I'm about 75% done rebuilding my 4.108 -- here are a few thoughts specific to that engine.
You need to ask the owner about the engine. If he did the rebuild himself, he should be intimately familiar with the engine; if he had someone else do it, then go talk to that person.
o Ask for documentation of the rebuild. There should be a lot more than just a receipt for an aftermarket rebuild kit.
o At a minimum, here's what should have been done using parts from a rebuild kit. Some of this can be done in a DIYer's garage, but some of it definitely needs to be done in a machine shop.
- Cylnder liners replaced and honed.
- Crankshaft polished.
- New pistons.
- New valves and valve guides; valve seats ground (not just lapped).
- New main, big and small end bearings.
- All new gaskets.
++ Red flags:
- Reground crankshaft. The Perkins cranks are hardened using a nitriding process that only penetrates a very shallow way into the crank. If the crank was reground, much (or all ) of that hardening was likely removed.
o The aftermarket rebuild kits lack a couple of things that also need to be checked:
- Oil pump. This is the only mechanical part of your engine that runs using unfiltered oil as a lubricant -- they tend to wear faster that other parts of the engine. Verify that the pump was either replaced or checked to ensure it was still within factory specs.
- Rocker arms and shaft. I found significant wear on the shaft. I sent the entire rocker arm assembly out for rebuild (new shaft, new bushings, arm geometry reground.)
- Fresh water cooling pump. Mine was leaking. Rebuild kits are no longer available. The only fix is a new pump.
- Raw water cooling pump. Also should have been torn down and rebuilt (new bearings and seals.)
- Fuel delivery components. The lift pump should have been disassembled and checked. At a minimum, the injectors should have been pop tested (better yet rebuilt using new nozzles.) The injector pump should have been thoroughly checked and seals probably replaced.
If you are interested, here's the thread that I'm using to document my rebuild (link