Piston Sizes

[admacp]

I had the barrel measured "properly" today and it read 76mm exactly (280 pro '04 model).

The piston measured 75.91mm which gives a distance of 0.09mm (or 0.00354 thou).

I have ordered the biggest size piston available (I 've gone up from a "B" size to a "D" size.

Can anyone tell me the difference in sizes between the "B" and the "D" pistons. I was told it was 1 thou but now whilst reading info on the american gas gas website I note they mention each size increase is 0.05mm (or 0.002 thou). Therefore if I am going up 2 sizes, that equates to 0.1mm (not 1 thou) and I am worried now that the piston will be too tight in the barrel.

[physco]

what a load of bull!!!!.does this guy know what he is talking about!!!!!.just ask the experts as this guy seems to be p!ss!!n in the wind abit!!.

[admacp]

That's a bit OTT!!!

Any help on this matter is valuable as I and I'm sure others are, learning a fair bit from all of the replies to any of the questions on this forum.

[admacp]

Thanks HondaRS. On thinking about it, you can't just go up a couple of sizes from measurements taken on the worn piston as a new "B" size piston (to match my own "B" size) will have a larger diameter.

Really what info I need then is - what size is a new "B", "C" AND "D" piston so that I can check that buying a new "B" size will still fit my barrel correctly. Any ideas where I could get that info from? It is a Mahle piston and I have checked their website and note that Gas Gas is not listed in their list of Manufacturers they supply!

[admacp]

I had the piston & barrel checked by 2 different machine shops with their own micrometres and they both gave the same results to within a thou which is accurate enough I would think.

[widrace]

The basics:

Typically a nominal bore size will vary by .001" due to manufacturing issues. To a lesser degree pistons have the same issues. As a manufacturer (assembler) you need to build to a nominal clearance with some tolerance. Lets say that nominal clearance has been determined to be .002" and your tolerance range allows clearances to vary from .0018" to .0022". You then take all of your pistons and catagorize them by size into 3 or 4 groups. Call them A,B,C and D. Do the same with the cylinder bore diameters.

Example - if 'A' is the smallest piston in the range, it would be fitted to an 'A' bore, which is also the smallest in the range. You will need to create as many groups as required to be within your tolerance range. When all is said and done, as long as you pick the same letter piston/cylinder assembly you will achieve clearance within your tolerance range. Typically, a piston will vary .0003" within any given range.

In service, you have the option to fit a larger piston to reduce clearance if you wish. Maybe you are on the loose end and want to reach the tight side of tolerance. Depending on the specific piston, you may find the next larger size is only .0001" larger. Another piston with the same letter might be .0003" larger however. If you have the complete range of pistons at hand to measure, your all set.

Measure like this:

Make sure everything (including the mic and bore gauge) is very close to 70 degrees F. Take a mic and measure the piston wherever your supposed to. Put a towel in a vise or something to hold the mic. Set the bore gauge in the mic and zero it. Stick the bore gauge in the cylinder (with some WD-40 on the wall) and it will directly read the clearance wherever you are in the bore. Look at taper and out-of-round. Pay particular attention to the area between the exhaust port and the top of the cylinder. Finding a .0005 hole here is not uncommon. Leave it. Recheck the gauge in the mic every now and then to make sure it didn't change from a bump, etc. Calculate what you may need to do from there.

Notes:

1) The old piston will not measure even close to the same 'letter' new piston. In addition to wear, skirts will collapse .002" or better sometimes.

2) If you haven't developed good technique using these tools don't bother. Find someone who has the gear and knows how to use it.

3) .0001" is nothing, .0003" is something, and .0005" is alot

4) Any other method sucks.

5) Just put in whatever letter came out and have a beer.

6) I have never seen a circumstance where you could not put the next larger piston (into a used bore) and be out of tolerance - see #7 however.

7) Every once in a while a piston is mismarked in repect to it's intended range - and it's always bigger.

Sorry 'bout long post.

[admacp]

Excellent reply - very helpful and very detailed - thanks!!!

[wayne thais]

I just rebuilt a 2002 Sherco, the factory piston was an "a" piston, I had the distributor send me a "c" and a "d" piston (at his suggestion) I used the "c", probably could have used the "d" but it would have been pretty tight. I rebuilt a very lightly used "01" a couple of years ago and replaced the "a" with a "b", it was tight so I broke it in very carefully. Started it up and then shut it right off. Did that several times and then ran it 5 minutes, then 10, then 15. After the extended break-in it was a very good running bike.

NO I did not measure the clearances, just used dead reackoning. Might not be the best but it has worked for me.

It appears that not only does the piston wear but the cylinder lining evidently does also wear a little, either that or the pistons I bought were slightly on the small size of the lot.

When you get your new piston I would suggest that you take it to the same place that measured your old piston and see what your clearance is with it.

Wayne