As it turns out, I will likely leave the bottom end of the motor alone. I’ve had a chance to examine things more fully and discussed the engine with the owner of a local machine shop.
First, I saw that the pistons are marked “60over”, so the cylinders have been bored .060 inches oversized—as far as they can be taken. Likewise, the valve guides were simply bored out and replacement valves with oversized stems (now long since discontinued) were used. I have no doubt that the crank was turned down as far as it could go as well.
I doubt that a valve job is absolutely necessary, since the engine was not using oil, but if it is, it will require new guides and new valves—pretty pricey. Likewise, once the cylinder walls are too far worn, sleeving and new pistons will be required. I think the only option for the crank, when the time comes, would be to find a new one.
What this adds up to, sadly, is an engine nearing the end of its service life It’s not there yet, though. With reconditioned heads, a new cam and proper maintenance, I can still get a number of good years out of this motor. However, there’s certainly no reason to pull it apart further right now.
I’ve spent some time this week pulling the 390 apart, a task that was not as easy as I expected.
I think the intake manifold is actually heavier than the heads. I had to use my hoist to break it free and lower it to the ground. The head bolts came free easily using a breaker bar. I also used the hoist to separate them from the block and lower them to the floor. (I know from my younger days that it’s a terrible feeling to drop one of these.)
I discovered that the water jacket openings at the back of heads, by the #8 and #5 cylinders, were full of debris and completely blocked. Also, the intake valve in the #8 cylinder is discolored like an exhaust valve—yet another reason to suspect that it’s the source of the misfire.
It took a few days to get the harmonic balancer off. I couldn’t budge the bolt with just a breaker bar. I finally found time to pick up a piece of pipe I could slip over the bar to get more leverage. This is normally a terrible idea, since breaking the bolt or the tool is a strong possibility. In this case, I got lucky and the bolt groaned free.
After that, it was pretty simple to get down to the thrust plate, which is held on with big phillips-head screws. I had to give these a tap with a hammer before I could turn them.
Thrust plate off, I was able to remove the camshaft. I expected it to have some problems, especially down on the end where #8 lives, but it was pretty horrible along the whole length. There was corrosion, pitting, flat spots and uneven wear on every lobe.
I don’t exactly know how the condition of the cam relates to the misfire I was experiencing, but I do know that it needs to be replaced. It also looks like a lot of debris (rust?) has worn off the cam and has been circulating in the oil, so I’ll tear down the rest of the block so it can be thoroughly cleaned and inspected.
Last week I performed some thorough diagnostics on the motor: checked the vacuum, tweaked the tune on the carburetor, looked at the timing, etc. In the end, the high rpm hot misfire persisted—maybe even got a little worse, as I managed to coax a backfire during one test run.
While I’m not 100% convinced it’s an internal engine issue, I strongly suspect it. (It’s unlikely, but it could still be an ignition issue in the distributor or plug wires.) Digging deeper, I pulled the valve covers and the rocker arm assemblies. There were no bent pushrods and nothing looked terribly amiss at this level, except perhaps an odd wear pattern on the rocker for the #8 exhaust valve. That cylinder was also the low outlier when I did a compression test some years back.
The diagnostic flowchart goes down the rabbit hole at this point, but one strong possibility is an out-of-spec camshaft. So, with help from my son, the engine came out of the car yesterday.
I’ve pulled engines from at least 5-6 cars over the years, and each one of them has been a challenge in some way. The tight fit in the Thunderbird engine compartment had me relying on every bit of that experience. (And I gained even more in this evolution.)
Getting the hood and accessories off was routine, and with the radiator out, the front of the engine is remarkably easy to access. Likewise when the hood came off things like the throttle arm and bellhousing bolts—notoriously difficult to reach—were right there for the taking.
Underneath, the exhaust flange bolts are buried in deep, greasy recesses, and even though these had been removed a few years back when the transmission came out, they were completely frozen. I ended up removing the 16 exhaust manifold bolts instead. (Many were, surprisingly, only finger tight! I guess the exhaust guy I used doesn’t believe in lockwashers).
While lifting the motor, we got hung up on the transmission cooler lines (hate those things, but no damage done) and the motor mounts snagged on the now free-floating (but unremovable) exhaust manifolds. After some head-scratching, we worked it free and the lift had just enough height to clear the radiator support. (The car being up on ramps to facilitate work underneath.)
There’s a paradox to the engine removal in the T-Bird. The motor has to come up a few inches to clear the motor mount studs, but it has to come forward an inch or so at the same time in order to clear the studs on the torque converter. It’s difficult to do both at once. In my case, the converter came partway with the motor and some 4 quarts of transmission fluid poured out the bellhousing until I pushed it back in. I’m not sure if that’s normal on a COM transmission or if I mucked up the front transmission seal.
In any event, with the lifting out of the way, the real work is just beginning.