SWHouston

Oh ok, but still a good opportunity to check/replace if necessary.

I've really never had a chance to take a close look at a "Hi' capacity pump. Taken several standard pumps apart though. It seems like for a standard pump, the gears are of equal size.

The only way I can think of getting more pressure or flow out of one is...
For Pressure, you'd have to make the Drive smaller, and the Driven larger.
For Flow, it would be the opposite. Large drive, smaller Driven.
The above assuming equal RPM and resistance.

Had you thought to maybe pull the Melling pump apart, to check just how they do it ?

SnoDrtRider

I know on a standard pump the gears and housing are taller... that would give more flow. As far as pressure that is upped by the relief valve spring having more tension.
On a Gearotor pump like this they could make the gears thicker (wider) I can throw a mic on both the old and new tomorrow to see what the difference is without taking the pump apart.
The housing on the Melling is Cast Iron vs aluminum for the stocker... that would probably allow them to have a thinner housing without sacrificing strength allowing them to fit thicker gears.

SWHouston

Yes, I think taller would be more likely to effect flow than pressure.

I did a test (or tried to) on this very issue, with an 1/2hp electric motor, on the bench. Tried to do a manual (stretch/cut off wraps) of the pressure relief spring for a Dragster I was running on 1/4 mile.

HA, total failure !
1/2hp was just not enough to get it past the start capacitor. It takes a LOT more HP than I thought to turn an Oil Pump ! Gave up since I didn't have a higher HP Motor. Wound up pumping at about 65 lbs, since we weren't running any coolant.

Looking forward to reading your findings !

SnoDrtRider

Well I didn't measure the gears in the pump... sorry but they look very close to the same as the OEM pump.
I did check the relief spring against the old one and it is longer and stronger.

Installed the pump, dropped a rod cap and everything is good so I didn't bother looking at a main.





Engine is cleaner than I would expect for 220,000



Got her most of the way back together today... a few hours tomorrow morning and the true test comes.

SWHouston

Well, I see a little Varnish there, but for 220K, that's not bad at all.
Somebody took real good care of it !

Great picks by the way

SnoDrtRider

Problem Solved! or at least fixed!

Completed the assembly this morning and with great anxiety tuned the key. She started like she had never been apart and the oil pressure gauge immediately went to 60-65 PSI!
After running a few minutes it settled at about 50 PSI idling.



Ran it for about an hour checking for leaks, topping off coolant and so on. Put about 100 miles on today just running around checking on jobs and errands and in gear at idle I see 40 PSI. 60-65 PSI driving down the road.

Now for figuring out what the exact problem was...

I took all the old parts including gaskets to my buddies machine shop for a forensic analysis.
We believe the problem is three contributing deficiencies.

#1 The pressure relief spring in the old pump was sacked and weak.
Here is a photo of the spring out of the old and new pumps. I measured the depth of the bore of the valve with the piston in it and they are the same on both pumps so spring length should have been the same.



#2 The oil pan gasket "O" rings at the oil passages between the pan and block are thinner by .001-.002 than the surrounding aluminum part of the gasket and there is no doubt that oil was bypassing the circuit and dumping back into the oil pan.



#3 The "O" ring that seals the pick up tube to the pump was a fairly loos fit in the pump and could have been sucking air along with oil into the pump.

SWHouston

FANTASTIC !

But that (idling) 40psi is with that real thin oil right ?
Even with that, given that the bearings have that kind of mileage on them, 40 is real good. I remember when I was doing that Pump Pressure Test, that even a few thousands inch on the length of that spring, made quite a bit of difference.

Glad you're on the road there, and "everyone" appreciates the nice clear pictures and your follow-ups.

S.W.