1956 Ford Thunderbird

I recently had my 56 Ford Thunderbird in for an engine rebuild. It has a 312 cu V8. When I got the car home I noticed an oil leak and brought it back to the engine builder. The oil leak was discovered and I got it home a few days ago. I drove it today and when I got home noticed it was dripping oil again. I am not sure if the rear main seal is leaking, or if there is residual oil dripping fro the frame and cross members. Any ideas? I definitely will have to bring it back to the engine builder for a check up. Hoping it’s something small. Any thoughts?

You’re doing the right thing in bring it back. It’s impossible to speculate from anywhere but under the car.

Isn’t the 312 one of the Ford small block family? 289, 302, 312, … up to 344 or something like that? The blocks are more or less the same, maybe even the exact same casting, and the displacement difference is only due to the various crankshaft designs.

If so and the 312 is a Ford small block, the rear main seal can be installed wrong, like backward for example. Apparently that is a common mistake made by inexperienced rebuilders. There’s a lip on it, and the lip has be face in the right direction. Likewise there’s a sort of ridge on it, and that ridge has to go into a corresponding channel. So something like that is a possibility.

Before I assumed that was the problem though I’d want to be sure that is where the leak is coming from, as there are more likely candidates for general oil leaks, like the valve covers, timing covers, etc. The shop can clean all the oil off, high and low, front and back, as best they can, then have you bring the car back every few days to see if they can spot the source of the leak. It may be in fact there is no oil leak, just oil left-over from the rebuild now making its way downward due to gravity. I wouldn’t bet on that, but keep it as a possibility anyway.

The other common way to systematically find oil leaks is to put in some UV dye into the crankcase, then a few days later use a UV lamp to look at the exterior engine. That method is said to be pretty effective at narrowing down the source of oil leaks.

Two piece seals can be a pain but the rebuilder should be able to handle it. There are other possible sources for the leak but none as likely.

Was there a 3/16" metal tube at the rear of each valve cover prior to rebuilding? Were the tubes reinstalled? They were prone to leak but unnecessary when the engine is properly rebuilt but the holes in the valve cover had to be plugged.

We wouldn’t be able to answer this one for you. Residual oil is a possibility…but it will be in decreasing frequency of dripping…in fact it will drip really slowly. A piece of new cardboard under the vehicle will be helpful for this. Only time will be able to tell you whats going on. If its residual…it will slowly stop over time… say 3 days till completion? Maybe sooner…but something like that.

If you have an active leak…you will see drops of oil continue past 3 days…that’s my thinking anyway…I’m probably pretty close on the time frames. I would inform your builder what you are thinking and then do your cardboard test over a 3 day period (While using the vehicle for daily cruises)…if it keeps going bring her back in


The 312 is a Y block, nothing like as 289/302.

The 312 engine came before the 289… In 1962, the first of that family started as a 221 V8 in the intermediate Fairlane. The displacement grew to 260, then 289, then 302
The 312 started in 1954 as. 239 in the Ford and 256 in the Mercury. By the next year the Ford version. Increased to 272 and. then. 292. The Mercury version jumped to 312. This,engine went onto some of.the. Fords. This engine was called the interceptor. This engine also went into the Thunderbird. I hope my .memory is close to accurate

I go back far enough to recall working on a few 292 and 312 Ford engines and the plumbing to oil the rocker shafts is the most memorable problem. The engine were notorious to leak when they got old and they got old long before 100,000 miles. I worked on a 1956 T-Bird with the 312 engine and a 3 speed manual transmission. It had no power steering or power brakes so you had to appreciate the car to deal with it. The spinner knock off factory wire wheels were very nice. A local motorcycle mechanic trued and balanced them.

@“Rod Knox” These Y block Ford engines were known for the oil passages through the block and cylinder head plugging up. There were kits offered to bypass these passages and run outside the block.

Man that brings back some old memories. The first engine I ever tore down and overhauled was a Y-Block 292. It was in high school auto shop, farther back than I can remember anything about the seals. The school sold most of the student engines at the end of the year, but mine went into a school bus. The instructor must have trusted my work. I’ve often wondered how well it served.

Did the seal look anything like this?

Triedaq: I also remember Y blocks with the bypass oil tubes running into the valve covers. Crude but effective and inexpensive.

Properly done, the engine should not be leaking oil anywhere. In my opinion the engine should be bone dry and there should be no residual oil anywhere.
There should also not be any issues with plugged oil passages if the job was done right as that would involve vatting the block out and blowing out all passages with compressed air prior to assembling anything.

Is the leak near the front or back of the engine or can you not determine this?

A second question (and I don’t mean this to be taken as too personal or nosy) is about how much did this engine build cost you? There’s engine builds and there’s engine builds…

These Y block Ford engines were known for the oil passages through the block and cylinder head plugging up. There were kits offered to bypass these passages and run outside the block.

I’m that old too. The oil return holes in the head were badly undersized, and unless the oil was changed religiously (sometimes even that didn’t help) small puddles of oil would bake in pockets in the head, then plug those undersized holes up. Then LOTS of oil would bake under the valve covers and really plug the engine up badly. Soon the valve covers were full of oil, overcoming the valve seals and smoking like a coal furnace. Very common in the '60s to see a Ford with 50 thousand miles smoking badly.

And I recall installing a few of those external quarter inch copper oil return lines from the valve covers to the oil pan, too.

Lots of great ideas here.

Allow me to make one comment on crank seal leaks; it’s not uncommon to have a seal leak while the engine is cruising down the highway but not when parked or even idling. Cruising down the highway develops more crankcase pressure, and with seal leaks the crankcase pressure can force the oil past worn/shrunken/improperly installed seals. In these cases there’ll be residue from the leak, but may not be drips left on the cardboard.

I agree with OK4450. The engine should be bone dry, but that’s assuming it was a true full rebuild. I also support his question about what was actually done. It’s not uncommon for someone to call even a valevjob a “rebuild”. Knowing would help.

But in truth, it needs to be assessed and fixed by the shop that did the rebuild. They’re in the best position by far to get it corrected, and if hey stand behind their work they will.

Let us know how you make out.

One of my early jobs was as a helper on a C550 Ford delivery truck with a Y Block. The lack of good oil flow to the heads woulf cause the pushrods to bind, bend and come out from under the rockers. More than once we would have to remove the valve covers, remove some pushrods and pound them straight with rocks and reinstall them.
That wasn’t as bad as the day the starter went and I had to push start it whenever the driver stalled it.
Perhaps because of that truck, I have never owned a Ford vehicle.

Some additional food for thought but what if this is a rebuild gone sour and excessive blowby is causing oil to puke out of the road draft tube…

Fix or repair daily
found on road dead
first on race day

Take your pick


Hey, it’s doing better at 59 than I was!

The Ford 292 was one of the greatest selling points for the Chevrolet small block.

Before we become too hard on Ford’s Y block engines and the problems with the oil passages plugging up that go to the rocker arms, there were other engines of that time period that had the same problem. The Chevrolet Stovebolt 6 had that problem as did the Ford 216 cubic inch six that replaced the Ford flathead 6.
I purchased a 1955 Pontiac back in 1962 that had been overhauled by the Rambler dealer before I bought it. I had all sorts of problems with the rocker arms chirping like crickets. The rocker arms were mounted on studs, so there was no rocker shaft and hence no outside oiling line kit was available. I had the studs pulled and cleaned a couple of times. The engine would then quiet down until I was on the road for a couple of hours. Then the rocker arms would begin chirping. I think part of the problem was that an oil filter was an option on the Pontiac in 1955 and the Pontiac I bought didn’t have that option. I did get the oil filter system from a wrecking yard, removed the block off plate and installed it. My guess is that the engine overhaul consisted of replacing the rings and grinding the valves. Later, I owned a 1965 Rambler Classic 550. It had the 199 cubic inch engine, which was a destroked version of the 232 cubic inch 6 engine that came out in 1964. I faithfully changed the oil every 2000 miles and I still had the oil passages to the head stop up. I had about 90,000 on the car at the time. The shop I took the car for the problem removed the cylinder head, attached a piece of a speedometer cable to a drill and rooted out the passage. The shop did a lot work on light trucks and the International Harvester trucks used that AMC engine, so the shop mechanic was familiar with that problem. There was a sharp bend in the oil passage where the oil sludge would build up. The estimate for doing the work was $37.50 back then. However, the repair did exceed the estimate–my bill was $37.75–head gaskets had gone up 25 cents.
All these oil passage problems to the rocker arms made me wonder if the flat head engines weren’t so bad after all.