Pressure Relief piston (again)

4sd911 · 01:45 PM

Update in post 4. This is the safety valve and not the relief valve.

I've received my case back from the shop. They said the pressure relief and safety valve bores are OK. Reasoning, "the piston drops in all the way freely". I'm concerned that these bores are OK and will work properly once put back together. There is shinny ware mark on one side of the bore from the piston moving when measuring the travel distance in the bore from back of seated piston to the ware line it looks to be traveling about 5.5mm, the distance from the bottom of the bore to the hole in the side that releases the oil pressure looks to be about 4.3mm. So the piston looks to be opening about 1mm - 1.5mm at the relief hole, does that sound like it is opening enough to release the system pressure at cold start up? The bore does have some pitting also but doesn't look to bad. I have two pictures of each side of the bore you can see the shinny side good and other not as good.

4sd911 · 03:21 PM

Picture a little big so I've resized them.

4sd911 · 10-17-2009, 07:02 AM

Any comments? Does it look / sound like the relief piston is moving enough?

4sd911 · 10-20-2009, 01:43 PM

I made a mistake, this is the safety valve and not the relief valve. As stated above I said it looked like the piston was opening 1 mm to 1.5 mm compared to the rubbing mark in the bore. I pushed the piston in so the rub mark is at the end of the piston and used a mirror to look in from the hole in the case, this piston is barely open if at all at this spot.

So, what does the safety valve do to regulate oil pressure at cold start up? If this valve is not opening at cold start up will this cause the pressure sky rocket?

DW SD · 04:50 PM

what year case is that?

Did it have pressure problems previously?

What symptoms might this cause, if any?

Reason I ask is I've had low rpm pressure issues on a 964 - based engine and even though the motor was apart and inspected by Ollie's (known as a competent machine shop), no issue could be identified.

Doug

4sd911 · 10-21-2009, 06:07 PM

This is a '74 2.7 mag case. I called Ollies today about having the hole fixed. They do this, they bore the hole larger and make a new piston to fix the new hole. My problem is high pressure, especially at cold start up, John at Ollies (I think it was John) said the problem with these bores wearing causes low oil pressure and he doesn't think it would be my problem with high pressure. He said some thing must be plugged, this leads to my other post on crank cleaning.

Walt Fricke · 10-21-2009, 06:51 PM

4sd

You might define "high pressure," and how it varies with temperature.

And did you have a match between pressure sender and pressure gauge. There are those made for 5 bar, and those for 10 (older system). Put a 5 bar sender into a 10 bar gauge car and you have high pressure indicated. Or so I am inclined to believe.

Walt Fricke

4sd911 · 10-21-2009, 07:15 PM

Well the gauge on the dash reads up to 150 psi. Before the problem I would see the cold start up pressure rise above or around 100 psi then I'd see a quick drop as if the valve had opened. What was happening was the reading at the gauge would rise to the top and wouldn't come back down until the car warmed up. I also checked this at the engine with a mechanical gauge with the same results, cold reading going up above 200 psi. I'd hate to see this go back together and see the same results. There were a few other issues that needed attention, like brass in the oil, pulled intermediate shaft stud, in need of spacers under cylinders. The case also had a problem with decaying magnesium, cleaned this by ultra sonic system.

Walt Fricke · 10-21-2009, 09:03 PM

OK

Had you not used a mechanical gauge I'd have suspected the OP sender. An "open" in that circuit (which could be in the rheostat windings themselves) will cause the gauge to peg.

Between the pressure setting piston (bleeds off oil to keep pressure from getting above the specified level) and the safety piston (does pretty much the same, just at a slightly higher pressure?), you'd not think even some blockage somewhere in the many places to which oil is fed would cause an overpressure. Lack of oil somewhere, yes. Not overpressure. Obviously you've probed, inspected, flushed, and blown out all the oil passages you can easily access.

Have you pulled some of the oil gallery plugs and run a bristle brush through them? Again, that is normally done to clean out any crud which could damage bearings, not because of a suspected full blockage.

Has this case had the oil pressure modification done to it? One hole blocked off, another added, and different pistons? That happened in '76 or 7. It is a useful modification if it hasn't been done, and you should do it or have it done. You get a kit with new pistons, springs, and some other bits, drill a hole, and tap and plug one. But I'm not sure it would help with whatever is causing this behavior.

I don't think anything is hurt by 200 psi for a while, though the safety piston I think is to protect the oil cooler, and it might burst eventually?

Time for someone with more skill to chime in.

Walt

4sd911 · 10-21-2009, 11:18 PM

All that you mention with the bypass and new springs and piston kit had been done. I also talked to Otto (Otto's Venice) when I told him of this he said the case was dirty and have it ultra sonic cleaned, the two main galley plug were pulled also. Since cleaning the case was sent back and line bored and the center main pinned. It will be ultra sonic cleaned one more time I'm thinking of pulling the spray jets. I don't know what chance there is of any shaving's from the line boring getting in the oil galleys, I assume that can be flushed out without pulling the main galley plug again.

Walt Fricke · 10-22-2009, 05:49 PM

Let's back up. What is important is how your engine was configured when it had the overpressure problem. Since it had been acting normally, what changed? Something happened. It wasn't the oil pressure sender, because you put in a mechanical gauge and still got 200 psi cold. The other stuff you have done this go around - line boring and whatnot, doesn't help the diagnosis.

So, had the oil bypass modification been done before you had the problem? Not sure it would be part of this problem, but it appears there isn't much experience with overpressure, so good to know.

Let's review what the teardown showed:

All you have found to date is apparent wear on the safety relief case bore.

And some miscellaneous brass in the oil. Don't think that means much as to oil pressure.

If you found that there was a ton of crud in a critical oil passage, that might be a clue (though my take is that the safety bypass ought to be adequate to deal with this kind of issue, and you'd not see high oil pressure, just an engine which destroyed something which wasn't getting enough oil).

Here is some information I gathered a while back from the little white 911 factory spec books about spring length and relief/safety pressures:

2.7 (before the mod) had 70mm free length springs. It took 10.6kp/104newtons/77lbs/ft to compress this to 52mm, and 14.1/138.3/102 to compress to 46mm. Full compression length of spring is 33.3mm.

The 3.0 had a safety spring 70mm long, and a pressure spring 86.6mm long (this is the one with a guide). Safety spring had the same specs. Pressure spring at 50.5mm is 80N/8.4kp/59 lbs/ft.

For the 3.2 (which I think has the same stuff as the 3.0): Safety opening pressure is 8 bar/114 psi. The pressure spring for the 3.2 is 6.2 +/- 0.8 bar, or 88.2 +/- 11.3 psi. All of which fits how your engine used to work before something went wrong.

Not sure what this may all mean for your issue, but may give you some ideas.

Also, your pictures show nicely an additional sideways hole, which is below the piston. It just dumps back into the sump. I believe its function is to allow the oil which gets around the sides of the piston (which has no seals or rings, so there will always be some) to dump into the sump. If there were no such hole, the piston would soon hydrolock - be unable to push down easily, due to trapped oil below. If somehow this hole got plugged, I could see the safety relief not working properly.

However, if something were stuck in that hole, you'd have found and noted it.

Walt

4sd911 · 11-04-2009, 05:07 PM

The case is at Ollies now. George (not John) said that the safety valve bore is worn out and will need to be enlarged, they will make a piston to fit. He doesn't think this is causing the high oil pressure. The crank is at a different shop (locally) and they have pulled plugs, they are reporting that there is gunk behind the plugs.