View Full Version : Piston Coatings for Alusil

07-16-2014, 05:48 PM
Hello fellow enthusiasts, I am looking for some input from those who have built or reconditioned their BHF engines.

Over the past year I have been working on a low mileage motor that I acquired from a wreck. When completed, this motor will replace the 165k mile motor in my Avant that has several galled cylinders. The replacement motor has perfect bores. It's been torn down to a bare block, and I've mic'd the bores for both diameter (84.510mmą0.005mm) and roundness with a Sunnen dial bore gauge - all good. Also measured the main bearings and checked for roundness - all good there too. Purchased new OEM bearings as indicated on the crankshaft (blue and yellows).

Here's my quandary then: how do I protect the bores from the pistons? Most of the OEM ferrous coating on the piston skirts has visibly worn away. In theory, this coating should be maintained to prevent like-metal contact and galling (I think...?). I have read about Porsche owners coating their pistons for longevity and performance in their Alusil blocks, but I have not heard about any stories in our Audis. There are many coatings available out there, but I am worried about coating thickness. In a block that measures to spec, the piston is 84.490mmą0.010mm. This does not give much room for a coating. I had been looking at Swain Tech's PC-9 due to the success they've had on similar Alusil engines, but at a thickness of 0.0008" (so, piston diameter +0.0016" or +0.04064mm) it exceeds the OEM piston tolerance by 4x.

Anyone have experience coating their pistons or doing something similar to an OEM ferrous recoat?

07-28-2014, 08:00 AM
I got my pistons coated for other projects from Dart Machinery here in Michigan. It is a dry film lubricant that works VERY well, as long as you properly coat the piston and have a proper piston to wall clearance. i dont remember the thickness, but it goes on quite thin and some people actually run a tighter Ptw by laying it on a little thick, but its air brushed on very thin. Id give Rick Zwieg a call at Dart Machinery and tell him what you need. my pistons were done and the work was top notch, as most places dont prep accordingly to Techline coatings procedure they showed me every step of the way in their process.