Cylinder bore size

[Guest]

I took the 23D out of allis and down to the machine shop to have it bored oversize .020. I got it back today and started re-assembly and definetely had a problem. The new piston assy. I got from Briggs, (piston, rings, etc), says .020 right on it, (stamped into the piston top, too), and the piston will fit the bore, but the rings won't even come close. The ends butt together. Keep in mind they came together as an assy. Now for the good part. A stock bore 23 series measures 3", we all know that. However my freshly overbored block now measures 3.010. Kinda strange for a .020 overbore. How is it that a genuine Briggs part that says .020 will fit into a .010 hole? Have I got a mis-marked piston and the right rings? Or are they both packaged wrong? Anybody else ever run into this? I've had a lot of engines bored, and this is a first for me.

[Kent]

Hey Tony - where did you get your parts - I thought they were NLA.

[dlcentral]

Tony, Sounds like your machinist either mis-understood or made a boo-boo!Thankfully a boo-boo that can be easily fixed! Briggs states that when boring oversize the finish size of the CYLINDER! is to be the oversize,in your case .020.Hence your cyl. SHOULD mike up to 3.020 By boring the cyl to the recommended oversize the piston then has the exact cyl. to skirt clearance the factory recommends.Hope this helps you! dlc

[Guest]

Okay guys, now it really gets weird. I went back out and measured the piston with a mike, and it is 3.009" Again the piston is stamped .020 right on it. Surely Briggs doesn't spec that much cylinder to piston clearance? I can't help but think that I have a .010 piston mis-marked to read .020. By the way, the part # is 294678, be advised that is only for the 23D series with 0129 spec #. Other spec #'s call for a different assy. thanks for your help, I'll let you know what I find out, if you have any more ideas, I'd welcome your input.

[Al]

Hi, Sounds to me like a bad part. The piston may really be .010 and the rings 020. The piston should be measured at right angles to the wrist pin and near the bottom of the skirt. I need to look but seems to me as I used to see about .002 to .003 clearance on those, Aluminum and iron blocks are different. Briggs doesn't really list skirt clearance, and my memory fails me right now. Since I sold my boring bar, I don't deal with much any more. Rings shoul always be checked in the bore and end gap measured. Occasionally they need to be "filed". To do this put the end of a fine flat file in a vise with the tang pointing to you. Hold the ring in both hands and push the ends of the ring against the file and draw towards you, check the gap each stroke. On many 2 cycles the OS rings often need to be fit. Better to be too loose than too tight. TOo tight and they will tear up the cyl when hot and break, then score it. Good luck, Al

[Kent]

I've only rebuilt one overbored motor (a bike motor) but it seems to me that the cylinders in your block should mike out to about 3.020, not 3.010. It does not suprise me that your 0.020 piston fits in the 0.010 overbore cylinder. The piston should not fit tight in the cylinder, it needs some room to expand. The rings make the seal. That is why your rings seem to big. I think you have the correct parts (pistons and rings) but I would take your block back to the machine shop and ask why didn't they bore it 0.020 over like you asked. Then again, maybe the Briggs motor is totally different from the Yamaha I rebuilt and I could be out to lunch. Good Luck.

[dlcentral]

If the piston mikes up to 3.009 that dosen't even sound like a .010 piston as it should be 3.006 the cyl. to skirt clear. MUST be at leat .004-.006 or the eng. WILL sieze up in 5 min. of running,GUARRANTEED!Air cooled engs. need more clear. than automotive engs.as they run hotter. Whatever you do make sure to maintain that cyl. to piston skirt clearance of .004-.006,the rings if needed can be file fit to the bore with min.[no less than] end gaps of .006 or they will touch under oper. temp. possibly breaking.My 1 1/2 cents! dlc