Minimum Compression

C

CJ Winslow

Guest
I have a 3hp Throttled Bean Sprayer engine. The engine has a 3 7/8 bore. I have put a compression tester on it and get 40 psi. The engine has professionally new valves and guides so that is not my source of problems. I put on new piston rings and still have low compression. I am not sure whether or not the new rings will break in enought to get sufficent compression to run well or not if I spin the engine over with and electric motor. I am not sure whether to sleeve and put a new piston in the engine or what to do. I have not had a chance to measure the bore diameter and piston diameter. I do know that the piston can be wiggled in the bore probabbly a good .010" Any suggestions would be good as to what I need to do to get this engine up and running well.

Thanks - CJ Winslow
 
M

Mark Thompson

Guest
Have you tried to break in the engine under a light to medium load? They normally won't break in very well running free.

Also, when you replaced the rings, were the ring side clearances checked? If the ring groove is too wide from wear, the rings will rock back and forth and never seat. You may need to cut the grooves wider to make them square again and get wider rings.

-Mark Thompson
 
K

Kevin O. Pulver

Guest
Mark is correct. Dave Reed made a believer out of me. My new sleeve and rings gave no copression. Cutting new, square, grooves and overwidth rings really woke up my air cooled chore boy. Kevin
 
A

Andrew Mackey

Guest
Check Harry's archives for my postings on measuring pistons and cylender bores. The piston and the cylender have to be reasonably round and straight in order for new rings to seat. Did you hone the cylender? If not, the rings will not seat in a glazed cylender. Did you check end gap? Std piston rings in a .020 over Cyl. will blow by like crazy. Have you run engine under load? If not the rings won't seat - too cool a cyl, and no real developed pressure to make the rings wear in. You need to make the engine work hard enough to boil water in the hopper, varying the load for up to at least 10 hours of running time, in order to gain any benifit from the new rings. for a Hit & Miss engine, that means making the engine hit at least 50% more than at idle - with periods of NO LATCH-UP lasting up to 3 minutes continious ! This means continious 4 cycle operation with out the governor locking out the exhaust valve. For a Throttled engine - 1/2 throttle constant load, with occasional full throttle sessions under load for 3 minutes at a time - in order to seat them in. Our engine club (North Jersey Antique Engine and Machinery Club) just re-ringed a 21/2 HP Lausen H&M engine - It had idled for about 20 hours running without seating the rings. At our local show - the Sussex County Fair, We belted the engine to a 3 cylender Kewanee water pump and ran the engine for a week - 10 hours a day, and the last 3 days, loaded the engine as above. By the end of the week, the engine compression was so good, that the exhaust valve had to be held open for the first couple of revolutions when starting! By the way, loading the engine to the point of no latch up was a neat way of demonstrating how a H&M engine works. It really drew a crowd when I loaded the engine up, and on the last day of the fair, the engine was really hard to keep from latching up, as it was making full power! Andrew
 
K

Kevin O. Pulver

Guest
Good post Andrew. All excellent points, I'm sure that very few engines ever get any load at all. Water pumps sound like a very controllable load to put on them as well. I always thought it would be neat to have a water pump on one, with a ball valve to restrict output and control load. You could even put an electric solenoid in there and be able to make your engine coast or hit with just the touch of a button. Kevin
 
T

Tom Cwach

Guest
I have never put a compression tester on an old engine. I have measured and calculated compression ratio. The ones I have are about 3:1 compression ratio. Off the top of my head I think atmospheric presure is about 14.5 psi. Multiply that by 3 and you only have 43.5 psi. You won't have 120 lbs compression like a modern engine.
 
D

DICK STAATS

Guest
IF YOU DON'T RE-GROOVE THE PISTON---YOU ARE "FLOGGING A DEAD HORSE". MY TWO CENTS WORTH. EH?
 
A

Andrew Mackey

Guest
Your 'modern engines are currently running at about 6 - 6.5 to 1 ratios due to air polution requirements. 110 to 125 PSI + or - 10% is the rule It is difficult to get a true comp pressure reading, unless you belt the engine to another, and check the pressure at speed. Just rolling the engine over by hand, will not give a true reading. As long as the piston bounces back on the compression stroke, you should have enough compression to run! A good test is to pull the engine up on compression, and let go of the flywheel. It should bounce back. If it just stops, or if the pressure drops off rapidly after a small rebound, then checking clearances and rings are in order. I had a 3 HP model E Hercules that had great compression - until you backed the piston up. The problem, when running, was the engine chucked oil off the piston so badly, that you could actually watch the oil fly out the engine bore, when it fired! I found that the bore was so badly worn out of round and tapered, that I ended up replacing it and the piston. It had worn .020 out of round, and was almost .040 oversized at the middle of the cylender! The rings had .015 groove clearance, and the best one had a .050 eng gap clearance at the center of the bore! The compression was pretty good at the top of the cylender though, the engine still ran well. Andrew
 
Top