alex_cristocea

Last year I started rebuilding my engine with Mahle pistons and as I read in the technical application guide they said that the 2618 alloy they use it has a high coefficient of thermal expansion. Mahle also gives some references, in the book they released in 2012 called - Pistons and engine testing, about the high performance alloy they exclusively use for forged pistons that is named M-SP25, witch also has the highest thermal expansion coefficient from the alloys they discuss in the book.
Now here it's what I don't understand: in the application guide the cylinder size, ring dimension, rod length, pin size it's given in metric system (mm), but for clearance it's not stated any measurement unit ( metric or imperial) - so, I assumed, if everything is in metric the the clearance is also in metric, but if you look in the application guide 0.0044mm clearance it's roughly 20% from the 0.02 mm clearance for 100.50 stock pistons so I said is something wrong. I measured the pistons and they are somewhere in the 100.42 area. so i said that the dimension in the application guide must be in imperial units witch means that the clearance wold be from 0.091mm to 0.11mm but this i find to be a bit to much.
Is there anybody who successfully used this pistons, and with what specs for the cylinders?
Unfortunately I already machined the block, I also did 300 km with the rebuilt engine and it does not sound to good.

edfishjr

I believe that the clearance numbers you have boxed with a red line are in inches, i.e. 3.6 thousandths of an inch minimum to 4.4 thousandths of an inch maximum. The Mahle guides specify the piston sizes in the customary units for that application, i.e. in inches for American V8s and in mm for German/Japanese four-cylinders, but all clearances are in inches.

These are very common clearance numbers for a forged piston made with a relatively high-expanding alloy in a motorsports application. Stock clearances would not usually be so large.

It is not clear to me what clearances you actually have in your motor?

blade7

I think there's been translation glitches in some German literature in the past. 0.004" (4 thousandths of an inch) would be the minimum clearance I'd specify if I was using aftermarket forged piston in the Alusil block. When I had the motivation to build a 3.0 engine I thought I'd use the Mahle Motorsport 104mm pistons. Then a few reports appeared about the skirt coating coming off, and the bores getting damaged. Not sure if that was ever resolved, or if anyone on here has used those pistons successfully?

PF

I have been through this process with the Mahle motorsport pistons bought via lindsey. Coating came off and bores were scratched (Alusil) with clearances supplied by Mahle used.
​​​​I would not use them.

gpr8er

I have used Mahle pistons that I purchased from Andail in Huntington Beach approximately 10 years ago I had the Machine Shop set the clearances spec by Mahle. I don't remember the exact spec but I do remember that it's an extremely tight clearance the machine shop I use is very experienced in Alusil bores Mike didn't bat an eye when I told him the spec
The car has been running fine for almost Ten Thousand Miles, recently had to head off to do a head gasket bores look perfect.
Back in the old days the coating was called ferrostan but because of environmental regulations it's now ferroprint.

PF

The pistons you have are not the same as the ones in question here I'm afraid, the newer is a lesser part imho. The OP and I wish they were like yours though. In my case it cost me tons of money and a full rebuild twice not to mention all the time I put in and all the time the car was not on the road. Neither vendor or manufacturer took any responsability in my case. I sent my pistons to Mahle in the US from Sweden for them to make a test on them only to get them back with the answer that they were fine.

So again, I strongly advise not too use the "new" version of Mahle motorsport pistons.

alex_cristocea

After oversize my cylinders measure
#cyl1 – 100.505
#cyl2 – 100.515
#cyl3 – 100.515
#cyl4 – 100.525
For the pistons I recived I get the following measurements:
#p1 – 100.411
#p2 – 100.419
#p3 – 100.410
#p4 – 100.410
So I installed in the following order resulting the clearances:
#cylinder1 - #piston4 = 0.106
#cylinder2 - #piston1 = 0.95
#cylinder3 - #piston3 =0.104
#cylinder4 – #piston4 = 0.105

blade7

The original Porsche pistons run less than 0.002" (2 thousandths of an inch) clearance I think. Perhaps those Andial pistons came from Germany, and were made of the same alloy. I wouldn't take a chance on the new Mahle Motorport pistons in Alusil now. I haven't read of any Alusil issues with the Wossner pistons, though one US supplier told me Wossner had told them not to supply to Europe.

alex_cristocea

I started the engine at the end of last year, changed the oil after 30 minutes, changed oil again did around 300 km and it runs fine. Passed the emission test. Did not go past 3k rpm mark during this break in period. The first thing that ring the bell was a knocking sound, something like piston knock and after that I checked the bores with an endoscopic camera and they look like this:



Now I'am preparing to remove the engine.

edfishjr

So, all your clearances are about 0.1mm or .004" Correct per the documentation you supplied.

blade7

It seems iron dry liners offer a less complicated solution. Though when I've asked about machining the liners for the windage ports on 104mm blocks, I've been told that causes a problem for the honing cutters.

alex_cristocea

Since I have the 2.5L block windage ports are not a problem, what I don't know regarding the dry liners it's the piston to wall clerance.

alex_cristocea

I opened the engine last week and found that the pistons are looking good and the bores are trashed. What do you think?

blade7

Were the rings gapped, and the correct profile for Alusil?

alex_cristocea

Yes. Done it myself ( 0.43mm for the first ring and 0.33 for second)