Piston size, A/B/C. How important?

[jdosher]

I've read mixed information on whether matching bore and piston codes (A/B/C, etc) is important or not. Some people recommend A with A, B with B, etc. Other people recommend "going up a size" each rebuild (how much does the plating actually wear between top ends?). And others just slap a Wiseco in, which is single-sized and somewhere in the "D" or "E" range without issue. The actual difference in size is quite small.

So does it matter? Does it not matter? If it doesn't matter, why would the manufacturers go through the trouble of taking precise measurements of bores and pistons in order to "bin" them into different sizes? Is it simply to provide them with a better margin of error and avoid worst-case matches?

What sizes have you seen on GG cylinders? Mine is an 'A', and from what I can tell, most are stamped 'A'.

Just curious what people's opinion is on this.

[twowheels]

A good manufacturer strives not only for ultimate performance but consistent performance of the end product. Rather than reject a lot of cylinders because they are slightly oversized, uniform performance can be achieved engine-to-engine by installing the appropriate sized piston and getting the same clearances. With uniform clearances not only is consistent performance theoretically possible, but error states like cold seizures and piston knock are also controlled.

If you want to achieve the same level of control in the field, carefully measure the cylinder and then buy the piston that maintains factory clearances. A slightly less scientific method is to inspect the cylinder walls and if they still retain honing cross-hatches then replace with the piston size designated on the cylinder. If they are slightly to moderately worn go up one size ... more than that strip and replate the cylinder and start from a known measurement.

FWIW, I've only ever seen 'A' cylinders, but have sold both 'A' and 'B' pistons, depending ultimately on what the rider wants.

Originally Posted by jdosher

I've read mixed information on whether matching bore and piston codes (A/B/C, etc) is important or not. Some people recommend A with A, B with B, etc. Other people recommend "going up a size" each rebuild (how much does the plating actually wear between top ends?). And others just slap a Wiseco in, which is single-sized and somewhere in the "D" or "E" range without issue. The actual difference in size is quite small.

So does it matter? Does it not matter? If it doesn't matter, why would the manufacturers go through the trouble of taking precise measurements of bores and pistons in order to "bin" them into different sizes? Is it simply to provide them with a better margin of error and avoid worst-case matches?

What sizes have you seen on GG cylinders? Mine is an 'A', and from what I can tell, most are stamped 'A'.

Just curious what people's opinion is on this.

[GasGas]

My cylinder has B stamped on the bottom edge. Unfortunately I have only been able to source A pistons. The stock piston from gg is far too expensive although they do have a and b. Worssner does list A B C and D on their web site and I will try and source the proper piston for next time.

[knobbyboy]

I just did a top end job on mine from original. Its stamped A but i had it carefully measured at a machine shop and based on the measurements decided to go back with a B wossner piston. The difference would be difficult to detect without good quality measuring devices and some experience using them. My friend at the engine shop took readings over and over again until we had a strong pattern. And thats from someone who does it all the time. In my case a C size piston could have been a little on the tight side. He was more concerned with proper ring end gap than he was with the small difference in piston size. I think he said the most important thing related to this sizing up is "slap" on the piston skirt against the bore, it should be kept to the proper tolerance. I have about 5-6 hours on the new top end and its running great.

[stay_upright]

IIRC I had an A barrel and was replacing a B piston (bike was probably on 2nd piston) - I had it measured and my barrel was under the minimum size slightly (i.e. tight) for an A barrel - this was after 3-4 years use so having a B piston was completely wrong - I think the PO probably followed the rule going up one size each new piston. The B piston I pulled out had some cold seize marks on it.

The next piston I put in was also an A and is running nice so my advice would be if you have an A barrel fit an A piston regardless of age.

I hear a lot of cold seize issues with 2 strokes so that's what I would be trying to avoid.

[GMP]

The difference in size is below the margin of error of most basic measurement devices. I have a very expensive, calibrated three point bore mic set in work. I'm sure most reputable shops have similar. Anything less and don't even bother. After measuring a few cylinders in many dimmensions, and noting the appearance along with that of the pistons, I now just do a visual, clean up, and replace the piston with a micro size up. Never a problem. I can tell you that the bores are not round or straight, as one would expect, but does it matter? No, not at the numbers we are talking about. Once I was in a jam and had to put a "D" piston in a used "A" bore, again no problem.