You are right. I don't think a deglazing hone is ok on nikasil. However, I have not really researched nikasil any further than its similarities to alusil. HDR, If you know something I don't know, please share.

Do a search over on the 911 rebuilding forum for some info on Nickasil refinishing. Yes, it is a layer that's electroplated (I believe...) to the cylinders. It is supposed to have a crosshatch pattern in it like regular cylinders and people said it can be restored with a bottle brush deglazing hone. Alusil is a totally different thing and is what the entire block is cast out of. As found above, it can be cut with regular abrasives, but must be finished to a fine surface and lapped.


With that said, I'm happy to report back with some compression numbers as the official conclusion to this project. The motor has 11,132.6 miles on it now and still runs very smoothly and strongly. I recorded both the pressure after the first cycle, the average peak pressure, and the number of cycles it took to reach the avg sustained peak pressure. I did not record the absolute max spiked pressure, but an averaged max once the pulses evened out.

#1 - 100 initial/170 avg peak/ 5 cycles to peak
#2 - 90 initial/160-165 avg peak/ 6 cycles to peak
#3 - 90 initial/165 avg peak/ 7 cycles to peak
#4 - 115 initial/165 avg peak/ 5-6 cycles to peak

My guage fittings weren't perfectly airtight, so that may have been contributing to slow the pressure build rate, but I'm pretty happy with the results.

The plugs haven't been touched since the engine was first assembled and they look pretty good. They are in order from right to left.

http://forums.pelicanparts.com/uploa...1281487550.jpg

I hope I can get another 150K out of this one. :D

Numbers look excellent.

Do you have a coolant leak on plug 1---far right? The seal looks rusted.

John

No, that's just leftover mess from the bleeder screw. Sticky mess in the #1 plug area and down in the block where the upper balance shaft housing meets the engine. I wish they designed the system like the ice resurfacers we've got at work that are built on 2003 GM 1500 truck chassis. The water pump recirculates enough flow through the expansion tank that it eventually "strains" the air out of the system.

I forgot to add that the small sample of cylinder wall I could see through the plug holes looked pristine perfect except for a small scratch on #2, but the cylinders had a few scratches already, which the honing didn't remove. The big thing is that they broke in very well rather than suffering some metalurgically catastrophic scouring/surface breakdown due to improper honing and lapping leaving a torn or smeared surface. Aluminum is just a pain to work with no matter what it is.

Good job, and very nice writeup. I'm glad that the lapping worked out for you.. Might try that on my badly scratched engine block and try to stuff a set of turbo pistons in there.

There's alot of good information in here, but I though I could add a little more.. Although, not clearly written.. the information might be helpfull in some way.

I think much of the issues regarding "cutting" or actually honing to a bigger bore, in the alu-sil matrix cylinder is that many home and even many shop based mechanics don't have the right equipment to get the required results for the surface of the bores. Once they cut through the factory surface with the "other tools" they sort of ruin the cylinder bore surface in a couple ways.. More on that below..

The shops with the propper tools for the alu-sil bore engines use a sunon multi-bladed boring bar? or boring head? with very sturdily mounted, very sharp cutting blades on the boring tool.

The sharp boring blades, while cutting the bore/aluminum to final dimentions, are at the same time "hopefully" cleanly breaking the silicon crystals on the new surface, so they are nearly flat at 90 degrees to the piston/ring direction of travel.

Getting the right number of crystals per square cm, and propper surface profile of exposed silicon crystals is the GOAL of the machinist in the boring process for these engines, all while trying to get to the final bore diameter to match as close to the piston group's clearance specs as possible..

Then comes the final lapping, using the lapping compound, and either cork backed, or special polymer based lapping tool pads, to only remove aluminum from around the silicon crystals, and possibly, slightly rounding off the sharp edges of the broken crystals.

The lowered aluminum surface between the crystals also has to be an appropriate depth, to provided a necesary oil holding surface.

In a standard ductile iron bore'd engine, the cross hatching depth and angle would be the oil holding surface, the ring seating takes place at the tops of the scratches, similar to ring seating on the alu-sil bore, but the soft iron cylinder is more forgiving to the rings...

Other boring processes, and tool/machine RPM/stroke degree rates, with smaller, less stout cutting bits "like a fly cutting head" or other offset boring tool are said to be "too weak" to cleanly break the crystals on an alu-sil bore.. This includes the stone based honing tools. Most shops have a heavier duty version of the stone tool, similar to the one you used for a lapping tool, as their primary boring tool.

Those work fine for normal iron blocks and even on freshly nickasil/hardchrome plated cylinders, to bore/hone to spec.. They just run them for a while and then measure the bore diameter/squareness.

The information that I could find regarding using those non multi-blade type boring tools on the alu-sil block/cylinder is that they can't break the crystals, and can gouge the crystals out of the aluminum, leaving pits and high spots of aluminum, and unwanted crystal profile.
Even with lapping to within piston specs after using one of the other non bladed honing/boring tools, would leave a weak metal to metal and/or agressively abrasive cylinder bore surface, due to high spots of aluminum and/or pointed silicon crystals at the surface.

When you do a re-ring, the factory finished bore already has a good number of exposed "propperly broken" crystals at the surface, that most re-conditioning/re lapping processes will not damage.

The already good exposed crystals may also remain in-tact, while using a scotch brite "abrasive type" pad.. Those pads may only remove "wiped" metal from the surface of the crystal, effectively cleaning it, in preperation for lapping, and eventually, installing new rings... Think of it like how a fouled file, or grey looking "white" ceramic knife sharpening stick, doesn't work as good as a clean one..

The cross-hatching that you see after using the abrasive type pad will most likely only be in the aluminum "host" metal, but you would have to use a very good microscope to see if that is in fact true..

Some other thoughts about the factory 944 piston/cylinder:
Since even the factory boring/lapping process still isn't perfect, and there is usually some aluminum still present at the bore surface, even after lapping (unconfirmed) likely, small ramps of aluminum leading up the sides of the silicon crystals. Since aluminum doesn't rub well on aluminum, the 944 pistons have iron somehow deposited on their exterior surface.
The iron is probly there to prevent the aluminum pistons and bores from burnishing/wiping/galling. I think the 944 rings are chrome faced, so that their hardness/resistance to abrasion is close to the hardness of the silicon crystals, in the cylinder bores..

When/if you overbored your porsche alu-sil cylinders for larger pistons, even with special coatings on your pistons, you overlooked the key feature of the propperly bored/lapped alu-cil porsche engine bore=piston ring's and piston body "skirt and crown" might actually contact the bore surface, through the oil. The main features there are that the piston rings face and the bore surface are a like hardness, and the piston body/crown and skirt, are of disimilar metals, to prevent "swapping of molecules" The piston special coating might work out just fine, but not having the factory finish "crystal profile" the ring face may suffer from premature wear, which could in the long run cause excessive oil burning, loss of compression..

Sticking with the factory bore diameter and just re-ringing, If you know your materials, you can get an abrasive pad that's particle "hardness" is below that of the hardness for the silicon crystals in the alu-sil bore. If the pads abrasive hardness is higher than the hardness of the metal that might be deposited on or between the crystals, it will speed up the cleaning process.. Knowing the hardness of the materials would help you to not worry too much about damaging the factory crystal profile with your abrasives... Those pads might not remove the metal that gets built up on the surface of the crystals if it's not "hard enough" but they will for sure bite at the aluminum between the crystals.

I would probly only attempt a final lapping/reconditioning with that recommended lapping compound, regarless of how long it takes to "clean" the bore surface, but that is just in the case of replacing rings. Since it is used just to "clean" the surface, and space between the crystals, to achieve propper ring seating. "scotch brite"~ing, in my opinion, would be less than desireable, but again, if you don't damage the crystals that are in the bore, those types of abrasive pads probly just speed up the cleaning "deburnishing" process.. So it's good info there if you know that you aren't causing damage.

I have spent some time considering what I could do with my extra block, in regards to boring/plating/sleeving etc.. The two most viable options included: overbore via someone that has the sunon boring/lapping tools for alu-sil bores, then put in larger bore factory 944/951 pistons.. In my case, since the bores are scratched pretty bad, I would have to go atleast 2mm overbore, to get at the clean/undamaged bore material, to expose/cut/break fresh crystals, below the depth of scratches.. If I understand the properties of the bore's (matrix) there's more going on in the scratch, "below the scratch" metalugical wise than can be fixed by just removing the ridges that can catch your fingernail. Thinking along the lines that if the scratches were just knocked down, that could lead to a ring building up a ridge in the softened scratch area, while the rest of the ring seated in, that could, in theory "push" the ring up, lessening the pressure on the bore surrounding it.. I dont' think it would happen very fast, maybe you'd notice some extra oil burning after 10k or so miles... but it's just a thought.... If the replacement parts were cheap enough "used pistons, new rings" the thought of a ring ridge wouldn't stop me from doing the work to get a runing engine in the car..

Going to a larger factory piston/ring=fairly expensive, but doable.

Or, overbore, then nickasil plating via the guys that did my honda cylinders, then using stock 944 N/A pistons, maybe even Euro type, higher compression pistons, with special skirt/base/top coatings, then rings; all compatable with nickasil.

This would be also doable, but not quite like "factory" and the added prices of compatible pistons and rings, would be more than having a good factory rebuild...

The other prudent thing to do, would be to forget about the costs (can't really do that) and go with darton sleeves, deck plate, larger studs, H-beam rods, and compatible pistons... and everything else to support the engine.. but I would be dumping my other three projects to do that.. for no real gain, other than a monster engine, and monster life support "air/oil/water/fuel" that I might not even be able to drive, "not street legal" The cool part about that is I could get a corvette rear end stuffed under the car, and put down some impressive rear wheel torque numbers, it would put a huge cramp in the style of the muscle car guys that are head set on "there's no replacement for displacement" and might just get some of them to accept that there really is no substitute..

Ah, yes, that's what the problem was with conventional abrasives. They rip the silicone particles out of the wall.

I tried just cleaning the bores of the blown engine (for practice/technique R&D) really good before lapping and it didn't seem to do much. Your theory on a mild abrasive that's abrasive to aluminum and not to the silicone particles is probably the key to the at least moderate potential for success for the scotchbrite pre-cleaning. I think the important thing is to not overdo the scotchbrite. I'm 90% sure it is definitely not abrasive enough to affect the silicon (pretty hard stuff). Just enough to clean the existing surface. It may also expose new crystals, which wouldn't be properly sheared. I think the Deves rings helped that case since they are not chrome plated...not quite hard enough to chip or destroy the rough crystals, which also helped for a very quick break in. They were very oil tight from day 1. The motor never smoked like people on the 911 rebuild forum described break-ins. Longetivity-wise, who knows, but a good oil may help them last.

The problem with just re-ringing used cylinders seems to be that crystals smoothed and covered with ring material (?) are not abrasive enough to wear in new rings, meaning well used bores will not break in rings and you will be worse off than just not messing with it (saw at least one of these stories to prove it on the 911 rebuild forum...10K miles and the thing still smoked badly). Even removing the pistons could also be enough to lose the precision fit the already broken in rings obtained, meaning they should probably be replaced in that case (every used cylinder will not be absolutely perfectly round, meaning the corresponding ovality worn into the rings will be impossible to re-index to the bores). I had one of those 1/10 scale 2 cycle nitro powered cars 10 years ago as a perfect example of this risk. Those things did not use rings...the cylinder was chrome plated brass with a aluminum piston, and the parts broke in to each other down to the microscopic crystal structure of the metals. Disassemble the motor...instant compression degradation that never re-broke in. Granted rings may have more leway in this case, and a deglazed iron cylinder could probably do ok re-seating used rings that are still in spec, but the Alusil loses much of its breaking in ability in the first 100 miles as I've heard (why they say to drive it like you stole it right away...the max combustion chamber pressures get behind the rings and seat them quicker while the fresh cylinder is still rough). My first break in oil change looked a bit pearlescent. After that, the oil always looked normal. It's complicated stuff and I think I got pretty lucky, but time will tell how much.

Thanks for that post. It summed up alot of what's been collected in my brain over time, only I never really intelligently assembled it all into one spot. I think this technique can work well if you start with already decent parts and are very careful (and a bit lucky). For me, it ended up being a "make it work or part out the car" scenario, as I definitely decided I couldn't afford to take it all and have everything professionally done. I don't even know if there is a shop nearby that has the correct equipment to deal with these cylinders. Would I try it again? I could give a better answer in about 60 or 70K more miles, but I would say yes. It would be interesting to analyze the surface after the process to see what flaws it has compared to doing it correctly (re-boreing oversize and starting from scratch, pretty much...), but the procedure itself seemed to be very dependent on "feel", and somehow I managed to assemble a correct enough gut feeling to make it work. I have a feeling the difference between how close this lapping is to The Correct Way is in the longetivity. It's already proven it is correct enough to break in rings well and retain compression for 10K. Oil consumption is about 1qt/2500 miles. My original motor had 90K when I got the car and it didn't do much better than 1qt/1500 miles, slowly degrading to 1qt/300 miles before I killed it. Compression was ok (~150) but not as good as the numbers pulled from the rebuild.

Thank you so much for this informative read! ! I was sick the other day when i pulled out my pistons the other day and saw scoring in cylinder 1.... but thanks to your post I'm sure I can salvage it now...the damage is only as bad as yours was.

No problem. She's done about 50k on the rebuild and still as good as gold. Still pulls strong and the oil consumption is roughly 1 qt/2000 mi. The valvetrain is a bit tired, as most of it is original to the car. I had to replace a couple of cracked valve spring retainers recently from a few overrev incidents over the years, but it still runs great. You can't kill these engines! (well, actually you can, just not typically...) ;)

I've been reading a lot on this topic lately as a rebuild of my own engine is inevitable. A few years passed since your rebuild, do you still have the car? How does it perform?

Cheers, nbK

I got about 70k miles on the engine and it still ran great. It used maybe 1 qt/2000 miles but also had leaking AOS seals and leaking main oil gallery seals on the oil cooler housing. I called it quits on the rest of the car shortly after fixing those. The car had spent the last couple years of its life parked on grass and it just rusted bolts and hardlines to the point it just wasn't worth it to me trying to get everything back to reasonable spec, so scrapped it out last year except for the engine/transmission and a few other worthwhile items for sale. I actually am about to deliver the engine to a forum member for their restoration and pass that torch along. She was a great car, but I've shifted into some Asian import projects and I'm ok with the fact that everything on those costs half the time and money to accomplish.

I did a silicon exposure in much similar way but different options and phases like 3 phase scotch Brite medium to fine grit with continuous hone oil feeding and then final with AN30 and felt method but intermediate hone oil feeding, 60 minutes, 250 to 300 rpm, 60 stroke. Ra I was looking for between 5 to 10 and i did measured it with Rapeak and everything using profilometer and achieved it great. I will show it in some time, and I started a project for the same in audiworld and i will replicate it in some time