Skip to content
HOME » MOTORCYCLE GRAB BAG » Pistons : A Big Boring Story

Pistons : A Big Boring Story

Pistons – Those two round plugs at the heart of your V-Twin need special care and attention if you’re planning to install new ones.

Whether you are looking at improved performance or refreshing a classic with an eye to reliability, there are a few things to think about for best results. I will use mostly the example of my 120-inch Ultima for no better reason than that’s what I am working on. These tips apply to most all brands, but V-Twins seem a little more sensitive (after all, they are big holes).

So, the Ultima … 

These big aftermarket engines require bigger machining setups than some bike shops can handle. The 4.25-inch bore was bigger than my usual go-to guy could even measure! (He was used to sub-four-inch jobs.) Few shops have torque plates for all brands, and depending where you live it can be an issue. I decided to go .005-inch over to take out some light scratches. In this case Jeff at Calgary Harley-Davidson came to my rescue. We (I) decided to proceed without the plates; (it would have been very expensive for him to make a set). I think these rather heavy cylinders will be okay as long as I am very careful with the break-in. It’s a bit of an experiment/gamble but I will learn something one happy way or the unfortunate other.  

First thing to do is make sure the cylinders are CLEAN! Put some oil on a white towel or lint-free rag and then rub the cylinder walls till they come out with no black showing. Resist the temptation to use WD-40 or other solvent, because some of it may hang around and knock off the oil you want on there for piston installation and run in. If you need to use something, hot soapy water is best. Just put oil on the cylinders right after drying so they won’t rust. If the pistons are coated there is no need to doctor them. 

If it’s an alloy, then lightly rub any high spots or sharp edges with some fine sandpaper. This will help prevent hot spots that can cause pre-ignition on a hot day or with some bad fuel.

Question One: How much piston clearance do you ask for?

Answer: Manufacturer’s specs should be safe most times, but know yourself. What I mean is if you are building a V-Twin you can ask for a little more clearance. Tim Mass of Sideline Cycles in Lethbridge, Alberta once told me he “knows” his customers—if the bike is going to get a careful break-in 2.5 thou is good for most brands. Another customer that will hop on and “ride it like he stole it” will get three thou at least.

Question Two: How much ring gap?

Answer: Your machinist-mechanic might be the best bet for setting this gap; they should have a ring grinder. This machine keeps the gap square but you can do it with a hand file if you are careful. File in toward the centre and rub the edges with some fine sandpaper—leave no burrs!

So, why the gap? Thermal expansion. As the engine heats up the gap comes close to coming together, but if it actually does that would be a disaster!

There are many specs and formulas out there. Mahle says for a mild street engine it’s .004-inch times the bore. 

At the other end of the scale is a supercharged engine, and it’s .006-inch times the bore. A nitrous drag engine is .007. The more power you make the more heat you make and then the bigger gap becomes essential.

So for me it’s: 

.004” X 4.255” = .017” 

It’s a polished engine, which retains a bit more heat (less outside surface area dissipates less heat). I think I will be safe at about .020-inch. Conventional wisdom says the second ring should have slightly more gap. This is not about heat but sealing. We do not want to chance a pressure build-up under the top ring … one possible cause of ring “flutter” at higher rpm. Remember: 6,000 rpm on a long-stroke engine means the pistons are really moving. So the second ring will do less work and stay cooler, which is a good deal if you are a second ring. (I set mine to .022.)

Put the rings in the cylinder and square them up (one at a time). Use a high quality feeler gauge to check the gap.

Next, fit the oil control ring. It’s different and its only real job is to make sure not too much oil stays on the cylinder wall. Oil is tossed up to the wall by the clearance in the big end rod bearings and the oil control ring scrapes most of it off. If it did not you would have a “smoker.” Just enough should get left on the cylinder to lube the two top compression rings. 

Put the expander in the cylinder and move it up and down. There should be just enough tension to feel the crosshatch left by honing. Different brands have different rules … be aware. Keith Black pistons require the installer to READ! They put their top ring up much higher than some other brands. This means they get hotter—if not given a lot more than standard gap the rings come together and the cylinder and maybe the head are damaged as the crown gets broken from the piston. Personally I think they work great and make excellent power. 

I once had an episode with Wiseco Pistons, normally never an issue. I decided to try their X-ring design in a high compression Evo. They made great power. The Evo certainly likes a bit more compression and it even makes them sound better. Anyway, the narrow, hard rings were not suited to the H-D’s “soft cylinders” and enlarged the bore at an alarming rate, a noticeable amount in about 15,000 kilometres. The hard, narrow rings were machining the cylinders. 

It would not have been an issue with brands that use hardening treatments in the liners. In this case the cylinders wore but the rings were like new. Guess that explains why there were always little black specs in the oil no matter how often I changed it. Wiseco is a great product but get the ones with “Hastings” type rings if you are doing a Harley.

This is just advice from a guy that has tried a few engine combinations. I do not pretend to know all the ins and outs of all the different brands. There are people that know vastly more about pistons, rings, and fits. If you are doing a rebuild it pays to ask questions of your local “guy.” If he is in a good mood he may gift you some priceless knowledge. 

FOOTNOTE: Green and red inserts in the oil control ring assembly are unique to Mahle pistons, as far as I know. They make it easy to see the ends of the ring are not overlapped. They stay in although they are not mentioned in any instructions I could find. I did confirm this with Paul Chirayath at RaceDyne. The plastic tubes insure the rings won’t get dinged.

by Rich Burgess Canadian Biker Issue #320


Keep independent motorcycle journalism alive! If you found this article interesting or useful, please consider sharing.