m42racer

Glad to hear everything worked out well. How come your builder left this to you to figure out. He owes you a cold one!

DAR951

M42...

As Tony, I appreciate the post, but I'm confused, as this is 180 degrees from what I understand, both in theory and practice with Alusil (specifically, one doesn't touch the cyl walls for any reason except with a Sunnen)... Do you have actual experience with this approach? If so, what kind/type of hone did you use, coupled with whos rings? Any problems with the rings seating???

Thanks

m42racer

You must not get the two processes mixed up. The process you speak of with the "Sunnen" is the process of honing or polishing the stock base material to bring the silica particles to the surface. Its done with special hining pads and paste that Sunnen amongst others supply for this purpose. Normally vertical honing machines have stones attached for honing material from the Piston bores. The amount is typically less than 0.020", after boring. The pads do the same as the stones but are designed to match the block material. Same boring and honing procedures typically apply. One the final size is acheived, the bores are round with no taper, a very quick hone is applied with different paste to apply a cross hatch to the surface. This is to promote the ring seating. On bores that do not need resizing, often they need this cross hatch replaced and the glaze removed. Otherwise the rings may take longer or in some cases not seat at all. This process on 951 Blocks should only be done by those that know how. It is normally done with special stones fitted to the vertical hone or a "bottle" type of ball hone on steel blocks, but on 951 blocks, a special grade of grit and mix of honing oil and paste is used to achieve the proper degree of cross hatch, and at the proper angles. The fact that we use soft rings allows this process often not to be done. That decision should be made by those that know. Failure could cause excessive blow by to occur. I will say this, I think it way more important to measure and make sure the bores are round, within spec on size and no taper, than to have the proper cross hatch. Of all the blocks I have seen, most are all out of spec and have taper. This makes sealing under compression and induction very difficuilt. I read so much here about different hybrid turbo's etc been touted with respect to HP gains, but in most cases the basics are forgotten or completely overlooked. After you have spent the time to ensure the quality of the Piston bores, Piston Ring sealing, Head Sealing, Valve seats and Btm angle stuff, that Turbo just may make some more power.

m42racer

Phew! Only four extra days and $130 out and I (with the board's help) remedied a certain person's (he shall remain nameless) FUBAR. That person kept assuring me "everything fits" with MY kits... That seems to be true except if your name is cpt_koolbeenz or MySwiss and you order 101mm pistons from incompetent "tuners." Sorry for my ranting... Long day - little sick of engine problems!

There is alot of stuff out there for sale. The problem is that not many know what they are selling. If it comes in a box ready to install, basic standard stuff, those ASSEMBLERS can sell anything. It takes a builder, someone who knows his stuff to help out when things get out of the norm. I know I will get hammered for saying this, but the proofs out there. You been one who has experienced it first hand.

Porsche-O-Phile

On a related note, what is the concensus out there regarding rings on used pistons? I have a set of used 951 pistons that I'm planning on transplanting into my 944 engine (they are 100.0 mm pistons, stock, Porsche part #'s will be posted if anyone want's 'em) in order to (1) reduce compression to 8.0:1 for the Callaway conversion (see below for more information) and (2) to ensure proper sealing / compression in the engine.

The engine is fine, I have no reason to suspect loss of compression, scoring or any other "bad" things. I'll do a compression & leakdown test prior to ripping it apart as part of the Callaway conversion, but I really expect the bores to be fine. The pistons have not currently been checked out by a machine shop; I will take them in at some point to verify their roundness and proper geometry. I cleaned them up, pulled the wrist pins & retaining rings off, inspected and reassembled everything. All looks good thus far. Connecting rod studs also appear good but I've been told they should not be re-used and I'll likely replace them with Raceware studs before they go into the engine.

Background: the original conversions done by Callaway Engineering included custom forged pistons to drop the stock n/a compression ratios from 9.XX to 8.0:1 in order to not run into problems with detonation under boost. Since the Callaway slugs are no longer available, I'm opting for the 951 pistons since they were readily available and a Porsche original part. The end result should be the same.

Bottom line is I'm wondering what to do in order to ensure this conversion is done properly. I'd kind of like to do it without pulling the engine out of the car, but if I absolutely have to, I will. My preferred method is to pull the head and oil pan at the same time, then push the old pistons out the top, inspect the bores visually, and assuming all looks good, install the new pistons & rings, new rod bearings, bolt them to the crank before the oil pan goes back on, and reassemble from there. The oil pan needs to be replaced to accommodate the turbocharger return line anyway (plus the current gasket leaks a bit, so I'm replacing it regardless).

So what's the best way to make sure I get the correct / best fit piston rings and do I absolutely need to pull the engine in order to have the bores honed even if they're good? Again, I'll do a leakdown & compression test to verify, but this engine pulls very strong and makes good power and burns no oil, so I suspect no problems like scored bores or anything.

m42racer

Just looking won't tell you the bore size. Whats the point in worrying about the ring seal if you don't know the bore condition.

As for the Pistons, do the following. Have them carefully cleaned. Zyglow and check the Hardness across the crown. Measure them, Pin fit he Bosses, and check the side clearances on the Rings. If to much, you may have to fit back up spacers. These are fitted to the tops of the Grooves. I have a feeling spacers are not available in our sizes. Any wear in the grooves and the Rings will not fit correctly and out the window goes the ring seal. If the grooves are worn some and spacers are not availabe, rings with a Tapered or Barrel face should be used. I'm no expert here, I've reached my limit, but call Neil @ Performance Developments as suggested earlier. He will tell you what to do. Be careful with production machine shops. They will tell you everything is OK, but when you have a problem, they will not care. Check everything first, one time and ensure you do not have a problem. You may have to have the ring grooves machined to use a ring width that offers the type of face design you may need.

Porsche-O-Phile

Well, the car is stock now. The bores should be 100.0 mm unless someone did me a favor at some point! There's nothing in the service records to indicate any modding though, I'm 99.999999999999999999999999% sure it's a stock 100.0 mm 2.5L 944 engine.

When I get it all apart I'll check the bores as stated. I guess I can rent a dial caliper and a few of the other specialty tools required to measure everything properly as well as well as a ridge reamer (I don't expect one, but you never know. . . ) Maybe I'll just pull the engine out, but given my current situation (apartment with shared garage) that's kind of asking for trouble. I'm technically not even supposed to work on cars in there, but have been for a VERY long time - and my neighbors don't care as long as I'm clean about it. It'd just suck royally to get the engine out of the car and then have the landlord start getting picky about the "no non-running vehicles" clause. I seriously doubt that'd happen but it's something to consider. . . If someone had a garage where I could leave the car for six months while I could pull the engine out to be tanked, rebuilt, etc. that'd be one thing, but it's simply not an option around here.

DAR951

I have no problem distinguishing between the two proceses. In fact, my "confusion" comes from quite the oposit... I believe the two to be incompatible. (as I wrote, my understanding is the only thing that should touch the bore of an Alusil block is a Sunnen hone or pad)... Your Post suggests otherwise in contradiction to a number of "authorities" (also noted by APKaos).

My question is, without intending offense, are you writing from direct, personal experience, or is this opinion?

I'm interested in this topic as I'm trying to determine/decide between simply re-ringing or a full overbore. For the purpose of discussion, assume I have done all the necessary measurements, the cylinder/pistons are within the wear limits and the bores are straight. They are, however, pretty well polished (no visible cross hatching).

Part of the decision will bear on whether I believe I can get a good ring seal... part of THAT determination depends on what, if anything, I can do to prepare the cylinder to seat the rings... So, 1) can I use standard engine building practice of honing a cross hatch? 2) If the answer is "yes", then is it: or is it: Thanks

m42racer

Dar951,

Your question would be best answered by an expert in this field. I can tell you I have not seen on many occasions the bores to be in good condition, so the repair has normally been the whole deal where 4 or 5 processes were done. the last been the paste and felt pads. The finish I have seen is a flat matt like finish which had cross hatching in it. The time spent on each bore was approx 1 minute or so. The bores were already to size, the paste was to finish the bores bringing the silica to the surface and put the cross hatch in.

On blocks that have been sent there for other work, Head Studs, Line Boring or whatever, they had the pads on a special honing bar that was driven by an electric Drill motor. They had some mix of the paste and some other special honing oil, and this was used just de glaze the Cylinder Bores and put a cross hatch back in. I seem to remember it took approx 30 secs per bore. Apart from those who have the correct pads etc, just know how and what to do is very important. I still think in most cases, making sure the bores are within spec is more important than any deglazing. Having a surface that brings in the ring seal evenly and quickly is still very important too.

Porsche-O-Phile

The factory manuals list some very specific criteria in order to re-hone the cylinders - they specifically list the use of a Sunnen CK-10 CV-616 machine with standard oversize values of 100.50 and 101.00 mm. The exact machine settings are listed - I can give them to you if you're interested. I'd check around with machine shops to see if they're familiar with the use of this machine / process. For my own car, I would NOT accept an answer of "oh, we use a different machine, but the process is the same. . ." as this would simply indicate a desire to make money off you with a minimum of concern about the quality. If they could specifically tell me details of why their equipment and the results it would give are comparable, then I might consider it. . .

The more I think about this, the more I'm thinking I'll check the bores once I yank the head and if they're within 100.00 mm spec, fine - simply plug in the 951 pistons (which I'll have checked per the above recommendations) and go. If not, I'll end up pulling the engine out and have a properly equipped shop go oversized to 100.50. This would be far less desirable from a cost standpoint and would also mean I'd have to get aftermarket oversize pistons (which are expensive as hell) but it beats doing a half-assed job, IMO.

I suppose alternatively I could just have them sleeved back to 100.00 mm with steel jackets - this was done by Callaway on the original conversions.

m42racer

The Sunnen equipment you speak of is one of many quality vertical honing machines. There are many different machines all of which will give the same results. The Sunnen machines are some of the most common. You may indeed find many good machine shops that may have Rottler or Berco or some other really good version. Its the operator, not the machine.

cpt_koolbeenz

I have the honing tools (not machines) from Sunnen for rent or purchace if anyone needs them... Many shops don't have the tools needed to properly hone our blocks. I have the holder, stones, paste, etc... Let me know if anyone needs them.

Porsche-O-Phile

No kidding? If I end up having to pull the engine out of the car I may take you up on that offer. I know nothing about the process though - I imagine these are something I would need to take to a machine shop along with the block to be used properly, right? I'm assuming a decent machine shop would know what to do with them.

A few questions, since we're on this subject:
(1) What is a fair price for hot-tanking / cleaning the block and boring / honing the cylinders as we've been discussing?
(2) What about sleeving?
(3) What is the exact purpose of honing anyway? Is it simply to create small, almost microscopic channels in which oil can sit to help provide a film between the bore and the rings?
(4) If I were to sleeve the bores with steel liners at 100.00 mm, why would such liners not need honing (and why do iron blocks not need it)? Wouldn't the same benefit be gained from honing regardless of bore material, or am I missing something here?

DAR951

Thanks M42... the further explanation helps a lot... much appreciated.

cpt_koolbeenz

<<<What is a fair price for hot-tanking / cleaning the block and boring / honing the cylinders as we've been discussing?>>>

hot-tanking - $80
honing - $80 the Sunnon supplies cost $360

if you take the engine out, you will most likely end up milling the head - $30
among other assorted rebuild processes.