Alusil bore with JE piston possible compatibility issue
Discussion
Hi, does anyone know the correct piston ring specification for Alusil bores? I have just had a 944 turbo engine rebuilt using JE pistons and rings. The engine has done 2k miles and already has significant bore damage. The parts were specced and rebore carried out by an Alusil block specialist, who assures me the correct process was used and the correct JE parts used. The guys who put the engine together, tell me the JE rings were the incorrect spec for an Alusil bore. Side note; the engine was carefully run in for 1k (of the 2k miles) on mineral oil. The oil was then changed for royal purple synthetic oil and the car driven normally. As soon as I started using it normally (full power), it was evident that there was excessive breathing from the engine. I am willing to accept that there might be another issue if the rings are not to blame. There is so much misinformation re Alusil so I am looking for a response from someone with qualified experience to help me try and figure out what has gone wrong here.
Mod note: company names removed, as per N&S rules
Goetze rings were OE fitment, and from memory are just plain cast iron. The key is in the bore finish. There's a huge amount of information about this in this thread..
http://forums.pelicanparts.com/911-engine-rebuildi...
http://forums.pelicanparts.com/911-engine-rebuildi...
I've reclaimed alusil bores for 944's several times and never had a problem with aftermarket rings. Think mine were ordered via Total Seal. What are the symptons and was the bore prepared properly?
ETA, it's the final polishing of the bores that's important. You can't just rebore them and leave them like that. As for heavy breathing I did come across an engine that had a pipe missing from somewhere which caused it to breath heavily. Once the owner refitted the pipr or sorted that out the engine was fine.
ETA, it's the final polishing of the bores that's important. You can't just rebore them and leave them like that. As for heavy breathing I did come across an engine that had a pipe missing from somewhere which caused it to breath heavily. Once the owner refitted the pipr or sorted that out the engine was fine.
Edited by Boosted LS1 on Friday 3rd September 23:28
Pumaracing said:
How do you know there is significant bore damage and what exactly does this damage consist of? Are you saying the engine has been stripped down again already?
What do you mean by excessive breathing? How was this measured or evidenced?
. What do you mean by excessive breathing? How was this measured or evidenced?
Damage is bore scoring or gauling which can be seen by the eye. Excessive breathing - blowing lots of oil out of breathers (pressurising the sump). Engine has been removed and stripped.
dom1102 said:
Damage is bore scoring or gauling
Ah! You mean the bores have been French polished 
Sorry couldn't resist. Anyhoo.
Excessive bore scoring or galling does indeed lead one to conclude that a faulty tribological process has been at work.
Alusil is not a primary area of my expertise but the main points as I understand them are as follows.
1) After reboring and honing to size the critical thing in bore preparation is the correct polishing operation with Sunnen felt pads and AN-30 silicon paste to expose hard silicon crystals above the aluminium base. This process closely duplicates the aluminium etching process originally used at the factory to remove aluminium from between the silicon crystals to leave them standing proud.
2) Pistons must be tin or ferro coated to prevent aluminium/aluminium contact which is not a good wear interface in this application. The piston skirts will be a golden or dark colour rather than the normal silver aluminium colour if this has been done.
3) Piston rings must be barrel shaped on the outer faces to prevent sharp upper or lower edges from ripping the exposed silicon crystals out of the aluminium matrix. Most ferrous ring materials will be suitable including chrome plated as long as the barrel shaped outer faces are present.
Conventional rings for iron bores are usually taper faced or parallel faced to assist in oil scraping and compression sealing. These shapes will not work with Alusil.
"
1) After reboring and honing to size the critical thing in bore preparation is the correct polishing operation with Sunnen felt pads and AN-30 silicon paste to expose hard silicon crystals above the aluminium base. This process closely duplicates the aluminium etching process originally used at the factory to remove aluminium from between the silicon crystals to leave them standing proud".
This is the kit I have.
1) After reboring and honing to size the critical thing in bore preparation is the correct polishing operation with Sunnen felt pads and AN-30 silicon paste to expose hard silicon crystals above the aluminium base. This process closely duplicates the aluminium etching process originally used at the factory to remove aluminium from between the silicon crystals to leave them standing proud".
This is the kit I have.
Some good information here
http://www.aera.org/ep/downloads/ep4/EP10-2008_18....
and here in the official Kolbenschmidt Guide to The Reconditioning of Aluminium Engine Blocks.
http://www.kolbenschmidt.com.tr/pdf/brochure/KS_50...
http://www.aera.org/ep/downloads/ep4/EP10-2008_18....
and here in the official Kolbenschmidt Guide to The Reconditioning of Aluminium Engine Blocks.
http://www.kolbenschmidt.com.tr/pdf/brochure/KS_50...
Edited by Pumaracing on Monday 6th September 09:09
In my experience the above posts about alusil are correct in terms of the process used to bring out the silicon etc. I would be asking the following....
1. You know what rings were provided (you bought them) so should be easy to confirm if they are the right ones or not.
2. The company who built the engine should also know that the rings were wrong so why fit them if they were wrong. They have built enough blocks to know the right spec?
I do know that it is hard to reproduce the factory Alusil finish as my experience between running in a new factory bare block with Mahle pistons & rings compared to one which has had the bores redone at a shop was very different. The factory block required virtually no running in whereas the blocks I have seen that were redone took ages for the rings to seal properly. There is obviously a fine art to getting the finish right. From memory when my last block was built I remember that the other engine my builder put together was Nikasil and that used different rings to my Alusil block.
Where I do think a lot of people come unstuck is that they do not always get the piston to bore tolerances right on these builds. The Porsche factory tolerances are very tight, there are group 0, 1 & 2 blocks (relating to bore sizes) and group 0, 1 & 2 pistons. Technically group 0 blocks should be matched to group 0 pistons. But you can in theory use a group 0 block that is on the high side of the measurement range with a group 1 piston that is on the low side if its measurement range. If you get the tolerance range wrong and the clearances are too far out this could possibly be a problem on a new build. Again I would have expected any competent Porsche engine builder to take these measurements of your block and make sure the pistons are within the right range before installing them. For what its worth I used a group 0 piston in a group 1 block after checking the sizes where within the Porsche recommended clearances and I had no problems.
Below I have pasted an extract from a post by a friend several years ago on another forum when we were discussing tolerance groups and piston to bore clearances. All the numbers relate to a 3.0 block with 104mm bores but the same tolerances apply to 2.5 blocks with 100mm bores:
Here is my thinking. I am no expert, so those in the know should correct me if I am off base on this. Based on the specs posted above, the group 0 piston spec is:
103.980 +/- .007 (spec range: 103.973 to 103.987)
The bore spec for a group 0 block is:
104.000 +/- .005 (spec range: 103.995 to 104.005)
From these numbers, the piston to wall spec range is .008 to .032; i.e., within the same tolerance group, Porsche accepts a piston as big as 103.987 in a bore as small as 103.995, which is just a .008 clearance; at the same time, Porsche also accepts a piston as small as 103.973 inside a bore as big as 104.005, which is a .032 clearance.
For what it is worth, the 944 turbo manual also lists the piston to wall clearance range spec as .008 to .032 – for the 951 anyway -- so it seems that Porsche did not build in any extra clearance on its turbo motors. Porsche did not take Bengt's suggestion in other words. Also, the wear limit for the bore is .08 on a 951, which is an order of magnitude bigger than the tolerances one could fuss over within tolerance groups.
Now, back to whether group 0 pistons will work in a group 1 block. The group 0 piston range is 103.973 to 103.987, and the group 1 bore range is 104.005 to 104.015. So, at the extreme ends of the ranges (smallest acceptable group 0 piston in largest acceptable group 1 bore) you get a clearance of .043, which is .011 over spec (albiet 1/8th of the acceptable 951 wear limit of .080). If you plot it out, about 2/3rds of the combinations in these tolerance groups fall within the clearance range of .008 to .032. If you assume that the distribution of actual group 0 pistons and group 1 bores center around 103.98 and 104.010, respectively, following some kind of bell curve, then the chances of an acceptable clearance goes up dramatically in the real world. For example, if both the piston and bore measure out at their target sizes of 103.98 and 104.010, you get a .030 clearance, which is within spec.
So, I agree you need to measure, but if the pistons and bores are within spec for their tolerance groups, you have a very high chance of the clearance being within spec (or, worst case, pretty darn close, well within wear limits anyway, and with room for coating the pistons perhaps)
1. You know what rings were provided (you bought them) so should be easy to confirm if they are the right ones or not.
2. The company who built the engine should also know that the rings were wrong so why fit them if they were wrong. They have built enough blocks to know the right spec?
I do know that it is hard to reproduce the factory Alusil finish as my experience between running in a new factory bare block with Mahle pistons & rings compared to one which has had the bores redone at a shop was very different. The factory block required virtually no running in whereas the blocks I have seen that were redone took ages for the rings to seal properly. There is obviously a fine art to getting the finish right. From memory when my last block was built I remember that the other engine my builder put together was Nikasil and that used different rings to my Alusil block.
Where I do think a lot of people come unstuck is that they do not always get the piston to bore tolerances right on these builds. The Porsche factory tolerances are very tight, there are group 0, 1 & 2 blocks (relating to bore sizes) and group 0, 1 & 2 pistons. Technically group 0 blocks should be matched to group 0 pistons. But you can in theory use a group 0 block that is on the high side of the measurement range with a group 1 piston that is on the low side if its measurement range. If you get the tolerance range wrong and the clearances are too far out this could possibly be a problem on a new build. Again I would have expected any competent Porsche engine builder to take these measurements of your block and make sure the pistons are within the right range before installing them. For what its worth I used a group 0 piston in a group 1 block after checking the sizes where within the Porsche recommended clearances and I had no problems.
Below I have pasted an extract from a post by a friend several years ago on another forum when we were discussing tolerance groups and piston to bore clearances. All the numbers relate to a 3.0 block with 104mm bores but the same tolerances apply to 2.5 blocks with 100mm bores:
Here is my thinking. I am no expert, so those in the know should correct me if I am off base on this. Based on the specs posted above, the group 0 piston spec is:
103.980 +/- .007 (spec range: 103.973 to 103.987)
The bore spec for a group 0 block is:
104.000 +/- .005 (spec range: 103.995 to 104.005)
From these numbers, the piston to wall spec range is .008 to .032; i.e., within the same tolerance group, Porsche accepts a piston as big as 103.987 in a bore as small as 103.995, which is just a .008 clearance; at the same time, Porsche also accepts a piston as small as 103.973 inside a bore as big as 104.005, which is a .032 clearance.
For what it is worth, the 944 turbo manual also lists the piston to wall clearance range spec as .008 to .032 – for the 951 anyway -- so it seems that Porsche did not build in any extra clearance on its turbo motors. Porsche did not take Bengt's suggestion in other words. Also, the wear limit for the bore is .08 on a 951, which is an order of magnitude bigger than the tolerances one could fuss over within tolerance groups.
Now, back to whether group 0 pistons will work in a group 1 block. The group 0 piston range is 103.973 to 103.987, and the group 1 bore range is 104.005 to 104.015. So, at the extreme ends of the ranges (smallest acceptable group 0 piston in largest acceptable group 1 bore) you get a clearance of .043, which is .011 over spec (albiet 1/8th of the acceptable 951 wear limit of .080). If you plot it out, about 2/3rds of the combinations in these tolerance groups fall within the clearance range of .008 to .032. If you assume that the distribution of actual group 0 pistons and group 1 bores center around 103.98 and 104.010, respectively, following some kind of bell curve, then the chances of an acceptable clearance goes up dramatically in the real world. For example, if both the piston and bore measure out at their target sizes of 103.98 and 104.010, you get a .030 clearance, which is within spec.
So, I agree you need to measure, but if the pistons and bores are within spec for their tolerance groups, you have a very high chance of the clearance being within spec (or, worst case, pretty darn close, well within wear limits anyway, and with room for coating the pistons perhaps)
Edited by Garlick to remove company name
Thanks for the technical feedback. Have to say that I left bore and piston specs and sizes unto Goznays. Fair to say that Promax certainly should have checked that the parts were compatable!! After getting nowhere on liability discussions I have discussed with Promax the possibility them speccing and building a replacement engine. They have suggested we use Nicasil coating on the bores and have said that they do not recommend sleeving as this has been proven to be problematic.
dom1102 said:
Thanks for the technical feedback. Have to say that I left bore and piston specs and sizes unto Goznays. Fair to say that Promax certainly should have checked that the parts were compatable!!
On strict legal liability then no. If you supplied Promax the parts and asked for them to be assembled it's not their fault if that's not what you really needed. You didn't pay Promax to spec the engine. The liability is on Goznay, not that you'll ever prove it in court.dom1102 said:
After getting nowhere on liability discussions I have discussed with Promax the possibility them speccing and building a replacement engine. They have suggested we use Nicasil coating on the bores and have said that they do not recommend sleeving as this has been proven to be problematic.
I would also not recommend boring out and sleeving a block which leaves it weaker than retaining the parent material. Depending on the depth of the bore damage you may well be able to hone and polish again and use proper Alusil suitable pistons and rings.As a general rule, and for your future advice, never ask one person to spec something and someone else to build it. You end up in no man's land.
When I look back over many years of race engine building the ones that have caused problems have invariably been when the customer supplied parts and said I want you to use these because I already have them/ I have been sponsored by a parts supplier/ I don't want to waste money on new ones etc etc and the damn things have turned out not to fit.
Just checked and the OE (Goetze) basic ring spec is as follows:
Ring 1 - 1.50mm plain compression ring with chrome plating and an inside bevel on the top side
Ring 2 - 1.75mm taper faced compression ring (no plating)
Ring 3 - 3.00mm bevel edged oil control ring with chrome plating
All cast iron
Ring 1 - 1.50mm plain compression ring with chrome plating and an inside bevel on the top side
Ring 2 - 1.75mm taper faced compression ring (no plating)
Ring 3 - 3.00mm bevel edged oil control ring with chrome plating
All cast iron
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