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All About The Right Gap – Piston Rings

CB Tech Advisor Rich Burgess has been doing top-end work on his old Airhead Beemer. Now it’s time for rings.

There is more to understand about piston rings than you might realize—some of the ins and outs, or should I say ups and downs—with a spin (they actually rotate at a fair clip when the engine is running).

A major part of any engine rebuild involves a look at the pistons and rings. We kind of have two worlds here. One is the iron liner crowd, which includes most vintage bikes and Harleys. The other group would include most modern Euro and Asian brands, which have induction-hardened or hard-coated bores. A subset here would be chrome plated bores (never use chrome rings in them or you get an almost instant seizure). 

The right piston rings are also absolutely necessary in the iron liner group. I know this because I once used a high-performance piston and ring combo in my Harley. The rings had a narrow face width (about half a “Hastings” type ring) and a hard alloy construction. These guys wore the bore to the point it need an overbore in less than 15,000 kilometres. The conventional Hastings ring replacements I used have left the bores in spec 50,000-km later. The reason: the narrow Wiseco X-rings were meant to fit a hardened bore—my experiment was not the best idea ever. You see, nitride treated bores don’t wear much; the idea is to just do a light (plateau) hone, change out the rings and you can carry on. In soft iron they were acting as little cutting tools, although power was very good while they lasted!

The piston rings gap is also very important, with the main concern being that the gap never closes up completely when the engine gets hot: that would be one of those catastrophic “fails.” The coefficient of linear thermal expansion of steel (depending on the alloy) is about 0.00000645 inches per degree F. So a ring on a piston that has a circumference of 10 inches (3.5-in. diameter) would grow 0.0000645 per degree F. A piston temperature of 450F above ambient would mean the gap would close by about .003. 

The second concern is getting a good seal but the gap needed varies with temperature and bore size. Keep in mind, more power = more heat! Gap closest to zero = instant destruction. Read the instructions!

I know folks that had bad luck with Keith Black pistons and I know why, they did not read the instructions in the box! KB’s require a lot more gap than normal on the top ring. This is because the top ring groove is higher on the piston closer to the heat (one of the features that helps make more power). The instructions are interesting. They say, for racing use an even bigger gap. This seems counter intuitive but they seal well enough once hot.

My latest engine adventure involved a Deves ring set for my Beemer (Nikasil bores). These are good quality aftermarket rings but a little different than the original equipment rings. First, the compression rings required a lot of grinding to get an adequate gap when test fitted in the bores. Lucky for me Paul Shore over at Motorrad Performance in Turner Valley, Alberta loaned me his ring grinding gizmo. It works great to remove a lot of material while keeping the gap square—hard to do with a file. A small fine file is still needed to do small chamfers on all ground edges. As is my usual habit I bought one so that I won’t need to borrow again, they come in around a hundred bucks. 

The second issue got me concerned enough to contact Deves (they are in California). Carefully mounted on the piston, well-oiled and using a good ring compressor it was difficult to compress them enough to go into the bores. Once “tapped” in (I like to be able to push the piston in by hand), they seemed a bit too tight. Well live and learn. 

Deves #1 FAQ: The oil rings fit way tight, what’s up? Their answer is, yes the tight fit is normal. Once the engine gets warm they relax and all is good. There’s something about Swedish Steel and its character. After turning the engine over with the kicker everything feels normal.

So having done the rebuild of the top end I now have a good engine, great paint, and new tires. It’s almost done. A few details (where the devil lives) and while not a showpiece it will be a very nice “rider.”  Well, a new exhaust would be nice. 

piston rings sizing ring tool set up

by Rich Burgess Canadian Biker Issue #313


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