Piston/bore clearance alloy cylinders

I would think alloy cylinders regardless of manufacture would expand a lot more than the cast iron cylinders therefore
piston clearance could be reduced somewhat . your thoughts
Don

Maybe so with Nicasil cylinders, but with the iron liners in most alloy Commando cylinders today, I'm not sure there is really any difference in clearance between them and iron cylinders. I've always used the same for both, and had good results. Good question, though. Maybe someone here has done some experimenting along these lines, and has a better answer.

Ken

http://www.motorcycleproject.com/text/c ... right.html
Honda air cooled engine piston/bore clearance WOW!!

madass140 said:

http://www.motorcycleproject.com/text/cylinders_done_right.html
Honda air cooled engine piston/bore clearance WOW!!

Click to expand...

Interesting article. But I'd be careful about trying to squeeze clearance in a Norton down to those sorts of numbers. I can guarantee from experience that trying to run any of the commonly available cast Commando pistons at this guy's idea of ideal clearances will result in seizures. And it's even more so with aftermarket forged pistons.

Still, if you had an alloy cylinder with no steel liner (Nicasil, etc.), you probably could safely reduce the clearance from that with an iron or iron linered cylinder. As far as I know, no on has yet made that work well on a Commando, except maybe Kenny Dreer, and I'm not sure how well his setup worked over time.

Ken

Tight clearances help keep the rings square to the bore, when the piston doesn't rock/stays square to the bore, the top edge, the corner, of the compression ring's face isn't worn down, especially the lower scraper edge of the compression ring.

The Norton's cylinder base and head clamping system is complicated and likely not symmetrical in the way it would distort a 100% alloy, nikasil or similar surface, cylinder. A liner attenuates those issues. All cylinders have hot spots and stud/bolt strains that distort the cylinder surface, some more than others.

By running it, taking it apart, and then reshaping the piston, you can enable a tight fit - and square rings.

Study the piston for witness marks that tell you what is going on, where it is tight, where it is loose, residue telling you where exhaust gasses are getting by, figure out why, look at the rings with a loupe for signs of ring cocking,
study the ring lands too, and so on. Rings flutter in their land, and forged pistons' lands bend. The OEMs use this kind of procedure before specifying the dimensions of their pistons. Most aftermarket pistons must have loose clearances because no one does this. It's expensive and doesn't matter in most applications anyway.

Since we are now discussing a variety of engines, piston types, and piston fit thought I’d drop one more into the mix - a late style air-cooled Hog that employs an aluminum cylinder cast around a spiny-lock iron sleeve. The factory pistons are a cast CNC machined design that provides a barrel shaped profile (top to bottom), thus the piston is difficult to measure (where do you measure on the barrel?) and the cylinder is therefore typically bored to a nominal size, which I believe is 0.001” + 0.00025”, larger than the quoted piston size.

So there is an example of a sleeved, aluminum air-cooled cylinder that runs quite tight. I believe the unique piston design in combination with the spiny-lock sleeve are the factors that allow such a tight piston fit.

Seems the clearance on a Norton sleeved Al cylinder should be able to be tightened ever so sightly, but don’t know that I’d care to be the guinea pig performing the experiment. I also concur with Lcrken that we are not going to be tightening up the piston fit on our cast iron cylinder Nortons any time soon.

Regarding the current pistons we use in Nortons, i.e., ubiquitous tapered cast and forged profiles, what are people’s opinions regarding a different piston profile in a Norton? Could we use a barrel shaped piston like the Hog, or is the barrel design for cast pistons only? Has anyone ever asked a piston manufacturer (JE, CP, Wiseco) if such a design has merit in a Norton application? Not trying to move us off to a different subject, just curious.

WZ507

I've been working with JE pistons for years. You can get a barrel shaped forged piston made to spec. The shape I use works out to give the best seal when the piston is warmed up. Different barrel/taper shapes are available for racing or street etc. Most forged pistons are wider front to rear to reduce friction. There's a compromise between high performance and high mileage use. There are other design details to consider such as how the oil is distributed to the skirt etc when you want to reduce wear.

Nortons get hotter than some other brands and you can only get so tight.

my understanding of this is that it is not necessarily cylinder expansion that should be considered, but piston expansion some pistons will expand more than other, forged pistons for example, so more ring gap is required when assembling. Most good manufactures will give you the required ring gap clearance, and in a lot of instances rings will require filling to prevent grabbing under expansion. I use JS light weight pistons with maney barrels and am supplied with min and max ring gap from the manufacture. Be carefull to tight and she will grab.

"I use JS light weight pistons with maney barrels "
and you are using what piston /cylinder clearance?

B73 said:

my understanding of this is that it is not necessarily cylinder expansion that should be considered, but piston expansion some pistons will expand more than other, forged pistons for example, so more ring gap is required when assembling. Most good manufactures will give you the required ring gap clearance, and in a lot of instances rings will require filling to prevent grabbing under expansion. I use JS light weight pistons with maney barrels and am supplied with min and max ring gap from the manufacture. Be carefull to tight and she will grab.

Click to expand...

How can piston expansion affect ring gap?