REF: General-MSR 04A

Modifying a Rubber Stopper for Oil Tank Pressure Testing

If you need a modded oil cap with a hose fitting for testing purposes, a rubber stopper can be modified to work.
The one below was modified to fit inside a 1998 Sportster oil tank (hole is tapered, app. 1.274“ to app. 1.224”).

A rubber stopper can be purchased at local hardware stores. The one below was bought from Lowes.
The Hillman 4.6“ black neoprene hole plug (taper 1-3/8” x 1-3/16“) will work by simply drilling a hole through the center of it.
However, if it can't be found, the Hillman 1-5/8” x 1-5/16“ x 1” can be slimmed down to work.
It is actually too big and the smallest part of the taper is all you can get into the oil cap hole.

The 1-5/8“ plug was used below. See the before and after pic below. The 1-5/16” end of the plug will go in the hole but not very far.
It is possible to simply hold it into the tank since it will go. But the one below was shaved to allow hands free operation.


The hole was drilled in three size stepped bits.
Then the fitting was clamped in a vise, the plug pushed hallway on the fitting and snugged up with a wrench.
To test how it would hold under pressure, 100 PSI was applied from the opposite side and the fitting popped out.
Another test was done after the fitting was reinstalled and it held app. 50 PSI well.
But, for testing crankcase pressure, you should be testing more than a 1-2 PSI anyway.

2) 3) 4)

The body was shaved by chucking the fitting into a drill and the plug was spun lightly on top of a rasp clamped in a vise.
Then the roughness was smoothed out by using steel wool wrapped around a file.
The fitting was pressed deeper into the plug during this process (not necessary but it happens).
This can now be used to test crankcase pressure in the oil tank.

5) 6) 7)

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photo by Hippysmack
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