Oil pump piston, some questions.

[elgyqc]

I bought the new oil pump piston (997-107-125-01) to replace the original piston (996-107-125-51) since this is recommended to help oil pressure at idle. I also bought a new spring but just noticed that the spring part number has not changed (996-107-127-53).
I wanted to compare the new and old pistons, so removed the piston from my torn down broken 3.2l engine. I understood that the old piston should just drop out, but that was not the case for me, I had to force it out from the top (which can be done with the pump removed) and once that it was I out I could feel that it was slightly mushroomed at the top, which I think is what made it stick in its bore.



Old on the left new on the right.

Questions...
1) Is a new spring recommended because the original one may have lost some of its "spring"?
2) Are the old and new pistions identical except for the chamfer at the top of the new part? If so why could a resourceful person not just machine a slight chamfer on the old piston to get the same effect?
3) Is the mushrooming at the top the reason for the chamfer?
4) What exactly do the piston and spring do and how do they do it? My understanding is that it is a pressure relief valve.
Thanks in advance.

[JFP in PA]

Originally Posted by elgyqc

I bought the new oil pump piston (997-107-125-01) to replace the original piston (996-107-125-51) since this is recommended to help oil pressure at idle. I also bought a new spring but just noticed that the spring part number has not changed (996-107-127-53).
I wanted to compare the new and old pistons, so removed the piston from my torn down broken 3.2l engine. I understood that the old piston should just drop out, but that was not the case for me, I had to force it out from the top (which can be done with the pump removed) and once that it was I out I could feel that it was slightly mushroomed at the top, which I think is what made it stick in its bore.



Old on the left new on the right.

Questions...
1) Is a new spring recommended because the original one may have lost some of its "spring"?
2) Are the old and new pistions identical except for the chamfer at the top of the new part? If so why could a resourceful person not just machine a slight chamfer on the old piston to get the same effect?
3) Is the mushrooming at the top the reason for the chamfer?
4) What exactly do the piston and spring do and how do they do it? My understanding is that it is a pressure relief valve.
Thanks in advance.

1. No, the new spring has a higher rate, which raises the oil pressure slightly at low RPM.
2. Yes, the chamfer is the trick, it stops the mushrooming and prevents the piston sticking in the bore.
3. See #3.
4. It is the pressure bypass system for the pump.

For around $20, you get everything new and do not need to machine anything...........

[elgyqc]

Originally Posted by JFP in PA

1. No, the new spring has a higher rate, which raises the oil pressure slightly at low RPM.
2. Yes, the chamfer is the trick, it stops the mushrooming and prevents the piston sticking in the bore.
3. See #3.
4. It is the pressure bypass system for the pump.

For around $20, you get everything new and do not need to machine anything...........

Thanks for the info. You make such a great contribution to this forum!

Because I am an ornery old fart I may just try experimenting with a machined piston and a washer added under the spring to see if I get the same result for free. It is easy to change the piston and spring and I have 3 engines I can experiment with.

[986tate]

Originally Posted by JFP in PA

1. No, the new spring has a higher rate, which raises the oil pressure slightly at low RPM.

Low rpm only?
So it won’t really help for track use?

[JFP in PA]

Originally Posted by 986tate

Low rpm only?
So it won’t really help for track use?

It will help slightly at higher RPM's as well, but was created to address pressures so low at idle that the oil light flickered, and address stuck piston's; plus most track oil issues are related to oil control and/or starvation,

[The Radium King]

it's a pressure valve. if oil pressure exceeds a certain value (as determined by the spring) then oil pump will bypass back to sump.

you will see high pressure at start-up with cold oil. the stronger spring keeps the oil circulating a bit more before bypass which will help lubricate the chains.

also, the oil pump pressure is dependent on rpm - pump spins faster as rpms increases which results in increased oil pressure. so when flogging it around the track at high rpm expect that some oil gets bypassed to keep pressures within spec. something to consider when trying to determine where to plumb-in an external oil cooler or accusump.

also, when oil starts to build up in the heads when doing higher-g turns, pressures will increase and your pump will start to bypass just when you need it the most.

so, stronger spring is good.

[986tate]

Cool, thanks