New rebuild won't start

Rebuilt a 72 360 for my CJ. Block was bored 30 over,crank turned etc. I assembled the engine and went to start it today and wouldn't start. It didn't want to fire so I assumed electrical. After a couple of hours checking distributor, wires, and plugs I didn't find a problem. I decided to check compression. Here's what I found. The first # is dry the second is wet

1, 80 125
3, 65 75
5, 80 86
7, 85 130

2, 85 90
4, 45 55
6, 86 90
8, 58 85


These numbers are obviously way low, The block was professionally machined, new pistons and rings installed. This is not my first rebuild, I staggered the ring gaps I used new Fel Pro gaskets and torqued the heads in sequence and in 3 steps. Heads were professionally rebuilt. What could cause low compression besides pistons, rings, head gaskets and valves.

 

I may be wrong but I thought the rings had to brake in to get proper comp. numbers. I was told to run some additive in the oil so the cam would break in for the first 100 miles or so. Timing is on right?

 

Cam and lifters were installed with moly lube that was included, I have double and triple checked timing. I confirmed distributor was set at TDC compression and firing order is correct. The rings do need to seat. but I think this is to low to even fire.

 

I agree with too many toys in that the engine needs to fire and seat the rings usually, although if using moly rings they should be pretty tight to begin with. Another possible cause for for the low compression may be valves adjusted too tight. If those check and you have verified the spark is there, then check for fuel. I like to squirt a little fuel down the barrel of any engine I haent started in awhile just to save the starter some work. Good luck!

 

From what I am able to determine an AMC V8 doesn't have any valve adjustment. You tighten the jam nut until it bottoms out.

 

lifters not pumped up yet ?
are the valve covers off so you can watch the valve train ?
prime the oil pump and system with a drill motor?
my guess is the valves aren't fully seating

 

Lifters won't pump up till engine starts, oil pump was packed and pressure runs between 20 and 40 psi with engine turning over. I wondered about the valves since the difference in compression, dry vs wet, isn't that much in most cylinders. My TSM is for a 75 and they have bridged style lifters. The heads are off a 72 which is non bridged but I believe they are non adjustable also.

 

You get the right pushrods? Are these the pushrods that came with the engine? Don't know offhand what the spec is, but I would expect the different rocker designs would use different pushrods.

 

72 does use a different push rod than 73 and later
I don't know the difference
and different rockers as well
the stud type with ball pivot

 

Sometimes the lifters won't pump up with a drill running the oil pump with the crankshaft static. An old trick is to run the oil pump with a drill while slowly rotating the crankshaft with a socket/breaker bar to allow oil to all the lifters and push rods. We'd do this with the valve covers off so we could watch when the rockers received oil. If the rockers/pushrods were oiling that's a good indication the lifters are receiving oil.

 

I checked the push rods on O'Rileys website as well as Bulltear, they list the same lifters and push rods on all 360's. I think the lifters have "pumped up" because oil is coming up the push rod and on to the rocker. The clearance between the rocker tip and the valve stem seems excessive. I measured one of the rocker tip to valve stem at .016 with a feeler Gage. The rocker pivot ball rests on a shoulder on the rocker stud, then the nut is tightened down, the rocker floats against the stud. The clearances seem to be all over the place.I'm thinking the machinist dressed the tip of the valve stem causing the gap between the valve stem and rocker.

 

Any luck with the start up yet?

 

NO, I have been adjusting the preload on the lifters. I might try again tonight.

 

[/IMG]I have gone thru everything, I checked the intake, changed the heads to a bridged rocker style nothing increased the compression. Checked the cam timing, everything. I finally pulled the pistons. I removed the top ring from 1 piston and inserted it into the cylinder and there is a 1/4 inch gap. Service manuel calls for .010 to .020 gap not .240. I checked the bore and acording to my super accurate HF caliper the bore is 4.105. Stock bore for a 360 is 4.08 add .030 overbore gives 4.110. That means assuming my caliper is accurate my .030 over bore is a little tight. I assumed I had the wrong rings so I ordered a new set. I got Perfect Circle E251K 30 rings. I placed a top ring in the cylinder and I have the same 1/4 inch gap. What gives ? What am I Missing.
The picture shows the block with the 2 top rings in the cylinder, the cylinder on the right is the old top ring the one on the left is the new top ring, both show the same gap.

 

I was able to start the engine by pull starting the vehicle and did get it to idle but had huge ammounts of blow by.

 

Well, just shooting from the hip, I would think the first thing to do would be buy or borrow a good caliper and get a dependable reading on the bore.

I looks to me like the cylinder has been bored at least .030 over, and if the pistons you have are for a stock bore, the only viable option is to buy another piston set that is the correct size.

But first things first. Work on getting a dependable measurement of the bore.

On another note, whats the difference if any between 304 and 360 bore sizes. Even if the motor is bored .060 over, it is tough to imagine such a large gap. Maybe the rings are for a 304?

 

The 360 is more than a 1/4" larger bore than a 304 (4.08" v. 3.75"). Unlikely you will mix up pistons or rings between them.

The 304 and 360 use the same crank, and have the same stroke. All the displacement difference comes from the change in bore size.

 

The pistons are new and have the part # cast in and are .030 over. It appears the pistons are correct. Here is what has happned. When you go to Summitt's web site you enter the information for your vehicle, year make model engine size then select piston rings, it displays all pistong rings not just the ones for your vehicle so I selected the rings it showed, thent selected .030 over. These are for a 4" bore. I guess you are suposed to scroll thru the several pages of rings, select the set of rings with a 4.110 bore and correct top middle and oil ring thickness. I have never seen parts listed this way. I guess it is my fault for not paying attention. At any rate I have the wrong rings and will order the correct rings today.

 

Ring gap with new rings .022

 

Much much better on end gap. Now for the push rod slop..