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I've got a used engine on its way to me. It's apparently around 140k and the seller says it runs well with recently fitted head gaskets (no paperwork). A seller will always talk up his wares though and the car was last MOTd in 2018, expired in 2019. It's likely not been run since then. MOT history shows braking system issues in its last few years and no mentions of engine lights, emissions issues or oil leaks.

While it's out of the car and I have good access to the whole thing, what should I be checking and replacing before installation?

Obviously the rear main seal, even if it's not leaking. My own engine had a brand new timing chain and oil pump fitted but has covered literally 0 miles at all since then - can I safely transfer that over or is it unwise to re-use them?

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+1 on rear main seal, as you have them, the oil pump and timing chain (if changing them involves dropping the sump, a new sump gasket too), core plugs might not be a bad idea, especially the ones behind the flywheel, any other gaskets (rocker covers and valley gasket?) that are easier to get at with it on an engine stand and, while you have it sitting in front of you, the water pump.

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Yeah, oil pump means taking the sump off so will need a sump gasket. Front cover gasket too. My engine has an almost brand new water pump on it, think it only covered a couple of thousand miles since fitting. I'll take that off and check it.

I'll add the other bits you mentioned too.

Is there anyway to check for compression and/or oil pressure while it's on the engine stand? I'm guessing not since it won't be able to run.

My own engine has ARP head studs that'd I'd love to transfer over. I'm reluctant to take the heads off though. Without paperwork I've no idea if the heads have already been skimmed or by how much. I don't really want to take them off and refit them without having them checked/skimmed. I also don't want to have them skimmed without knowing what any previous skimming has removed. I'd hate to have them skimmed and refitted with a standard thickness head gasket only to find too much has been taken off and now valves are hitting pistons.

Maybe I'll just keep the ARP studs in the garage for the future. Taking the heads off to see if it's been top hatted would be nice though!

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Compression testing is simple on an engine stand - just fit a starter motor and spin it over. If the compression is OK across all cylinders I would leave the top end alone.

Oil pressure will not be such a concern if your newer oil pump is to be fitted. Indeed, once the sump is off (and assuming it’s not full of sludge) I would remove the main and big end bearing caps to check the crankshaft and if OK re-shell the lot. Whilst there do not forget to replace the cruciform seals on the rear main bearing cap. I’d also replace the front crankshaft oil seal as well as the rear. These are all relatively inexpensive items as insurance for the future.

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Garvin wrote:

Oil pressure will not be such a concern if your newer oil pump is to be fitted.

Oil pressure is why I need a new engine! My current engine has low oil pressure that the new pump didn't fix. That's why the new oil pump has covered precisely 0 miles since fitting. The car was off the road, I replaced the pump, it didn't fix the issue so the car stayed off the road.

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I regard dropping the sump and lifting the rocker boxes as no-brainer inspection jobs on a new-to-you used engine. State of the sludge, crud or oil stains are an excellent clue as to how well the beast has been looked after and whether its been on light duties / short run work (bad for a motor) or had a decent chance to stretch its legs. Arguably inspecting the camshaft makes sense given that they are known to wear.

But my view is that any used engine automatically gets a proper rebuild before swapping in. To me its worth getting another motor to sort in reasonably slow time whilst the one fitted still works rather to be forced into a fast fix. If I'm going to the effort of swopping motors I want my "150,000 miles with only routine servicing" in exchange.

Clive

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Clive603 wrote:

But my view is that any used engine automatically gets a proper rebuild before swapping in. To me its worth getting another motor to sort in reasonably slow time whilst the one fitted still works rather to be forced into a fast fix.

Current motor doesn't work. I'm doing it a bit backwards to your idea. Cheap motor to go in to get it running and rebuild the current one :)

Clive603 wrote:

I regard dropping the sump and lifting the rocker boxes as no-brainer inspection jobs on a new-to-you used engine. State of the sludge, crud or oil stains are an excellent clue as to how well the beast has been looked after and whether its been on light duties / short run work (bad for a motor) or had a decent chance to stretch its legs.

Added to the list

*Clive603 wrote:

Arguably inspecting the camshaft makes sense given that they are known to wear.

I have a genuine LR cam in the garage waiting to go in the current engine that I plan to rebuild. Need to get hold of some pre-finished bearings though. I bought some genuine LR bearings that I gather need to be line bored before fitting the cam. Need a cam bearing installation tool too I think.

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In your position I wouldn't go so far as replacing big ends and mains or doing too much work other than making sure it is oil and coolant tight. Otherwise you may as well not bother with a replacement engine just rebuild yours. If the sump has to come off you'll get a pretty good idea what the bottom end is like just by what you find in the sump. Maybe try giving the big ends a twist to see if there is any movement that would show excess dry clearance.

As I see it you have two extremes. Either take what your supplier says at face value, drop it in as is and get back on the road as soon as possible (done an engine swap from 4.0 litre to 4.6 on a GEMS in a weekend) or start on a complete rebuild. With the former that then gives you the time to rebuild your original engine at your leisure. Check for anything obvious, the state of the outside of the engine will give you a good clue, is it largely clean and dry or does it have oil stains everywhere?

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If I’ve got it right, Rutland is going to do some minimal work on the second hand engine before replacing his existing low oil pressure engine in order to rebuild the existing low oil pressure engine and then, for a second time, swapping that back in. Why? If a complete engine rebuild is on the cards why not just rebuild the second hand engine and do the swap once? If the existing engine is that low on oil pressure that a complete failure is imminent then I think a rebuild of it might be a tad risky!

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Garvin wrote:

Why?

Mrs wants the car running lol.

We recently moved house and now live in a nicer area and she doesn't like the idea of having a car just rotting on the driveway where we live now. We also want it up running again so we can get back to using it for off roading and camping now our kid is getting old enough.

I get what you're saying. Seems like redundant work but from a weird, slightly sentimental, reason I'd like the car to have its own engine back. Can't really explain why lol.

I also hope/plan to build a kit car in the future so might even end up rebuilding both engines eventually. I've had most of the parts for a few years and have been waiting for time/a garage.

Another reason (and probably the most accurate): bad decision making?

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OK, I didn’t get the ‘project’ bit from your earlier posts, more a what’s the minimum to swap engines to keep the P38 running for some more years. However, what is the evidence that the original engine has low oil pressure and if confirmed and not the oil pump then what else can it be? For really low oil pressure there must be a hell of a leak somewhere (internal or external) or the bottom end must be rattling like a good ‘un. Either way, the original engine must be a bit of a risk to refurbish if the cause isn’t known.

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If the original engine is going to get a full rebuild, top hats, ground crank, new big ends and mains, etc, then no matter what the cause of the low oil pressure problem it is going to be sorted anyway. As long as it is done properly, and everything is carefully checked, then I don't see it being a risk at all.

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Garvin wrote:

OK, I didn’t get the ‘project’ bit from your earlier posts, more a what’s the minimum to swap engines to keep the P38 running for some more years.

Yeah, that's my fault. I didn't indicate at all that that's what I was doing. I might even have decided on the current plan after I made this post lol.

Garvin wrote:

However, what is the evidence that the original engine has low oil pressure and if confirmed and not the oil pump then what else can it be? For really low oil pressure there must be a hell of a leak somewhere (internal or external) or the bottom end must be rattling like a good ‘un. Either way, the original engine must be a bit of a risk to refurbish if the cause isn’t known.

The oil pressure light started flickering on and off at idle. It would go away with revving the engine. Over a period of a few weeks the light came on steadily and stopped flickering. The amount revs needed to make it go off increased too.

I took it to a garage and had them test the pressure with a gauge. They confirmed it was low. I don't remember the exact figure but I think it was near 0 at idle and barely made double digits when revved. Just enough to put the light out. I had a new oil pressure sensor anyway so I had them put it in just in case. The light stayed on.

I parked the car up for a while and when I was furloughed at the beginning of COVID I bought my own oil pressure gauge and I changed the oil pump. The light stayed on and the guage read almost nothing. Then I left the car alone until now.

There's no oil leaks and there's not any serious knocking. There's a slight rattle that I thought was more cat/exhaust as pushing on the exhaust changed it. A few thousand miles before the pressure dropped I did the head gaskets. I guess it's possible I screwed something up or loosened some crud that has floated around the system and eventually blocked something. The oil pick up in the sump is clear though.

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Did you check the pressure relief valve ?

As far as the replacement engine, I would definitely spin it over on engine stand with spark plugs removed & and check compressions as well as oil pressure. It will also be easy to see any oil leaks & fix them before installing.

If you're really creative you could rig up some pipework & do a coolant pressure test as well ?

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Yeah, I checked the pressure relief valve. It was a bit of a pig getting it in and out! It seemed to be ok though.