I sometimes tend not to be the best decision maker, so I thought I'd ask for advice.
I'd like input on whether I should swap my current engine for a used one, rebuild it then swap back... or rebuild a replacement first... or?
Current Engine Situation --
The Canyonero! currently has a tired, but still starts-and-runs-fairly-strong 360 in 'er. PO claims it was rebuilt about 80k mi ago, and I found a rebuilder's tag on it, so I believe him.
However, it's making a growing number of uncomfortable sounds -- lotsa ticks, occasional knocks, etc -- uses a lot of oil -- leaks + burning -- and seems to be losing coolant. Oil pressure wasn't great, but adding the Lucas magic goo helped a lot.
Vaccuum is around 17mmHg and the needle wavers quite a bit when < 5 at heavy throttle. While I don't know quite what that means, it might be helpful? Seems to want a while to warm up, and otherwise just doesn't instill a lot of confidence. I've put about 30,000 hard miles on her, so I'm not upset that she needs work.
The New Engine --
I'm about to pick up a used engine from a member on this board -- it reportedly runs well, and just passed smog. Other than that, I don't know anything about it.
SO... the question is: Whadda I do?
1 - Swap in the used engine, rebuild mine, swap it back in later. My original plan, as I feel a bit of urgency to get my current engine out... but maybe I shouldn't? Hmmm... Pro: Cheapest plan, at least right away. Con: Most time consuming.
2 - Rebuild the used engine, swap it in when done, and just hope my current one hangs in there -- anyone wanna venture a prognosis on how long it has to live? Pro -- Least time, only one engine swap. Con: More money right away...
3 - Do a "mild" rebuild on the used, swap it in, then do a Big, Bad rebuild on the old one... I'm thinking, like, timing chain, water pump, valve spring seals... what else is quick/cheap/important while it's out on a stand? Pro/Con: Happy medium??
4) Something Even Wiser??
I'm gonna do the rebuild myself, using a machine shop minimally... since I'm unemployed, darnit, time is not important, but $$ is...
Thoughts? Wisdom of the crowd?
if the about to purchased engine is still in a jeep and is able to be run, it may behoove you to do some diagnosis. an oil pressure check, a compression check and even a vacuum check. would give you a good idea of its condition. BTW, the same checks on the present engine would be helpful too.
If I had a line on a decent motor and my current one still got me around, I think I'd try to nurse it until I could be sure about the replacement. Like Andy said, it's of course best to do as much diagnosis as possible before the new motor leaves it's home, but I suspect that it's already out if it's for sale.
My situation wouldn't lend itself to bench running the motor on a stand, so I'd invest in a gasket set and pull the pan & heads for a look-see. Actually, I'm so poor that I'd just stick the thing in there and hope for the best, but presumably most folk have a few more bucks than I.
Check rod side clearances and crank endplay against specs, and look for uneven wear in the cylinder bores. Pull a main and rod bearing cap(someone please jump in here with which ones on an AMC wear first -- I'd guess #4 main & #8 rod) and Plastiguage them for a rough idea of the general health of the bottom end.
Uneven cylinder bore wear (crosshatch gone in places) is a good indication that you'll need at least new pistons, if not an overbore. A piston top with carbon on the center, but clean at the perimeter is an indication of poor ring seal.
A quick-easy on the valves is to fill the ports with gasoline and watch for leakage into the combustion chamber. Before doing that, look for dark, damp deposits on the back of the valves. This can indicate poor oil control from cheap stem seals or more expensive valve guides.
Bottom line, if the cylinder walls seem good, then it's easy to renew the bottom-end bearings if needed, and not terribly expensive to have a valve job done. If it seems that the motor needs pistons, then your budget and timetable will determine whether to slap it back together with the new gaskets and hope for better from the original motor, or to go on with a full build.
As I thought I'd finished typing this hyperbole, I recalled that you said the replacement motor purportedly just passed smog. I'd think that a fair indication that the cylinders are building decent pressure. Here's my new analysis: Stick the other motor in and forget about it.
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