Oil relief piston

Mark Ennis · 01-12-2021, 01:21 PM

I am rebuilding a 2.7 MFI engine. I have carried out the oil pump bypass modification. I have replaced one of the relief valves with the piston guide and new solid piston with chamfer. Do Inalso need to replace the second piston with a hole with the new piston design?
I have also noticed some corrosion on the magnesium valve walls. Is this a serious concern? If I needed what would a fix be

Walt Fricke · 01-12-2021, 03:04 PM

Mark - you should replace both pistons with the new type. The kits typically come with both, as well as one longer spring and the guide (for the vertical piston) and a shorter one just like the two you have.

Magnesium valve walls? What do you mean? Valves aren't magnesium, and don't have walls.

stownsen914 · 04:51 PM

I assume the corrosion in on the galley walls in the case where the pistons go.

Mark - what does the corrosion look like? White dust on the surface, or pitting? The dust might be ok if it cleans up well. You do want the galleys to be in good condition for the pistons to properly regulate pressure. Both so the pistons don't jam, and also so not much oil squeaks by unregulated.

Walt Fricke · 01-12-2021, 05:09 PM

What makes me think that the cylindrical hole the pressure setting piston rides in is a steel tube cast into the case? Am I incorrectly thinking of the cast in tube in the SC which makes some sporadic appearance outside the casting on the rear of the right side case half, just forward of the oil cooler?

Corrosion seems unlikely in either piston's hole, though - the area below the piston is bathed in the oil which gets past the piston - which is why there are small relief holes by the external plugs for these holes, so that oil can get to the sump and not create a pressure resisting movement of the pistons. Maybe a motor recovered from a flood?

But if it is there, it would be of concern. It wouldn't matter toward the plug side, as only the spring is there and it wouldn't be affected, but would if it were in the part where the pistons move up and down. And if there, a fix wouldn't be simple. With the engine fully disassembled a machinist might be able to bore and insert a steel tube?

stownsen914 · 01-12-2021, 05:30 PM

Hmm, now I can't remember. It would make more sense for it to be a steel tube.

Re: the leakage, correct that there is a path for it to get out so no hydraulic lock, but too much leakage due to excess clearances, etc. would lower oil pressure which isn't ideal.

Mark Ennis · 01-13-2021, 08:06 AM

Thanks for the feedback. On the 1974 2.7 MFI there is no steel insert. It is bored out of the magnesium crankcase. The engine was in a car that was left in a barn in the UK for 12 years, hence some pitting. The pistons move fine my main concern was the surface it mated against.

cgarr · cgarr's Garage

There is no steel insert in the mag case unless someone sleeved it? I have repaired these before but using an aluminum sleeve before.

Walt Fricke · 01-16-2021, 03:48 PM

So - is the area where the piston moves up and down the area which looks funny? If what you are worrying about is lower down, below the lowest piston travel, it isn't going to affect oil pressure. If you run a bristle brush through, does it clean up (just some surface oxidation or something)?

Craig - what have been the indications that called for resleeving? Wear you can measure with a bore gauge? Or scoring from an engine blow up?

cgarr · cgarr's Garage

I had pitting in the area of the piston