The Heart of the Honda L15B's Oil System: The Oil Pump

Have you ever wondered what's actually inside the housing of your oil pump? Well in this article we're going to dive into the Honda L15B oil pump and how it functions to keep your engine alive.

The L15B oil pump is bolted directly to the front of the engine block behind the timing cover. The pump type is a directly driven gerotor pump, since the pump is driven directly by the snout of the crankshaft as opposed to being driven by a chain or belt. This is an important detail because it bears a large influence on how the pump survives (or doesn't) when the wick is turned up. Pictured below is an L15B oil pump, there are 6 bolts that mount the pump to the block and note the two opposing pairs of tangs inside the inside of the pump, this is what rides against the crankshaft to spin the pump. The pickup tube is bolted to the bottom of the pump with two M6 studs and nuts.

To open the pump there are 4 flat head screws which retain the backing plate to the pump housing. These screws use a #3 phillips head, using any other size greatly increases the likelihood of them stripping. The 8mm allen plug next to the pump inlet at the bottom of the pump is the retaining plug for the oil pressure relief valve. With the backing plate removed there is access to the pump gears and the oil pressure relief passage can be seen as well.


Gerotor Pump Function

The internal gear of the pump is driven by the crankshaft and the motion of it spinning causes the outside gear to spin as well. As it spins the gear teeth separate as they pass over the pump inlet, creating a vacuum to draw in oil. As they continue to spin the oil filled cavity passes over the outlet as the cavity shrinks, pushing the oil out creating flow.

This repeating process at high speed is how oil pressure is built inside the engine. When a certain pressure is reached the spring behind the relief piston is overcome and compresses from the greater oil pressure. This causes the outlet of the pump to be connected to the pump inlet and bleeds off excess back to the inlet oil to keep oil pressure from getting too high.


Why Pump Go Boom

Part of the reason for this article is to cover what causes these pumps to fail and what can be done to keep it from failing. The main reason oil pumps fail is due to torsional vibrations in the crankshaft. Torsional vibrations exist in the crankshaft of all running engines and the simple way of explaining it is the crankshaft is trying to speed up when subjected to a cylinder's power stroke and slow down when there are no power strokes. These extremely small speeding up and slowing down motions manifest as torsional vibrations. If severe enough, torsional vibrations can break crankshafts in half and wreak havoc on anything connected to the crankshaft, like the oil pump. Torsional vibrations are heavily influenced by engine speed, and as the engine spins faster they get worse. So suffice to say, if an L15B is revved high enough the torsional vibrations can break the oil pump gears.



How To Keep Pump From Booming

This is where Boundary Engineering comes in!Boundary makes billet gerotor style oil pump gears for a multitude of applications including the L15B.

The factory gears are cheaply made sintered metal. Similar to how many factory connecting rods are made, these gears are made by filling a mold with powder metal and then subjecting it to immense heat and pressure to make the powder metal fuse together. This process is far cheaper than machining a set of gears from billet steel like Boundary does, but it leaves small stress risers in the form of microscopic voids on all of the surfaces of the part. These stress risers are the weak link that breaks first resulting broken pump gears when those pesky torsional vibrations enter the room. Boundary gears on the other hand are made from solid billets of 4140 steel which is a chromoly steel. This means that there are no voids in the part and the material itself is significantly stronger than the mild carbon steel used in powder sintering. The result is pump gears which can handle far more abuse and torsional vibrations than the weak OEM gears.

Here are a few photos that display the visible difference between a billet part and a powdered metal sintered part. We'll note this is a like new factory pump, so none of this is worn surfaces. Also, notice the nicely chamfered edges on the Boundary gears, this is done to avoid stress risers from forming at those edges and creating cracks in the gear that can lead to failure.


Hurray! Now Pump No Boom

The process of installing Boundary gears is as simple as removing the factory gears and dropping in the Boundary ones. Before we do that we want to double check things fit properly because catching any issues now could save us from an engine failure. We want to check that there is sufficient clearance in both the radial and axial directions of the gears in the pump housing. To check the radial clearance use feeler gauges between the outside of the outside gear and the pump housing.

To check the axial clearance put a straight edge across the pump housing and use feeler gauges to check between the top of the pump gears and the bottom of the straight edge.

We found our factory gears had .004" radial clearance and .001" axial clearance. The Boundary gears had .005" radial clearance and .002" axial clearance. This is perfectly acceptable and honestly probably more desirable than factory clearances since this will give a tad more room for the gears to flex when being rattled by strong torsional vibrations without tearing into the pump housing. The only downside to the increased clearance in this case is a bit more leakage around the gears, but these engines don't struggle with oil pressure so that's not a concern.

With the clearances checked we can coat the gears with the supplied lube and install them in the pump housing. We ran all the parts of our pump through our ultrasonic cleaner to get any and all dirt out of the pump which also left it dry so we also lubricated the relief valve piston prior to reinstalling it.

Finally, put a light coat of lube on the backing plate where it comes into contact with the pump gears and reinstall it along with the 4 flat head screws. Honda does not list a torque spec for these bolts because the FSM says not to open the pump so we torqued them to 10ft/lbs as that felt sufficiently tight without stripping out the heads. The relief valve plug gets torqued to 30ft/lbs according to the FSM.


So Why'd We Do This?

Well, we are focusing on some engine development projects with the L15B and our 10th gen Civic to show what this engine is capable of with the right parts combination and hopefully prove that it is a viable option for builds and swaps alike in the coming years. Part of this project involves revving it higher so we wanted to insure that we won't have any unexpected oil pump failures while we're exploring higher redlines!

If you'd like to get one of these Boundary pumps for yourself we have them available for order HERE on our site, or if you'd like to buy them preinstalled in a new Honda pump housing send us an email at and we'll get you taken care of!

We've got plenty of more L15B7 engine content coming up so be sure to check back for new blog posts and follow us on our social media accounts for Facebook, Instagram and TikTok!

Until next time!

- Austin and the Wunderladen Team

1 comment
Back to blog

1 comment

Fantastic write up! Anyone who experienced a failed OEM pump on B-series knows this is a great insurance policy.

Ryan Kristoff

Leave a comment

Please note, comments need to be approved before they are published.