I am currently running a M55 pump it came with my motor. I took off the oil pan because the pickup fell off. I took apart the pump to check it for damage and found the impellers were chipped and damaged.

I was going to get a new one from autozone and replace the impellers or use the new pump depending on what the casting look like.

I had 45-55 PSI while cruising before and about 12 at idle even while missing the pick up.

Any reason I should not go ahead with my plan?

 

Don't worry about replacing the impellers, just replace the whole thing.

 

Sounds good I wish I had noticed the damage before I welded the tube to the old pump bottom.

At least new ones are cheap.

 

There have been problems with the "cheaper" melling oil pump castings breaking in motors with decent power.
I always use the old (now new again) "heavyduty" casting pump ..

 

So my old casing is much thicker than the new pump. I was thinking I would just swap the spring and impellers out. Any issues with this?

 

DO NOT DO THAT.

Buy a NEW standard volume, standard PSI melling pump but order the heavy duty casting. Yes it will cost more but it won't break in half.
If you have a 100% stock motor, you can order the basic casting.

 

Sounds good thats why i asked because i did not know.