questions on boring a cylinder

[Pontisteve]

This thread seems dead, but I couldn' resist answering. Your solution is to perform a leakdown test. With both valves closed (motor at TDC on compression stroke), air should not be able to leak out of the engine, except a small percentage past the ever-present ring leakage.

Not only will performing a leakdown test tell you the percentage of cylinder leakage you have, but the escaping air pressure will also point out where it's leaking out at. Inevitably, it's almost always out one of the two valves (due to valve or valve adjustment issues), past the rings (due to ring sealing or bore issues), past the head gasket (due to bad gasket or warped block or head), or out the piston (thru a hole in the top or side).

With a leakdown test, you can't miss.

FYI, because of the fairly low cost of small engines to be replaced with a new or used one, spending big bucks on properly machining a new bore is rarely worth it unless you do it yourself. And even if you're not great at it, as long as you have a dial bore gauge to use, you can do it good enough to make plenty of compression to get a lawn mower engine to run adequately. It's not a V8 race engine here.

 

[jetrail]

I got the motor broken . Never got it running and i. Took a shot at it just for the experience . Once my customer repairs die down i might tinker with it again . It had a brokend connecting rod when i got it ect

 

[SeniorCitizen]

The leak down test is an excellent way to get down to the nitty gritty and tell exactly where the leak is.

Often a quicker way to tell if the rings only is where the leak is, squirt about a teaspoon of oil in the spark plug hole and repeat the compression test. If the rings are the culprit the oil will usually temporarily seal the rings and increase compression. If there is a leaking valve or other opening the added oil usually doesn't increase compression.

With only 40 lbs. compression one can usually hold his thumb on the plug hole. With compression in the good enough to start range - 70 -- 80 - it's difficult. That's just another quick way to tell if your endeavors have accomplished anything.

 

[chance123]

A good way to check for cyl wear is to get a new set of rings, set one of the compression rings squarely half way down in the cyl (using the piston) Then using a feeler gauge, measure the ring gap. I don't know what engine you have, but ring gap reject specs are readily available.

On another note and a cool tip is when your con rod gave out. If this was due to lack of oil and some of the aluminum from the con rod was transfered to the crank pin, DO NOT USE ABRASIVE! Get yourself an empty quart oil container and cut the top off leaving about 2 inches from the bottom. Then cut a "V" in each end. Set your crank in the "V" cut outs in a mannor that the crank pin is in the down position. From your swim pool supplies, get some muratic acid and fill that container so the acid covers the crank pin. This acid will eat the aluminum off the crank but will not attack the steel crank pin. (I hope this was clear)

 

[jetrail]

Thanks for the tips everyone. I know my way pretty well around a small engine but the tips you guys are giving me ive never used . Plus with repairs dying down in the next two months or so i like to tinker with equipment that is mine and i have replaced connecting rods and crankshafts but never brought one back that was like this so i have always wanted to learn how to bore a cylinder just for the experience ect .