Which lapping compound for oil pump work?

[rang-a-stang]

I went to McParts this morning and the only lapping compound they had was a 2 pack with a tube of coarse grade and a tube of fine grade. Which one should i use for my oil pump surface? none of the wear marks catch a fingernail so I don't think its very deep...

[babywag]

Depends on how long you want it to take...
If it was me I'd start with coarse, and finish with fine.

When you move on to the cover check it carefully for being flat.
Both the ones I've done were NOT. Think concave...DIY remedy was LONG flat fine file, then lapping compound.

[rang-a-stang]

Awesome! Will do!

[SJTD]

Why lapping compound rather than fine paper? Seems a lot slower.

[FSJunkie]

Lapping compound on a piece of glass will likely get the surface smoother than sandpaper on a piece of glass, but I usually use sandpaper and it's always worked fine. Something like 600 grit.

[rang-a-stang]

Why lapping compound rather than fine paper? Seems a lot slower.

Could be. Both tutorials I saw said to use lapping compound. the oil filter housing is done (took about 30 mins total). I used the coarse for about 20 mins and got it all cleaned up then polished it off with the fine for about 10 mins. Looks great!

These are the tutorials I read:

http://www.ifsja.org/forums/vb/showthread.php?t=175526

https://www.jeepforum.com/forum/f8/amc-v-8-oil-pump-front-cover-rebuild-thread-1050446/

[440sixpack]

It would be interesting to do a before and after on this with an accurate gauge. I bet you couldn't read the difference. at the speed those gears are turning the amount of bypass on those light scratches would be extremely minimal.


If you do it be sure you get the entire surface perfectly flat. just cleaning up the scratches and not bringing the entire surface down with it will just increase the tolerance on top of the gears and accomplish nothing.

For this reason if you're really concerned the plate is the best solution it keeps it perfectly flat. and gives you another place for a leak..

[babywag]

Whole point of this is to check/correct bad clearance. A midplate won’t do squat if end clearance isn’t good.
On my ‘88 the difference in hot oil pressure was substantial, especially @ idle speed. Gained pressure across the board.

[440sixpack]

The end plate gives it a new and harder surface. that's better than a dished or uneven surface that increases end clearance is my point.

[babywag]

The timing cover is often the reason for bad spec. on end clearance.
Generally the end plate isn’t the problem @ least on the ones I’ve done.
Adding a midplate would have done nothing.
If cover isn’t flat, and/or end clearance spec. is off, the midplate is useless, the end clearance will still be wrong.

[440sixpack]

Yes, but if you don't screw up the base the plate will take care of any end wear and it will be flat. and the end clearance will be whatever it was when the engine left the factory. not that the plate is a great idea or necessary but it's a cheap fast sure fix if it's bad enough to need a fix.

I agree the housing is where the real problem lies and there isn't much you can do about that. maybe someday there will be a repop cover that isn't a POS.

[babywag]

I agree the housing is where the real problem lies and there isn't much you can do about that. maybe someday there will be a repop cover that isn't a POS.

That isn?t exactly accurate, you can correct a problem with cover that is how you get good clearance. The cover on my ?88 was pretty bad, after making it flat and some shaving & resurfacing was in spec.
It was likely never ?in spec.? even when brand new.

[rang-a-stang]

I am addressing my pump in a 3 fold manner: Lap the filter mount so its's perfectly flat (done), buy new pump gears (on order, expect them to show up early this week), and lap the housing so the clearance is almost nil (can't do this until my gears show up) and is totally flat. I think if we do those 3 steps, hot idle pressure will be as high as it can be. The only clearance we can't really do anything about is the pump to side wall clearance but I don't think the sidewalls wear much anyway (other than scuffing for contaminates).

I decided against installing a midplate (even though I have one) because it is another potential oil leak/gasket surface (Ristows point) and I had to lap the housing anyway (babywags point).

I am sure AMC had some minimum specs for oil pump clearances but I seriously doubt they took the time we do to truly spec these pumps out like this. I am pretty confident that the 3 steps I am doing (along with new bearings) will get my hot idle oil pressure up plenty high.

[440sixpack]

The AMC oil pump is one of the worst designs ever made. what you're talking about isn't going to hurt anything but keep your expectations realistic.


And the need for oil pressure needs to be kept realistic as well, these engines never ran great pressure I've owned them off the showroom floor and they didn't. 10 PSI per 1000 RMP is sufficient that's why these crappy pumps worked in the first place.

[Ristow]

agreed on pump design. just horrible. however,you better be doing better than 10 psi per 1000rpm if you want the engine to last. a 2.73 rear GW does what...1800 rpm at 55? you'll be wanting more than 19psi.


you should be on the relief spring by about 2000. and they can do it even with this asinine oil pump and design. these gotta suck oil for what...2 feet before it even gets to the pump? lol! brilliant!



the whole 10 per 1000 came from smokey yunick i believe? and it was regarding racing engines at race rpm. an 8000 working rpm engine would dictate 80psi.