The requirement for a rebuild varies a lot so it is a little difficult to answer the question definitively.
I've realised we have a wide spectrum of owners that range from extremely hands on and excessively maintained, to those that own the cars and don't really know much mechanically and tend to treat their car like an old Toyota; never service and just keep driving. So the cars tend to be in varying states of condition. But the common issues that would require some form of rebuild would be
- broken headstuds
- burnt valve seats
- worn valve seals
- excessive oil consumption
- excessive oil burning
- oil leaks
- low oil pressure
For me, rebuilding an engine is a total rebuild; split cases, check all tolerances, new main and rod bearings, rod sizing checked, wrist pin bushings replaced etc.
Usually a top end only, is just having the heads refurbished, check cylinders and pistons for being within spec, and replacing rings. Might replace headstuds too. (if you don't have proof of headstuds being replaced at some time, then always replace them) But a 'top end' only is not always a wise choice. and here are a couple of examples below.
Recently, I completed just a 'top end' rebuild for a 3.2 at the request of the owner, due to 3 broken headstuds. Now, my preference was to split the case and inspect but he assured me that oil pressure was adequate, and that he actually had receipts for a FULL rebuild by a well known shop only approx 20kms ago (but it was about 15 years or so). Why did the headstuds break? Well, when they rebuilt it back then, they did not replace the dilavar exhaust studs.. maybe it wasn't the done thing back then.. I removed all studs and replaced with steel.
Upon dissassembly, we found 3 cylinders to have excessive scoring and the nikasil plating had been compromised, so all cylinders were replated and honed. Yes, the cost went up, but now, the owner has a car with refurbished heads, good compression, and frankly a bit of a rocket. I'm really happy with how that car performs.
Another engine was targeted for just a 'top end' rebuild. This engine was very leaky... top and bottom.. so the owner just wanted the top end disassembled, checked and then reassembled Upon disassembly, I found 1 broken headstud that the owner was not aware of, and I saw enough evidence to persuade the owner to let me split the case. He had no history for any rebuild work done before to that engine. When I split the case, 1 of the crankshaft bearing shells fell out and was worn down to the copper.. it was trashed. Likely in a previous owners care, back when these cars were not cared for. We were lucky in that the crankshaft needed just a polish and was still straight with no cracks etc. The case however, needed a LOT of fettling to bring the bores back to within spec. This allowed the used of stock size bearings (not the $2500 oversized ones from porsche) and the engine was rebuilt back to stock. We also found the rockers and cams to have mild pitting and 1 of the cylinders to have some delamination of the nikasil.
I always keep owners well informed of where I am in the process, and if something crops up that wasn't expected, then I am straight on the phone and send photos and /or video. When I rebuild fully, I have all the parts ultrasonically cleaned. Depending on budget, my preference is to have cylinders, heads and cam towers vapour blasted and for rocker covers and timing chain covers to be powder coated. Engine tins are sandblasted and powder coated too.
So basically, you end up with a brand new looking and performing engine. Process takes a few weeks to complete and I only like to work on one engine at a time.
Hope this information helps.