The diesel has very tight piston-wall clearance requirements. The piston-wall clearance is something like 1 or 2 thousandths, so usually to do a complete and correct rebuild, purchasing a new set of pistons are required. Typical gasser piston-wall clearances are much less tight, so you might be able to get away doing technically complete rebuilds with just a re-hone and re-use the existing pistons. That is rarely the case for a diesel. Sure lots of folks get away with it, but it is a cut corner that will manifest itself in reduced lifespan of the motor. So, in diesels to do a correct complete rebuild you usually always need to replace the pistons.
The diesel pistons are also expensive, as the diesel pistons are a pretty specialized design and also probably cost more to manufacture. Specifically, they have a steel strut inside them that is there to control thermal expansion (making them called "autothermic".) The top ring carrier also is also cast with a different metal called resistal (to promote longer life than just leaving aluminum touching the top ring). I don't know if the gas pistons have that feature, maybe the newer ones for turbocharged application do, but my guess is the naturally aspirated gasser pistons are pretty much just a plain hunk of homogenous aluminum - and so, much cheaper to manufacture.
Indeed, a big chunk of the cost to rebuild these motors is just in buying the pistons. Figure about $100 / piston for a 1.6lTD piston, four of them come out to $400. Naturally aspirated diesel pistons are much cheaper, and 1.9lIDI pistons are much more. TDI pistons are the worst. If you think is costs a lot to rebuild these older diesels, for kicks go price out a new set of TDI pistons sometime! (And TDIs still have the same tight, piston-wall clearance specs I believe as the older diesels do, too.)
As for head work, I don't see how a diesel head would be more costly than a gasser head to rebuild, really. Although there are a couple things that come to mind: an IDI diesel head can't be resurfaced without special equipment / techniques, because of the hardened prechamber inserts. This isn't a problem if you just find the right guy with the right technique and tools (and there are at least a couple of ways it can be done...) Although some of us here pay extra to have the cracks between the valves welded up, it is really optional and will not cause a problem unless the cracks get wide enough to the point there is risk they could break through to the water jacket.
When assembling the head, you can't just pick a standard head gasket. You've got to measure piston projections and pick the correct thickness of head gasket. That is an extra step the engine builder will need to take.
Add to the technical difficulties discussed above, all the stuff you mentioned about supply and demand, which I think is more or less true. I think that old gasser motors are not in demand as there are a lot of them out there in the market, while old diesels are more rare to start with, and are in even more demand than they've ever been now, as there are lots of folks wanting to find vehicles suitable for bio-diesel or waste veggie fuel conversions. Or there are folks who just want to take advantage of the fuel economy and reliability of the diesel, at a time when gas prices are high, as is the public visibility of our nation's dependency on overseas oil, and since the internet bubble burst, people are increasingly cost-conscious. This makes finding say, a rebuildable TD core, on short notice when you find that you need one, next to impossible. Luckily, the aftermarket has kicked in at least readily available sources for buying outright brand new stuff (including replacement new cylinder heads, and complete motors.)
Topic: Rebuild costs on diesels (Read 12229 times)
