Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
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CAUTION!! After having my pistons coated, I've come to the conclusion that I wish I had never bothered. Here's why:
I started out with a nice set of Nikisil plated cylinders and a matching set of quite nice pistons. There was nothing wrong with the pistons, but I figured an additional layer of wear protection on the skirts couldn't hurt anything. So, I sent the pistons off to Swain Tech and they did their thing. Did a good job, too. Certainly no complaints from me about the work they performed.
It wasn't until I received the pistons back from Swain Tech and attempted to fit them into the cylinders that I realized I had a problem. The pistons would not fit because the coating on the skirt was too thick. Again, Swain Tech had done nothing wrong. I had failed to properly calculate the clearances required.
Swain Tech coated pistons, TBC on Dome and PC-9 on Skirt.
Photo courtesy of Gregory Bender.
Swain Tech coated pistons, TBC on Dome and PC-9 on Skirt.
Photo courtesy of Gregory Bender.
UG! What to do? I called Swain Tech and they told me this was a common occurrence and to simply sand off the extra with 400 grit sand paper. Oh joy, now I get to hand sand pistons to get the proper clearances. Not exactly my idea of a good plan. I ran this past several experts who said sanding or Scotch-Brite the pistons with these Moly type coatings was not a bad idea. BUT, if I remove too much of the coating, it could get down to the aluminum of the piston that Swain Tech glass bead blasted to encourage better adhesion with the coating. That would be a very bad thing.
At the end of the day I figured I had already made one mistake by having the pistons coated in the first place. I certainly did not want to make another mistake and destroy my cylinders and rings. So, I sourced another pair of properly sized pistons.
In my final analysis, I'm not sure what good coating the piston skirt can do with the clearances that are supposed to be run on our Guzzis. Certainly one could have the piston skirts coated and then have the cylinders bored and nikisil plated to match. The skirt would then fit in the cylinder properly, but standard rings would then be that much more expanded yielding larger ring end gaps and less support of the ring in the ring groove around the perimeter of the piston. Seems to me like a lot of work and effort for nothing more than a tradeoff in gains/losses.
At the end of the day, I do not recommend piston skirt coatings.
