Bore Measurements

lefty2cox

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I'm just looking to improve this machine. It's one of those craftman vacuum/chippers. It was given to me and was in tough shape as far as oil consumption. Was leaking out the crank seal as well. I'm looking to use it a couple times a year but am suspecting the rings are worn. The cylinder is not scored but i haven't checked it for trueness. It runs but leak down isn't so good. Losing quite a bit of air into the crankcase. Right now, I'm just trying to ascertain whether or not I'm even going to get parts for it or if it's even worth it. Having trouble cross-referencing the numbers. I'll post a new separate thread appropriately titled for that stuff. Thanks.
 

cpurvis

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The gap is related to bore diameter by a number called 'pi' which is approximately 3.14159

A .00314159 change in gap indicates that the bore has changed .001"

So, roughly a 3:1 ratio.
 

Hammermechanicman

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I'm just looking to improve this machine. It's one of those craftman vacuum/chippers. It was given to me and was in tough shape as far as oil consumption. Was leaking out the crank seal as well. I'm looking to use it a couple times a year but am suspecting the rings are worn. The cylinder is not scored but i haven't checked it for trueness. It runs but leak down isn't so good. Losing quite a bit of air into the crankcase. Right now, I'm just trying to ascertain whether or not I'm even going to get parts for it or if it's even worth it. Having trouble cross-referencing the numbers. I'll post a new separate thread appropriately titled for that stuff. Thanks.
Have you run a leakdown test with a leakdown tester? Even a good engine will pass 100psi air past the rings that you can hear.
 

bertsmobile1

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If I understand correctly, I'm guessing that process would be performed with a new ring?
Nope an old ring is fine
The very bottom of the bore will not be worn so you measure the gap in say 1/2 incriments to the bottom of the bore
The gap should get smaller the closer to the bottom you get and the last 2 should be the same
Now google "Ring Gap to Bore wear conversion "
Lots of cleaver people have written spread sheets so all you have to do is drop in your original bore diameter ( from the manual ) and your ring gap measurements and it will do the maths for you.
Otherwise the circumference is 2 pye R and the
 

upupandaway

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I like that idea of using a ring, squared up with a piston, and comparing the end gap at it goes down the cylinder. Genius, or at least ingenious
Thing is, that wouldn't tell u if the cylinder is oblong would it?? Last time i did this, I measured from 12 to 6o'clock and from 3 to 9 o'clock at top middle and bottom of cylinder.
 

bertsmobile1

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Thing is, that wouldn't tell u if the cylinder is oblong would it?? Last time i did this, I measured from 12 to 6o'clock and from 3 to 9 o'clock at top middle and bottom of cylinder.
Ring in bore backlit
Ring is always round so any where that is not round will show light under the ring.
Essential for detecting pincushioning from bad boring practice.
 

lefty2cox

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Have you run a leakdown test with a leakdown tester? Even a good engine will pass 100psi air past the rings that you can hear.
I did. It wasn't wonderful. I didn't take it up that high though. Just enough to see what's up and confirm what I think I already knew based on the other symptoms.
 

Born2Mow

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I'm new to mower engines, but I've attended 4 Honda schools and certified to build their motorcycle engines.

Honda us taught to "map" the bore in 9 places, top, middle, bottom... in-line with the plane of the rod, 45° and 90°. This because bores do wear with a taper AND to an oval shape. Since mowers run a constant RPM, they may not be as critical as motorcycle engines. The ring method is a cheap way to get a handle on the Taper, but IMHO a ground ring is not going to readily adapt to the ovality of the bore.
 
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