I probably have more experience with crank walk than anyone. We have rebuilt 7-bolts for a very long time and rebuild many engines a week (yes, we are a trans shop but also rebuild many engines). What I have learned over the years is any engine can walk and for a number of reasons. When you think you know what the problem is, an engine will come along which will walk for a different reason and bite you on the ass again. These are the reasons I have found to cause a 7-bolt to walk:
1) When using the 2-piece thrust, the alignment between the two halves have to be perfect or the edge will act like a squeegee and wipe off the oil from the crankthrust surface. You would be amazed at how cleanly and perfectly an edge can wipe down a crank. Anyone that follows the correct engine assembly procedures will center these and not have this problem, though.
2) If the main bearing clearance is tighter than the rod clearance, the engine will walk.
3) The narrow style of the 2G rod bearings tend to make a 7-bolt walk more than if 6-bolt bearings (the wider early bearings) are used.
4) Excessive aftermarket rod clearance will make an engine walk.
What we have discovered is when a 7-bolt engine walks, the thrust and mains are wiped out, but in most cases, the rod bearings look to be perfect. It looks like the mains run dry and the rods continue to have oil fed to them. The question due to this observation is how is that possible? How can an engine, which feeds the rods via forcing oil through the mains to begin with, lose oil in the mains yet continue to feed the rods? You would think the rod bearings would be the first to go, but that's not the case.
The answer appears to be that centrifugal forces are the cause. The rod orbits the crank, and due to this orbit it can pull oil away from the mains. So much in fact that it can create a vacuum at the mains and starve the thrust. What appears to cause this are 2 things. 1) The oil passages in the crank are too large and 2) the rods are leaking too much oil away too quickly for the system to be able to keep up.
The oil pump is linear with engine RPM, so as the engine RPM increases, so does the oil pump flow, but it is still not enough to counter the oil loss at the rods. The higher the RPM, the higher the oil pressure, but you also get more centrifugal forces, so an engine can walk at idle or at any RPM, which we find to be true on walking engines. Hold an idle on a walking engine and push the clutch, you will see the RPMs dip. Hold the engine at a steady 3k RPM and push the clutch, the RPMs will also dip there too.
I thought I had a solution several years ago, but did not discover the centrifugal force issue until a few years ago. For the past few years I have been using this knowledge for my 7-bolt engine rebuilds. I would say during that time we have rebuilt around 80-100 7-bolts and 1 has walked giving us a 1% failure rate. Yes, we rebuild a huge number of engines here on top of transmissions, not too many people know that. That's not too bad and worth the loss from a business stand point. What I do to counter the walking issue today is this:
1) I use only 6-bolt rods as they are wider than the 7-bolt crank. I will then narrow each rod at the big end individually to match the crank journal in which it will be assigned to. This way I can assure a snug and perfect rod side clearance with the crank to prevent excessive oil loss due to centrifugal forces. Both the factory and aftermarket 7-bolt rods are terribly stupid loose, so that's not good. Modifying a 6-bolt rod to fit as it's supposed to assures everything is as it should be.
2) I will resize the big end of the rod as most (not all) aftermarket rods are too loose with bearing clearance. I believe cheap rod manufacturers do this to cover their ass, but excessive clearance causes other issues to the consumer which is not good.
3) I will use only the wider early style 6-bolt rod bearings as they tend to not only live longer and have more surface area, but they hold more oil in to prevent loss.
4) The crank oil passages are too large. So large that they can continue to feed oil to the rods as the mains don't get enough. I modify the passages so as to increase oil pressure at the mains yet still give the rods adequate oil to live a long and trouble free service life.
I have been thinking of adding my modified crank, rods and piston sets to my website for those willing to build their own 7-bolt, but I am a bit afraid to. The DSM community is very hard on those which try to come up with a repair that elitists believe there is no possible solution for, and anytime I think I have a solid solution to this walking issue I get a sharp bite on the ass. Not only do I get a bite on the ass, but the bite clamps down hard on my butt cheek and follows me around for years afterward. It's not worth the hit to my companies good reputation which we have worked so very hard for. I have been quiet about my latest modifications which appear to be working well for this walking issue, but I know as soon as I offer something on my website, I will get hit again. The 1 engine which failed had 7-bolt rods in it. I would be willing to bet that if that engine had my 6-bolt rods fitted to the crank, that it would have lived. That was one of those odd deals where the customer already had 7-bolt style forged rods. Either way, it did walk, so I count it against my totals for the past few years. I have to admit it's an impressive number, though.
On another note, people seem to think that heavy pressure plates cause crank walk too. I found that to be untrue. If your engine is going to walk, it will walk with a factory clutch. I have seen 7-bolts walk in factory OEM trim and even with automatics. Thrust pressure has nothing to do with it, it's a lubrication issue. If you have no oil reaching a bearing, it's toast.
I personally believe that the 7-bolt engine is a superior engine over the 6 and have spent insane money and time trying to figure the walking issue out. I believe my solutions which appear to work today not only help prevent the engine from walking, but also offer the customer a much higher quality engine with very accurate and meticulous machine work which many others don't feel the need to perform to their builds. I do these same things with the EVO engine too and they appear to also perform very well. I suppose there is nothing wrong with a little more attention to detail, but maybe I'm just wasting my time. Either way, I feel more comfortable fitting everything to my specs and in the way it was intended to be to begin with...
I swore in the past that I would not share this information, because to be honest the DSM community can be very critical at times and due to this i didn't feel like sharing my hard work and findings to the public. I have since changed my mind. At this point, rebuilding engines is no longer a very profitable thing for this company. With the large number of high end complex transmissions coming in from $100k+ exotic vehicles, the profit margin and quantity of units we receive make the engine building something which is no longer worth it for us. If you see engine build options on the site, we are still performing the rebuilds at this time, but who knows for the future. Engine building via Jacks Trans may soon be a thing of the past. I have decided to share this info now as any of the DSM elitists out there which would claim this info as their own and use it to their own advantage will no longer affect us, so I decided to let the info go. I'm sure it will help more people that would appreciate it over the ones that don't.
Good luck with your 7-bolt build and we hope you stick the the 7. They are a far better engine in many ways and to install a 6-bolt in your 2G has always been something we thought was a move in the wrong direction.