Secret (oil) escapes

1 Mar

Many engine designs are compromised somewhere, often by packaging or manufacturing process constraints; the XPAG is in my opinion no exception. So, having taken my engine out to fix 3 weeping core plugs, hopefully once and for all, I decided to take the opportunity to improve a critical aspect of the oil gallery which has concerned me since I first noticed it when building the engine. Ignored, it has the potential for a significant internal leak which cannot be seen and a pressure drop when hot.

Unlike the distributor, the location bore for the oil pump body is machined clean through the main oil gallery; the body is waisted where they intersect to allow oil to flow around it. Any leakage outwards is prevented by the joint face gasket, but inwards, which cannot be seen, relies on just a short (10mm) metal to metal fit between the end of the tubular pump body and the machined bore in the block. The issue is referred to briefly in Michael Sherrell’s red book, page 139, but no solution offered.

Cross section through oil pump

As best I could measure it I had a 3 thou gap, so I resolved it by machining a groove in the body to take a 25mm x 1.5mm O ring to provide a more effective seal.

O ring in groove

Reassembly will require care and a good lubricant because the edges of the gallery which the O ring has to slide past are sharp and will it make any difference? I will probably never know but with the engine out it was relatively easy and makes me feel better, peace of mind.

I like to build an engine where maximum oil flow is delivered to the bearings and where the pressure is managed by the relief valve, not by unintended leaks which replace its function when the oil is hot.

For the same reason I had already blanked off the vertical drilling in each rocker with an M3 grub screw. Dictated by the shape of the rocker arm, these holes had to be drilled down from the top to complete the lubrication path between the pressurized bush and the horizontal drilling out to the pushrod cup. But I am convinced the open end should then have been plugged in manufacture.

Tapped M3 to take a grub screw

Ed’s note:

Regarding the issue of blanking off the vertical drilling in each rocker with an M3 grub screw, this was covered in an article entitled Mods and Rockers by Brian Rainbow in Issue 27. Here’s an extract (the diagram referred to has not been reproduced).

I found an article by Don Jackson in an old ‘Octagon Bulletin’ #198 dated July 1986. It explains that “these holes should have been blanked off in production, so you are only correcting a fault that shouldn’t have occurred in the first place. The result will be proper pressure-fed lubrication of the rocker bushes, improved lubrication of the tappets, and a vast reduction in the amount of oil swilling around in the rocker cover. You will also gain a few pounds per square inch oil pressure as a bonus!” The diagram that Don drew is reproduced below (diagram 1) and explains the problem. If you have ever run your engine with the rocker cover off, the eight holes look like mini ‘oil geysers’, with loads of oil oozing out!


« Previous: Bits and Pieces Next: Lost & Found »

One Response to “Secret (oil) escapes”

  1. Tim Parrott 16. Mar, 2019 at 10:48 am #

    I assume that the same issue applies to the MPJG engine, however my pump was a very tight fit when I removed it. It was tight putting it back so hopefully OK.
    I did recover 10psi through the range by putting a rubber seal in the flexible joint in the oil pick up in the sump. This does not apply to the XPAG as it does not have the joint.

Leave a Reply to Tim Parrott