Low compression
 

The bargain rebuilt motor I purchased a couple of years ago has low compression on number one cylinder,it measures 40 lb.The other cylinders have 60 and 70 lbs.I lapd the valves in and adjusted the tappets,pulled the piston and replaced the .080 rings and honed the glaze off the cylinder wall.I still have 35-40 lbs. compression.I had removed the carbon ridge off that cylinder and measured the bore(cheap Harbor freight digital mike) came up with 3.94,does this sound right with .080 pistons?Thanks in advance.

Re: Low compression
 

Yep pretty close .080 would gave you something like 3.955


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Re: Low compression
 

Thanks Tinkirk,I'm going to pressurize the cylinder with 60 lbs.and check for leak down.Possible the new rings got to seat yet.

Re: Low compression
 

My rebuilt motor took about 2000 miles to fully break in, using conventional 15w/40 Shell T4 multi weight detergent oil. 1st readings at 250 miles were all over the place with one cylinder at 40 lbs. Now at 4000 miles bored over .080 on a stock head am getting 78 lbs on all cylinders. I also asked about this, and was told to start worrying only if readings were bad after a couple to 4,000 miles break in.

Re: Low compression
 

Air pressure for cylinder leakage test only need about 30-35 psi. Any extra pressure will only make the crank turn away from TDC.

Re: Low compression
 

A compression test will probably show where the problem is, leak-down is even better. A squirt of oil in the cylinders will show if it is rings or valves, I'm thinking valves.

Re: Low compression
 

I pressurized the cylinder and air escaped from the inlet valve,I hand cut the seat with the flea market cutter I had picked up a few years ago and valve held the pressure.I'm now getting a slight air leak from the exhaust valve,I will have to do the same.Lesson learned,if the engine seems like a bargain,it is not!!!