028 AV Super project

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“A” would be the smallest bore cylinder, as well as smallest diameter piston. The piston and cylinder must match to get the proper clearance as I’m sure you know. So A piston with A cylinder, B piston with B cylinder and so forth.
I find this very interesting. this is only my 2nd saw overhaul, but I've done scores of 2 stroke dirt bike top ends, and it has always been that way, A is smallest diameter then B. etc.

I just measured my "C" marked cylinder this evening (piston is marked "C" also but is too damaged to get accurate measurements). using a snap gauge and a micrometer I took 4 measurements in the intake to exhaust port axis and 3 in the transfer-to-transfer port axis (less cylinder wall to measure with the double transfer ports). I also took 2 measurements diagonally each way about halfway up opposite the transfers. on each of the 9 spots, I checked the snap gauge with 3 instruments, micrometer, digital caliper and dial gauge caliper. most of the measurements were very close to each other. I added the 27 measurements together and the average was 1.8114" or 46.00956mm. let's call it 46.010mm.

if a worn, possibly 38-year-old, freshly ball honed "C" cylinder is 46.010mm (1.8114") diameter than I am curious what new "A" and "B" cylinder diameters would be.

I cannot find any info anywhere with measurements of Stihl A, B or C pistons. in fact, I can't even find a C piston online for this saw. I do find NOS AB and B pistons. btw, I would assume that an AB piston should fit between A and B in diameter.

as far as aftermarket pistons, I looked at them all as far as I know and the only AM manufacturer that lists piston diameters on their website is Meteor. their "A" piston is 45.96mm (1.809") and their "B" piston is 45.97mm (1.810"). and so far, I could only find the "B" meteor for sale. so, if my bore is 1.8114" minus the Meteor "B" piston diameter of 1.810 then I only have a bore .0014" (.039mm) bigger than the piston. that would give me a piston to cylinder clearance of .0007" (.0195mm). ain't workin. hope I'm missing something.
PXL_20220726_183830671.jpg
 
You're wrong with your clearance calculation. Piston to wall clearance = jug diameter - piston diameter

A 0.05 mm clearance is a usual value to look for.

So you need a 46.010 - 0.05 = 45.95 mm piston.

Capture.JPG
 
You're wrong with your clearance calculation. Piston to wall clearance = jug diameter - piston diameter

A 0.05 mm clearance is a usual value to look for.

So you need a 46.010 - 0.05 = 45.95 mm piston.

View attachment 1006059
great. I guess if the piston I ordered is too small I will send it back. thanks.
 
piston just arrived. using my Japanese digital caliper and checking that with my Japanese digital micrometer the piston measures 45.96mm/1.809 in. when I drop it in the bore with the spark plug in it drops all the way in but slows down the last half from air pressure.


 
are you saying 0.002-0.0025" piston diameter smaller than bore or 0.002-0.0025" each side of cylinder?

I have a cylinder diameter of .46mm (1.811") and the meteor "A" piston is 45.96mm (1.809") which is .002" smaller than my cylinder bore.
You want 0.002-0.0025" clearance. Differences in diameters.

Make sure you measure pistons at the skirt. They have significant taper, 0.005-0.006".
 
ebay. use the part numbers to search. I couldn't find them OEM just searching using the model number of the saw. by using the seal part numbers I found out a series of Stihl backpack blowers uses the same part number crank seals. they are Stihl BR400 BR420 BR320 BR340 BR380. I looked up an IPL for the BR400 and sure enough. do your own research though.
 

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