Caution: Oil Relief Passage in Crankcase

[NWade]

All -

While re-assembling some engine components following my oil cooler blowout, I found something I wanted to share with other AeroVee builders.

As I was pulling things off the engine to check them, I figured I'd pull the oil pressure control and oil pressure relief springs and plungers and examine them for free movement in the crankcase. When I pulled the oil pressure relief plug & spring, I found that the plunger remained in the engine. This was surprising to me, since I tested the plunger in the crankcase (oiling it lightly and then checking to ensure it slid up and down in the passage) before assembling the engine - per the AeroConversions assembly manual and DVD. The plunger was way up near the top of its travel, but seemed to be sticking only lightly so I was able to wedge an angled dental-pick in the underside of the plunger and pull it out (over a series of several attempts). When the plunger got about halfway down, it suddenly slid freely out of the engine. Examining the outside of the plunger revealed no gouges, rust, or deformations that would explain why it stuck, so I was perplexed. I stuck my pinkie finger up in the hole, and couldn't find any issue, either. But then I shined a flashlight up the hole and everything made sense:

As you can see, just beyond the reach of my finger (where the horizontal galley joins the vertical hole for the plunger) there is a step - a change in the diameter of the hole! When I tested the relief plunger in the crankcase during assembly, it is likely that I was sliding it up and down inside the lower (and larger) section of this hole and never pushed it in far enough to see it jam in the smaller-diameter upper section.

My guess is that this step was caused by some secondary drilling action during the manufacturing of the crankcase, which slightly widened the lower hole (probably when the passage was drilled and tapped to accept the threaded plug that holds the relief spring & plunger in).

If I had detected this before engine assembly, I would likely have gotten a machine shop to bore that hole to a consistent diameter all the way up so that it had no step. However, since my engine is basically complete and I'd have no good way of removing chips. My solution was to use a fine scotchbrite pad to polish the outside of the relief plunger and slightly smooth/round-off the chamfers on both the top and bottom. I did this over a couple of iterations - polish a little, then thoroughly clean the plunger and reinsert it (pressing it to the top of the shaft using the spring). I was looking to see if the plunger slid/fell out on its own. Given the oil in the passage, the plunger could stick at the top just due to the viscosity of the oil. So I would give the side of the crankcase one or two light taps with a rubber mallet. If the plunger didn't slide down, I removed it (using the pick technique) and polished it further with the scotchbrite pad - then cleaned it and did another insertion/slide test. After 2 or 3 passes I could get the plunger to slide all the way down and out of the engine after a couple of mallet taps.

Once again: The key takeaway for other AeroVee builders is to note that sticking the plunger in with your finger doesn't test the plunger's full travel. You want/need to examine the bored hole for smoothness and use tools to check the plunger's free movement along the entire length of the shaft.

Take care,
--Noel

P.S. I do not believe that this is the "smoking gun" for why my cooler failed; but it may have contributed to some of the stresses my oil cooler was seeing right at start-up.

[lutorm]

Yeah, I noted the same step. I think that's the difference between the part of the bore that is reamed to precision fit the plunger and that which is just the size for tapping the threads. I do not think you'd want to enlarge that part, it would leave the plunger with a very sloppy fit and allow oil to leak past it.

I pulled out the plugs for the oil galleries to clean them out and when you tap the holes for the npt plugs to plug them back up, you run the tap into the plunger bore. This caused some burrs to intrude into the bore so what I did was to tape some fine sand paper on a socket of appropriate diameter on a socket extension and pull that back and forth across the hole until the plungers ran freely.

Compressed air in the oil cooler hole is a great way of getting the plungers out. If they're really jammed in there you may have to plug up the holes in the bearing journals to get enough pressure, and cover the plunger bore with your hand when you do this or you may have to go searching for the plunger. Ask me how I know... ;-)

[Rynoth]

Doesn't the fact that the plunger came out with a little tug from a pick indidate than even a few psi of oil pressure would overcome any friction in this galley?

For what it's worth, I've removed pistons twice now from my oil galleys to troubleshoot what seem like high oil pressures (80-100psi) in my Aerovee Turbo, and both times I found what you are describing... slightly more friction near the top of the galleys, but nothing much more than enough to keep them from free-falling when the springs are removed. I even got myself a pluger removal tool:

https://www.handsontools.com/Schley-805 ... gJTNPD_BwE

so I can attach to the plunger and feel the actual friction in the galley, and it is still very slight... I don't think my own high oil pressures are due to this friction. But, I still don't know why my oil pressures are also so high. I'm following your progress very closely and appreciate your posts and updates very much.

[NWade]

Doesn't the fact that the plunger came out with a little tug from a pick indidate than even a few psi of oil pressure would overcome any friction in this galley?

To be clear: It did not come out easily the first time. And I was also worried the first time that it had gotten cocked at an angle inside the bore. Wasn't sure how, but the way it seemed to be "hung up" in there had me worried.

Again, the biggest thing I wanted future builders to note was that the plunger may seem to slide just fine in the lower area (i.e. as far up as you can push it with your finger), but that doesn't mean its a good fit all the way to the top.

--Noel

[Don_P]

Thanks for this post. I'm in the process of assembling my AeroVee and came upon this oil relief/control piston issue pretty quick. The discontinuity in the cylinder wall of the pressure relief valve is about 1 3/4" up in the cylinder, and when the piston and spring are inserted, one or the other can snag at that point. Then when enough force is applied in trying to get the cap on, the spring and piston can break free and the piston will be driven up into the narrower part of the cylinder. It could easily jam there. Best way to test (and adjust) the fit of the piston is to attach it firmly to something (the tool cited by Ryan is good) and then slide it all the way up into the cylinder. If it's too tight, adjust it with emery paper.