowest point. This is so that a tappet retaining tool 44 can be introduced through the threaded bore 42 to retain the tappet in its lowest position, ie that furthest from the centre of rotation of the cam 30.

Claims

1. A valve gear for an internal combustion engine comprising a valve gear casing having a bore therein, valve means incluaing a tappet slidably mounted in the bore with shim means on one end, and cam means having a cam surface arranged to act on the shim means to move the tappet in the bore between two extreme positions, wherein the shim means is of substantially the same diameter as the tappet and guide means is provided to retain the shim means in position over its full travel.

2. A valve gear according to claim 1 wherein the guide means is formed integrally with the valve gear casing.

3. A valve gear according to claim 1 or claim 2 wherein the guide means comprises an extension of the side walls of the bore.

4. A valve gear according to any one of claims 1 to 3 wherein the shim means has a curved surface on its outer edge.

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5. A valve gear according to any one of claims 1 to 4 wherein the tappet has retaining means thereon to enable it to be held away from the cam means.

6. A valve gear according to claim 5 wherein the retaining means comprises a notch or annular groove arranged to receive a tappet retaining tool.

7. A valve gear according to any foregoing claim wherein the valve gear casing has an aperture therein to allow access for a tappet retaining tool.

8. A valve gear according to any foregoing claim wherein the shim means has tool engaging means thereon for engaging a shim removing tool.

9. A method of replacing the shim on a tappet of an internal combustion engine having cam means arranged to act on the shim to move the tappet, the method comprising the steps of rotating the cam means until the tappet is at or near its furthest point from the centre of rotation of the cam means, engaging the tappet with a retaining tool to hold it stationary, rotating the cam means away from the tappet and replacing the shim means.

10. A method according to claim 9 wherein the step of replacing the shim means includes the step of dislodging the shim with a shim dislodging tool.

11. A method according to claim 9 or claim 10 wherein the step of replacing the shim means includes the step of removing the shim means with a shim removal tool.

12. A valve gear for an internal combustion engine substantially as hereinbefore described with reference to the accompanying drawings.

13. A method of replacing the shim on a tappet of an internal combustion engine substantially as hereinbefore described with reference to the accompanying drawings.