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MG MGA - compression check question

I am planning to purchase a compression test system for my 57 MGA.
Is the size of the spark plug threads 14 mm? Long neck or short neck?
When checking the compression I understand that you need to remove all 4 plugs and then turn the engine over 6-10 times.
Is it correct to leave the ignition key in the OFF position and then use the starter switch to turn over the engine?

What is the safest type of oil to use for the "wet" compression test? Is standard 5/30 motor oil OK?

Any recommendations for a brand of compression tester?

Thanks -
Bill from Ann Arbor


W Chandler

"Is the size of the spark plug threads 14 mm?" >>YES
Long neck or short neck? >>Plugs are long, but gauges are all short, or rubber cones you have to hold in. Get screw-in one.

"When checking the compression I understand that you need to remove all 4 plugs and then turn the engine over 6-10 times.
Is it correct to leave the ignition key in the OFF position and then use the starter switch to turn over the engine?"
>>All plugs out, battery fully charged, throttle blocked wide open, coil LT disconnected or ground the coil to cap wire. (on screw in caps, fit a temp coil lead to ground) Stray sparks can bite you or cause fires. Watch the gauge as you crank - it will reach a maximum after about 8-10 turns.

"What is the safest type of oil to use for the "wet" compression test? Is standard 5/30 motor oil OK?"
>> Any motor oil. On most MGA and other engines with dished pistons, a common error is to squirt the oil in such that it just sits in the dish, which accomplishes nothing as to ring sealing, but does give erroneous (high) compression readings. leading people to conclude that it has improved and therefore you need rings - NO!. You must squirt the oil against the cylinder walls so the rings seal, and get as little as possible in the dish.
In fact, I rarely do the "wet" test, because of this problem, and because worn rings are pretty apparent from low comp + oil use + smoke.

"Any recommendations for a brand of compression tester?"
>> Doesn't matter much. You are only looking for comparisons between cylinders. Absolute pressures vary from many causes, immaterial. So long as the gauge gives repeatable readings it's good. I've been using cheap ones most of my professional life, and the result is no different from the high priced spread. It is nice to have the easy release button, but since I always write the numbers down, I always have a pen to push the release valve on the cheapest gauges. Harbour Freight!

FRM
FR Millmore

>>...coil LT disconnected or ground the coil to cap wire...etc, etc...

Or just leave the ignition key turned off while you spin the engine using the starter pull switch.

Andy Bounsall

You should have the throttle wide open when doing the compression check. Have some-one in the drivers seat with foot flat on floor, or wedge the pedal with a brick or similar.
Cheers John
John Lambie

Is it possible to check the compression if the engine is out of the car? Perhaps by spinning the crankshaft with an air wrench?
Joe Holtslag

Not likely, and quite likely to damage the bolt on the crank, and the threads in the crank.

If it has a flywheel and a way to mount the starter, you can hook up a battery and solenoid, which is just like in the car. On MGA/B, the starter mounts to the rear engine plate, but you need some nuts and such to screw the bolts into if the trans is not to hand.

FRM
FR Millmore

What type of ignition do you have? If it's electronic, you might want to check with the owner's manual to see if there are any special procedures to follow when cranking with spark plug wires unconnected. I have heard of some electronic ignitions being sensitive to this type of thing.
JM Morris

>> I have heard of some electronic ignitions being sensitive to this type of thing.

I hate to repeat myself, but...just leave the ignition key turned off while you spin the engine using the starter pull switch.
Andy Bounsall

I'd love to try all that, but currently the engine is sitting on a bare chassis with no body, no battery, and no wiring. The starter is attached but hasn't turned over in over 35 years. I guess I'll just pull the motor and let the pros look at it.
Joe Holtslag

Joe-
All you need is the battery, with a ground cable to the engine, a hot cable to the starter switch (which you can anchor on any convenient bolt, and a cable from the switch to the starter. Or you can substitute any stand alone solenoid for the OE switch.

FRM
FR Millmore

Hummmm....If I took the valve cover off and hand cranked it a couple of times to make sure all the valves were moving, do you think I could possibly damage the engine by cranking it over with the starter? Sounds like it might be fun!

I don't really need the starter switch do I? Assuming it is still positive ground, wouldn't I just clamp the negative jumper cable to the to the starter terminal and touch the positive to the motor frame (it still has some 35 year old oil in the crankcase).
Joe Holtslag

If you are trying to check compression, holding cables while you do it is difficult. Also, jumper cable clamps suck for making secure connections, which gives bad readings. Polarity does not matter for the starter.

If it has been sitting, spray some penetrating oil in the cylinders and hand turn a couple of turns. If it turns, you are not going to hurt anything that isn't already dead, but you don't want it dry as you do it. Spray the valve stems too, and let it sit a few hours for the oil to dribble down.

FRM
FR Millmore

This thread was discussed between 06/06/2012 and 14/06/2012

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