XPAG Oil Pump Circlip Replacement

This follow up to the short article in issue 54 reveals the progress made. The oil pump issue stems from the concern about the diminutive circlip provided in some oil pump rebuild kits. Alarming numbers of damaged engines due to the failure of the circlip are coming to light, so identifying those pumps that have been rebuilt and rectifying any potential problems becomes a cause célèbre.

My first consideration was, can the oil pump be removed from my TC with the engine in situ? Yes, it can, but quite a bit of dismantling is called for. The pump needs to clear both the chassis rail and front wing skirt to allow some 4 ½” of the pump body to be withdrawn. Lifting the front of the engine to the limit where the bell housing collides with the ramp plate, just about allows the pump to clear obstructions. However, this means removing the bonnet, the radiator, oil pipe to the filter, front wing fixings but excluding the stanchion fixtures and front engine mounting bolts. I also removed the exhaust down-pipe and some of the bolts securing the rear gearbox flange to the rubber mounts, but now suspect this may not be necessary.

The front of the engine should be jacked up, using a chunky bit of wood to spread the load on the sump.  Partially undo the pump’s securing bolts but ensure they can be eventually returned to their original position or better still change the bolts to HT socket cap types. If any threads need cleaning up, use a very blunt 6 mm plug tap (1 mm pitch) to reform rather than re-cut the threads.

Leaving the end cover loosely in place, try to withdraw the pump’s body. It’s likely some encouragement is needed to split it away from the engine block, in which case use a soft aluminium drift to jog the body away from the block. Once free, undo the bolts and remove the pump’s end cover making sure the idler gear doesn’t drop out. The relationship between the gears ought to be kept the same. The pump’s body can now be slid out, clearing the chassis and the wing, which can be swivelled outwards around the stanchion’s pivot.

If the oil pump’s driving gear is secured to the shaft by the replacement circlip shown in photo 1, then, having gone to all this trouble to remove the pump, a replacement is probably advisable.

Photo 1 – the EN16 squeeze clip was made by the author (Eric Worpe) and is referred to later in this article.

Two choices can be considered:-

  •  Doug Pelton recommends and stocks a HD Spiral Retaining Ring (CCL 094) (see Spirolox), which consists of two coils of a thin flat spring shown in sketch 1. These should be more robust as they have a 360 degree engagement in the shaft’s shallow groove. However, they are not easy to install.

Sketch 1 – the Spirolox retaining ring stocked by Doug Pelton of ‘From the Frame Up’.

  • The grooves on replacement 13 mm dia. drive shafts are rather shallow, as advised by the circlip specifications, eg. Dia. of groove =12.4 mm, internal dia. of circlip =11.9 mm, suggesting a groove depth of only 0.3 mm to allow some spring pressure of the circlip to be applied to the groove diameter when installed. This shallow depth of engagement is unfortunately a consequence of the need to prevent the possible overstretching of the circlip when being expanded over the 13 mm dia. shaft. I must admit to being uneasy over such a shallow groove, so my feelings are that the groove should be deepened for an alternative fixing.

Deepening the groove may be complicated if the end of the shaft has also been “case hardened”. If this is the case (sad pun!) then the hardened skin needs to be ground away with a diamond abrasive disk driven by a Dremmel type electric motor. The motor is mounted on the tool post of a lathe and the shaft is held in a rotating chuck, (photo 2).

Photo 2 – deepening the groove.

The groove depth on an original shaft was about 1 mm, so that seems a good value to aim for. All of this does assume the availability of a lathe which can then be used to make a suitable “squeeze to fit clip” from a non-springy material.

For the EN16 squeeze clip shown in photo 1, an EN16 alloy steel bar was initially chosen as it is both high tensile and easy to machine, an important consideration when coming to “part off” the machined clip, (photo 3). A ¾” dia. (19 mm) bar of EN16 was drilled to fit the shaft dia. of 13 mm and then “parted off” to give the required thickness.

Photo 3 – making the EN16 squeeze clip.

The thickness of the clip was matched to the open groove width after the shaft and gear are assembled in the pump. Sometimes the groove width is partially covered by the gear so feeler gauges are used to measure the available groove width, which should be at least 1 mm.

A 60 degree segment was cut out of the ring which should allow the clip to be closed around the 11 mm dia. groove with pliers, (refer back to photo 1)

As this idea is a leap into the unknown, it seemed prudent to test the 3 types of clips i.e. (1) the original replacement circlip. (2) Doug Pelton’s Retaining Ring, and (3) the home made “squeeze in” type. A 13 mm dia. bar with 3 grooves of 11.6, 11.6, & 11 mm dia. simulating the pump’s drive shaft was machined up and a mild steel block with a 13 mm bore with a slight chamfer was used to simulate the spur gear. Pressure to force the shaft into the block, stressing the circlip under test, was provided by a hydraulic press.

Photo 4 shows the test set up with the original replacement circlip about to be tested. The hydraulic press provided a force of over 4 tons and photo 5 shows how the clip deformed into the chamfer but still remained in the somewhat distorted groove.

Photo 4 shows the replacement circlip about to receive some metallic torture and photo 5 shows that it didn’t submit!

The next test on the Pelton Retaining Ring (photo 6), also went to over 4 tons force and again deformed the ring into the chamfer and distorted the groove leaving a short portion of the ring displaced out of the groove.

The final test on the EN16 “squeeze in” clip located in the deeper 11 mm dia. groove (photo 7), ended with a “bang” at just over 4 tons force. The clip had neatly sheared with part of the ring still located in the groove.

Photo 6 shows that the Pelton Retaining Ring withstood the force, but the EN16 “squeeze in” clip shown in photo 7, didn’t.

The final Photo 8 shows all 3 tested clips and the dummy shaft, the only clip that failed was mine.

All 3 clips were able to sustain forces far greater than would be found in practice, the chamfered bore of the spur gear does not seem to compromise the security of the clip as I had originally thought. It might even allow a wedge effect to develop, although this could vary from batch to batch depending on the machining of the chamfer. So why do the replacement circlips fail? My own circlip had been in the pump for 10,000 miles and seemed fine. I’m left wondering if some circlips have been overstretched when being inserted. Care is needed to only expand the circlip enough to just slide over the shaft, anymore risks deforming the circlip, resulting in a loose fit within the groove.

My choice of EN16 steel needs reconsidering as its shear strength seems low. An 18 mm HT bolt may provide a suitable material to try out. How do these tests compare with the original clip? I was able to try one out and took the press up to 6 tons force. Again, the clip deformed to the chamfer profile but was otherwise unmolested, so I’ll reuse it in the pump once it’s flattened.

Photo 8 – shows the three tested clips and the dummy shaft.

Eric Worpe

Ed’s note: A big ‘thank you’ to Eric for his analysis of the problem and the tests he carried out. Based on the tests, it would appear that some replacement clips sold as part of the oil pump rebuild kits could be OK if carefully fitted, although we do not yet know how they fail. Doug Pelton’s HD Spiral Retaining Ring (CCL 094) seems sound, based on a static stress test, but then so does the original replacement circlip if both are properly fitted.

However, without the security of a retaining system based on a deeper groove, those shafts with a shallow groove system leave something to be desired. Knowing Eric, he will continue to experiment.

4 thoughts on “XPAG Oil Pump Circlip Replacement

  1. Brian W. says:

    So. You don’t remove the front engine mounting bolts…but you have to loosen them, right?
    Seems to me the front of the engine must be free in order to raise it up from the chassis rail and wing sheetmetal. Am I missing something?

  2. Eric Worpe says:

    The front engine mounting bolts were excluded from the list of items as I thought I was already overstating the obvious, but you’re right I should have mentioned that they need to be removed along with each headlamp bracket’s attachment link to the radiator, draining the radiator, loosening some of the jubilee clips on the hoses, removing the nuts securing the radiator to the cross bracket, unplugging any temperature sensor in the header tank and disconnecting the stay bars.

  3. Stephen Postlethwaite says:

    I have a 1938/9 TA with XPAG engine, and have tried lifting the engine up as you describe to remove the oil pump. I cannot get it high enough to avoid the chassis rail. Is the answer to take off the front engine mountings from the chassis and lower the engine to facilitate pump removal?

  4. Graeme Hogg says:

    Just to share my experience whilst recently rebuilding my XPAG. My replacement shaft measured 12.96mm OD and the groove 11.09mm ID x 1.31mm width. I have used a spring steel snap ring, SW12-M2400 from Simply Bearings. I replicated Eric’s test using MS test pieces I machined and easily achieved 4Tons (the “bang” came just under 5T). For that purpose my dummy “gear” was a slip fit on the shaft not the interference fit of the actual parts. I took care to only expand the ring just enough to ease it into the groove and avoid over stretching. Hope this may be of interest..

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