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304065 304065 is offline
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Now some comments about the springs.

The original springs were both 70mm long.

From the workshop manual:

Free length 70mm
Diameter of wire 1.4mm
External diameter of spring 12.2mm
Pressure exerted at length of 52mm 76.66 ft/lb
Pressure exerted at length of 46mm 101.98 ft/lb
Length fully compressed 33.3mm

Those are the specs for the original 70mm spring. Here's an illustration of the change.

In 1978, some SC 3,0 motors were having oil pressure issues, so the factory came out with a service bulletin, increasing the length of the PRESSURE spring to 86mm, including a spring guide inside the lower end so the spring didn't buckle, and changing the depth of the plug to 9mm.

Notice that this bulletin about the springs came out much later than the oil bypass mod-- there is some disinformation out there that the springs must go together with the bypass mod, but this isn't necessarily so. They are often done at the same time, but aren't required. (This becomes important when chasing oil pressure issues high or low).

The original SAFETY valve opens at 8 atu, or 113.7 psi. The original RELIEF valve opens at 5.4 atu, or 76.8 psi.

Here is some information from Walt about the spring rates:

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.
Here are some actual spring measurements from Aaron Burnham:

Walt, here is some data. If you need a specific let me know. I used a spring compressor tester to get the rates

Long spring(dims include piston and guide tube)
60mm=6 lbs
50mm=12 lbs
41mm=19 lbs (guide tube limits travel @ 40.5mm approx)

short(dims include piston)
60mm=6 lbs
55mm=12 lbs
50mm=20 lbs
Implied spring rates-- for the long spring
85mm-60mm = 25mm of compression for only 6 pounds of pressure-- so 6/25 =0.24 pounds/mm (6.154 pounds/inch) (first 29.4% of length)
85-50mm = 35mm for compression fo 12 pounds = .343 pounds/mm (8.709 pounds/inch) (first 41% of length)
85-41mm = 44mm for compression of 19 pounds = .432 pounds/mm (10.968 pounds/inch) (51% of length)

So this is a "progressive" spring that gets increasing resistance as it gets more compressed.

For the short spring:
70mm- 60mm = 10mm for 6 pounds = 0.6 pounds/mm (15.228 pounds/inch) (14.3%)
70mm-55mm = 15mm for 12 pounds = 0.8 pounds/mm (20.305 pounds/inch) (21.4%)
70mm- 50mm= 20mm for 20 pounds = 1 pound/mm (25.413 pounds/inch) (28.6%)

This is also a progressive spring, the rate increases as it gets shorter.

Spring rates tend to lag for the first 20 per cent of compression, then be relatively constant for the 60 in the middle, then increase for the last 20 per cent.

As Walt's photo shows, the original plug has a 12mm depth. The second plug has a groove around the head, and is 9mm deep, as is the final hex-head plug 9mm deep.

For the VERY early engines (up to 901282) there was a 2mm thick washer used between the first version plug and the case. The depth of the bore from the face to the piston seat was 57.5mm. Later, from 901283 onward, this was revised to be 59.5mm.

A standard crush washer is 1.5mm thick, it gets compressed down to about 1.3 on installation to 60NM.

* * *

An interesting aside: the original 944 used the same type of piston/spring arrangement, which was later changed to something like this:

Rather than relying on a potentially imperfect bore in the case, this is a complete insert that slides inside, with the o-ring intended to seal the end against the bore. When pressure builds up inside a machined steel piston inside this assembly, it overcomes a small spring, venting pressure into the original vent passage. I suppose they did this more expensive solution (this part is like $400) to have more precise tolerances and better control over oil pressure.

To continue the aside, this is the 944 diagram with the oil cooler and filter in the pressure circuit. They are a lot more effective this way, there is less air than on the scavenge side.

While we're on asides, the apple doesn't fall very far from the tree. . .

The 356 used a variety of springs. The A case lists:

547 07 113, 11 x 65mm for type 692/0 engine
999 521 016 00, 12,2 x 66mm for type 692/1/2/3 (same diameter as modern spring)
999 521 012 00, 12,5 x 61.5mm for type 692/1/2/3

* * *

Now, the 993 actually used a spacer between the plug and the spring, Walt, did you ever measure this one? See illustration below.

Also, I hear that the GT3 uses an even deeper plug, >12mm depth, along with a huge 102mm spring, but I haven't seen the specifications on this. Can anyone help?
'66 911 #304065 Irischgruen
'81 R65
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Ex-'89 944 Turbo S (Sold 8/21/20)

Last edited by 304065; 05-07-2013 at 06:35 AM..
Old 10-22-2012, 11:51 AM
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