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Oil filter cover has oil passage restriction

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I was in the middle of an oil change and I happened to look into the oil passage of my filter cover (the one at the edge of the cover, where the small o-ring is). There is a cap that covers the end of the hole drilled to make the passage within the cover, and that cap extends far into the oil passage, forming a restriction where the oil has to take a 90 degree bend. I imagine this creates a pressure drop, and reduce the amount of oil flowing into the engine.

I'd like to take a dremel tool to the thing; unfortunately this is downstream from the filter and there is no way to positively get the grit out of there. So, I guess not...

Also thought about drilling the cap out, threading the hole and putting a short cap screw in. Who knows what makes sense?

Maybe I should just forget about this, but it bugs me to have a restriction like this in an oil passage. It's not like these engines have a huge surplus of oil pressure.

It's a risk to work on this part, but not a huge risk. It's a $40 part...

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I was in the middle of an oil change and I happened to look into the oil passage of my filter cover (the one at the edge of the cover, where the small o-ring is). There is a cap that covers the end of the hole drilled to make the passage within the cover, and that cap extends far into the oil passage, forming a restriction where the oil has to take a 90 degree bend. I imagine this creates a pressure drop, and reduce the amount of oil flowing into the engine.

I'd like to take a dremel tool to the thing; unfortunately this is downstream from the filter and there is no way to positively get the grit out of there. So, I guess not...

Also thought about drilling the cap out, threading the hole and putting a short cap screw in. Who knows what makes sense?

Maybe I should just forget about this, but it bugs me to have a restriction like this in an oil passage. It's not like these engines have a huge surplus of oil pressure.

It's a risk to work on this part, but not a huge risk. It's a $40 part...

Since this is a 1991 bike, I would think that this would not be a problem if it made it through the first 19 years... what do you think?👍

Perc

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Sure. Any bike can run with any flaw, for a while. Mine has under 4k miles on it, probably in mostly cool temperatures. I'm just wondering if I can have more oil pressure just by taking care of this little flaw. More oil pressure is better than less oil pressure, I think. 👍

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I'm having a hard time visualizing what you are talking about... could you post a picture?

If you think about it, pressure is caused by restriction. The more restriction, the higher the pressure will be. Less restriction would mean oil can flow more easily and there would be less pressure involved.

I would leave it alone!

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Here is the cap that plugs the end of the passage drilled from this point inward:

6.jpg

Here you can see in the hole at right (which carries the oil into the engine), the cap protrudes about 40% of the way into the 90 degree bend, causing the constriction. Sorry for the fuzzy photo.

2.jpg

You misunderstand me. The pump generates oil pressure. Every time the oil passes through a restriction, be it at this corner or through a plugged filter for example, the pressure drops some. In fact just going down a straight pipe reduces pressure too, but if the pipe is big enough the drop is minimal. By the time the oil actually gets to the bearing you want to lubricate, you have less pressure than you had right after the pump; less pressure means less protection for the bearing. Get low enough and the bearing is in danger of seizing. Of course the bearing is itself a restriction, and the oil pressure (what's left of it at that point) drops to zero after passing through it. But the ideal situation is large passages and few corners and few restrictions (and a clean filter), so maximum pressure is delivered to the bearing.

Of course the obvious question is, what percentage of the pressure is lost before we get to the bearing? If it is 1%, we don't care. If it is 50%, we do. I don't know the answer to this question.

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The XR pump is positive displacement. Unless the pressure drop is great enough to make the pump blow the gasket it will pump pretty much the same amount of oil.

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I was looking in the shop manual for a pressure relief valve, but I don't see it.

If there is a valve somewhere, having a constriction in the oil passages will result in more oil going to the relief valve and less to the engine bearings.

If there is no relief valve, having a constriction in the oil passages will not change anything as you say, except that more oil will leak around the pump rotors. Whether this amount is significant is another question. It is possible the pump was designed to allow a certain amount of leakage in higher pressure situations, similar to a relief valve.

I notice there are two sets of rotors. Can anyone tell me why? Is there some online document explaining how lubrication works in this engine?

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I disagree with your assessment Paul. I think you're confusing pressure with flow. As XR250rdr stated, the XR pump is positive displacement. The restriction in that oil passage doesn't reduce pressure. It acts to increase pressure in the system downstream from the oil filter exit.

By removing the restriction, you might get more flow, but at a reduced pressure. The pressure on the system is what carries the oil, no matter what the flow rate is, to the far reaches of the system.

The filter media itself and the sheer volume of the oil filter space is a restriction to flow and pressure seems to me. It acts to want to "stop up" flow in the system as oil is filtered thru it. I think the restriction in the filter outlet passage is there to ramp up pressure after oil is filtered.

Edited by Trailryder42

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Well, your motorcycle must operate in defiance of the laws of physics, then. 👍

Any flow restriction acts to reduce pressure, downstream from the restriction. That is, there is a pressure drop across the restriction. You cannot increase pressure or flow to the bearings by putting a restriction in the path. You may increase pressure upstream of the restriction, but there is no point in doing that, unless you like blowing out oil seals. If there is a bypass valve upstream of the restriction, more oil will go there. If not, more oil will leak past the pump rotors.

It's just like having a garden hose with an open end. If you start to close off the tap, you get both less flow and less pressure out the open end.

It's possible I am getting confused with this, but not too likely, since I spent my working life as an electronics engineer. Fluid dynamics has an almost exact analogy with electricity. Flow is like current, a restriction is like resistance, and pressure is like voltage. A positive displacement pump is like a constant current source (not like a battery), but there are imperfections (leaks) in any pump which makes it an imperfect, that is, not quite constant, current source.

This plug is not there to make a restriction. It's there to plug the end of the drilled passage, but they goofed and created the restriction. However maybe the pressure drop is small enough that it doesn't matter.

Anyway I got my new o-ring in the mail and finished my oil change, so the question is moot for now. But I may some day get my tap and die set out and rework that plug area, and put in a short screw.

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The larger rotor pulls oil from the main sump, the smaller rotor evacuates the crank tray and left side cover.

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Paul, I'm still going to stick with my original assessment and say "don't fix what ain't broke"!

You are talking about a 19 year old bike. I don't believe any engine could have a problem with the oil delivery as you are describing and get to be 19 without major bearing failures.

Do the cam bearing show signs of lack of lubrication?

The oil pressure/flow balance game is a well engineered one. Robbing from Peter to pay Paul, is what you can do when you make changes to the oil system. If you increase oil flow through one passage, it slows through another.

I still think that the age of this bike has proved that there is no problem.

Perc

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