Compression check

Today I ran a compression check on Legs. First time since I've owned her. Here's what I found:

Test number: #1 #2 #3 Median

Cylinder 1: 132 135 130 132
Cylinder 2: 135 135 135 137
Cylinder 3: 109 125 120 120
Cylinder 4: 135 135 135 135


This is actually better than I was expecting. That number three was the lowest of the four also wasn't any surprise. The compression checker I used was from a box of free stuff that was on the sidewalk outside Volks Cafe in Santa Cruz about three years ago. But today was the first time I'd used it and the first time I'd looked at it closely.

A homebrew contraption, it was made of four fixtures. Down at the sensing end, there were two threaded inserts. The endmost one was probably just an extension...not sure why. It added about two inches to the length of the tester. It fortunately ended in a 18mm nut, which allowed me to thread it out of the sparkplug hole when it came disconnected from the next piece on the tester. The second piece had a check valve and was also threaded for the spark plug hole. Both this tip and the extension had O-rings at the base of the threaded ends. Nice. Its other end was fitted to a 2-ft tube that appeared to have washing machine hose fittings at both ends. Hey--works for me, they're strong clamps. Into the other end of the washer hose was a pressure relief valve. It's a press-button to relieve the air pressure in the rig. Teflon taped into that is an air pressure guage that goes to 200 lbs. That puts the typical range, 100-150 right in the middle of the gauge, helping with accuracy.

I ran the test with five "pfut!" cranks at each go. I started with three, but it wasn't gettin up to full go. Theoretically, it should, but the gauge hadn't been used in three years. At one point I put some silicone lubricant into the pressure relieve valve and re-ran cylinders 3/4. The results were more consistent after the lubrication, so those are what are recorded here. The time on the starter motor was very similar for each cylinder. So even if the numbers aren't absolutes, they're consistent and good for that inter-cylinder 10% target.

The final indication that compression in the engine is good, is that while the starter will turn over juuust fast enough to start the motor in run-ready trim, with all four plugs pulled, it will spin the motor fast enough to make the oil pressure light go out. Note to self: if you ever have to limp home (or across the finish line) on the starter motor, take out as many plugs as you can....