Interference fit piston coating

I'm trying out a new piston coating by "line 2 line" on a high RPM high compression motor. The idea is to set the clearance as normal for a bare piston and not account for the coating. When the piston heats up there will be an interference fit but it won't scuff because of the coating - even if it drags down the RPM a little from friction. Instead it will wear off some coating around the interference fit area during break in. Then after the piston skirt collapses a little (as always happens on break in) the piston will still have some coating on it and will be a little tighter than it would have been if it was left bare. After a half hour or hour of break in the piston interference is gone and you have zero clearance. Supposedly you get twice the piston life with this coating.



I'm only adding .001" thickness of coating on each skirt which will give .001" or .002" interference when the pistons if fully hot at WOT. They recommend more thickness but I'm not willing to try that on such a hot temp motor as a Norton.

Ever thought of ceramic coating on the crowns?

With the tight fit, are you trying to keep compression up, or avoid piston slap and collapse of the skirt ? I never worry about excessive bore clearance unless there is a step due to wear.

Technically, an interference fit is when the part going in is slightly larger than the part it goes into, (like valve guide-to-head or rocker spindle-to-head) but I think you're referring to reducing the normal .004 or so piston-to-skirt clearance being reduced by .001 and then bedding the results in for a slicker fit. You'll still need some clearance to start out with or the piston won't slip into the bore.

Don't do it.

grandpaul said:

Ever thought of ceramic coating on the crowns?.....

Click to expand...

I believe that our very own Nigel aka: Fast Eddie tried that with less than desirable results. GP
concours-- wha da ya mean "don't do it". That's crazy talk
Just look at that beautiful JSMotorsports piston. Shoot if it works out, then here is new technology and technique that could possibly double the piston life. For race bikes that's maybe an extra season or two and for my street bike that means that my next rebuild would be ...like sometime after I'm dead or forget to turn on the in line anti-sump drain valve. :shock:

The piston slips into the bore when cold but the coating means it will initially have a little interference fit when hot at full throttle. After a short break in and the usual piston collapse the coating wears a little in the interference area and conforms to the bore size. You end up with a tighter piston than usual. This coating has been developed on the race track and was recommended by "Bore Tech" who coated the cylinders with carbide. I'm taking their advice with the coating. There is a racer on this forum who has 30 races on these pistons without the coating and there are a lot of bikes on the street with the same. But I'm always looking for improvements and lower maintenance. Its not like I'm the 1st to try this coating technique. It seems successful so far. If it really increases piston life then it will be worth it. We'll see how it goes.

I know of some who have used the ceramic coating on the piston crowns and some have had problems with it coming off - loose ceramic in the motor is extremely abrasive/destructive - I'm not taking a chance with that.

cjandme said:

grandpaul said:

Ever thought of ceramic coating on the crowns?.....

Click to expand...

I believe that our very own Nigel aka: Fast Eddie tried that with less than desirable results. GP
concours-- wha da ya mean "don't do it". That's crazy talk
Just look at that beautiful JSMotorsports piston. Shoot if it works out, then here is new technology and technique that could possibly double the piston life. For race bikes that's maybe an extra season or two and for my street bike that means that my next rebuild would be ...like sometime after I'm dead or forget to turn on the in line anti-sump drain valve. :shock:

Click to expand...

Yes, 'twas I.

I have used it many times in the past with good results, but the last time, on the Commando, I had issues with bits flaking off.

Interesting to note however that non flaked off of the Pistons, only the head. Nevertheless, it was enough to deter me from from using it on since!

I'm using a special anodising treatment called Diamondyze currently, will see how it holds up.

Not a good idea, i'm pretty sure WOT is not when the engine is at its hottest, when you come of WOT and slow or stop shortly afterwards is when your engine will get hot. Any pinching on the piston will be felt on the conrod small end - any slight twisting action and it won't be long until it goes expensively wrong.

Piston speed is very high in Commando engines and apparently ring flutter can be a problem under that circumstance. If the coating reduces friction, it could be good. However I wonder about the fit. I use methanol fuel, so I always warm the motor up well before riding the bike.