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Out of round cylinders


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Larry mentioned before in the Briggs/Kohhlar discussion that he has found cylinders to be out of round. I have heard of this in vertical shaft engines, but not in horizontal shaft engines. Does this occur in horizontal shaft engines, and if so why,since the cylinder is vertical and the walls would not be influenced by gravity. Also I have a 16HP Briggs, serial number 326431 0190-91 7506251.
What series engine is this. It is horizontal ahaft, single cylinder. Please continue these great discussions! Thanks, Rich.
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Rich,

It's not so much a factor of gravity as it is the side force generated by the throw of the crankshaft and the rod... The rod does not move straight up and down, but the base moves in a circle as the crankshaft spins. This translates into the top of the rod (and attached piston) moving up and down. Meanwhile the rod has been also pushing at an angle as it pushes the piston up to the top of the cylinder. Only at the very bottom and very top (i.e. top dead center, or TDC, it's called) of the stroke is the rod straight up and down. After TDC, as the crank continues to turn, the angle of the rod reverses and the piston is either forced or pulled (based on whether it's the power or intake cycle of 4-cycle combustion)toward the opposite sidewall of the cylinder. Over time, the sidewalls that are 90-degrees away from the direction of rotation of the crank will start wearing, as do the rings (which try to hold the piston tight and perpendicular in the cylinder). The piston actually can begin to "rock" in the cylinder. It can get so worn that the bottom (i.e. skirt) of the piston can actually hit the sidewall of the cylinder. This is called "piston slap"....

The only solution is bore the cylinder and go oversize with the piston...

By the way, to illustrate the point of the next item in the list regarding dating Briggs engines, yours was made on June 25, 1975....

Hope this helps,
Kent
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Rich, I'll tell you what I heard at a Tecumseh seminar a few years back. As the piston travels up and down the bore, it rocks back and forth slightly. Halfway up the bore on the compression stroke the rod is not just pushing up, it's pushing a little sideways as well. At the completion of that stroke, the piston starts back down and rocks a little in the opposite direction. When you combine this with the natural stresses relieved in the block from alternating heating and cooling, and normal wear, you end up with an out of round cylinder. That's why good pistons are made egg shaped from the factory. (It's called cam-ground) When they get hot they distort into a round shape and fit the bore tighter. Just ask your favorite Nascar engine builder! Your engine is a 320000 series. the 32 is cubic inches, the 6 is the design series, the 4 means horizontal crank with Flo-jet carb, the 3 is flange mounted ball bearings, and the 1 indicates set up to use rope starter. Whew, am I too long-winded guys? Let me know. Tony
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