304 compression?

[Hedgehog]

After dribbling the question of swapping out my AMC 304 for a 360 I made the decision to make the change. But while getting ready to remove my AMC 304 I decided to do a compression check. More to know what to tell potential buyers and to have a general knowledge of the condition of the engine. All plugs were wet, but the thing hasn't done anything but idle for a very long time. The engine is producing 138 lbs. (3 cylinders), 132, 135, 128 and I can't get to the two by the battery with this gauge. That didn't seem to bad until I checked my Chilton's manual (piece of the thing doesn't even mention a T-15 Transmission ). I hate that manual, but the charts are copies from the shop manuals so at least they are reliable. It says that '72-'75 should have AMC 150 lbs. MINIMUM and the '76-'81 should have 120-AMC 150 lbs. Quite a difference if you ask me. So, is a '75 with 129-138 lbs. pressure an engine in need of a rebuild?

 

[Scooter402]

Considering that 138 psi is 92% of AMC 150 psi, I don't think it's in that bad of shape. I understand it says minimum cranking compression, but I think it's really not that bad off. Since all the cylinders you were able to check were within 6% of each other, I'd say the engine is in decent shape.

Question -- when you did your compression test, did you remove ALL the plugs, or just for each cylinder you tested?

Also, are you certain that your engine is a 75? I'd get the head casting numbers and look them up. You could also get the casting number off the block, if you were that curious.

 

[grozam]

What wt. oil are you running? Heavier wt. oil will bring up the compression.

 

[Hedgehog]

Scooter402 - This Jeep is new to me so no I really don't know the history of the engine. The AMC serial no says it has a AMC 304 . When I pull the motor I will check the numbers.

I removed all the plugs.

Agreed, ~135 doesn't seem bad at all, but as you pointed out, with the manual saying MINIMUM I had to sit back and scratch my head a little.

Grozam - I haven't changed oil in this engine yet. The Jeep was a project from the beginning and the oil looked very fresh when I got it so the oil hasn't been changed. I was gong to do that just before putting it in active service.

This motor certainly does seem to run well, while sitting in the driveway anyway. Put the plugs back in and he fired right up, no hesitation, no smoke. I think it's a good engine, but the 360 talked to me so that is the engine that I will run.

The AMC 304 just might get a good going over though......

 

[Scooter402]

Great! Sounds like that will be a good engine until you do pull the trigger on the 360. I know I would like the extra grunt that the 360 has.

When you say AMC serial number, are you referring to the casting on the side of the block near the motor mount, or by the VIN? The CID of the engine will be cast into the side of the block, in case you haven't looked there.

Kind of like this:



Or some later models had the displacement in liters cast into the side:



Sorry, couldn't find any pictures of one with either "AMC 304 " or "5.0L".

 

[Hedgehog]

The 360 motor is hanging on the engine hoist in the garage as I type this. It has the 360 cast in the block.

Yesterday I changed the ring gear on the used 360 flywheel I just bought. Never done that before. Just place the flywheel so the ring can fall down. With a punch or wide chisel smack the ring with a sizeable hammer, you can feel it move until it falls off. The new ring won't go on until it's heated. I don't have a torch, but I do have a kitchen stove. I baked the ring at 500 degrees for an hour while freezing the flywheel. Guess what. It didn't work so the stove was turned up to 550 degrees for another hour. By golly it almost didn't work again, but all it took was a few taps and I mean taps with a hammer and the ring started on the flywheel a few taps later and the ring settled with several taps all around. Then that satisfying ping of a well set ring.

The flex plate has been removed and the newly machined 360 flywheel is now in place. Just set the wheel in place, turn it until the holes line up properly, they will only do it in one position. 105 ft./lbs. on the bolts was fun to accomplish alone, but is easier the second time after remembering the Lock Tight. The pilot bushing went in like it wanted to be there, I like that. It's interesting to set things like pilot bushings. Tap them in and they make a sort of hollow sound, when in all the way the sound instantly changes to a solid hit sound, you know your bottomed out.

 

[Scooter402]

The sound of progress -- it's like music!