help on overhaul failure

[Plainsman30]

When started the rebuild last spring, the found the old block was cracked, so we had to find a different block. Rebuild included magnifluxing, cooking, and decking the block. Cylinders were rebored 80 over, new pistons, counter balanced crankshaft, inserted rods and mains, new stainless valves, and 5.5 head. The pistons were a modern type with Teflon coating and thin rings. The carb, distributor, cooling system, exhaust system, and drive train are stock and unchanged since motor was overhauled.

At 530 miles the head gasket blew and coolant filled the back 2 cylinders. At 1100 miles the back 2 pistons scored. When they scored, I was driving 45 against wind, GAV open about ½ turn, advance lever down about half, water temperature at 180 and I was running about a 50/50 mixture of 10-30 and 10-40 oil.

The scored areas “face” each other. In other words, they are on opposite sides of the cylinder wall and are in the same area where the head gasket failed.

In trying to figure out what happened, machine shop checked the cylinder bores and rods, which checked out OK. We checked all the water passages in the block and the head.Everything was clear, except one hole in the 5.5 head was not open.We checked a stock head and the hole is open. We contacted the supplier of the 5.5 head, who said the hole were dead end and does not serve as a water passages.

I am not trying to blame anyone, but would like the engine to hold together next time.So far, we plan to put it back together using a stock head and stock type pistons. It was also suggested to run the GAV open further.

Any other ideas would be appreciated.

[Jim Brierley]

I think your piston choice was a good one. Pistons set too tight in the bore often result in the problem you describe. Ford said a maximum of .002" clearance. Today that is too tight, I usually set .0035 as a minimum. I use more in my engines because they have OHV conversions and I run them hard.

[George Miller]

Hard to say. As a Guess I would say pistons to tight in the bore. 3 and 4 run hotter than 1 and 2.
Also could have a vacuum leak on the intake on 3 and 4. Also slow timing will make it run hot in the cylinders, or lean mixture.

On the head gasket they need to be re-torqued several times.

[1930artdeco]

Can't help with the head question but when I run my car on the freeway I only run the GAV at about 1/8 to 1/4 open depending.

Mike

[Mike V. Florida]

The Gas Adjusting Valve provides a manual adjustment for additional gasoline to the Cap Jet, bypassing the metering effect of the Compensator Jet. The carburetor was designed slightly lean to allow for high altitude driving and, with the GAV, is able to provide additional fuel as needed. This extra fuel is normally needed when the vehicle is cold and at low engine speeds.

[Tom Wesenberg]

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The scored areas “face” each other. In other words, they are on opposite sides of the cylinder wall and are in the same area where the head gasket failed.

In other words the score areas are in line with the wrist pins. Did the engine lock up, get tight, or make any noise? Any change the wrist pin keepers came loose and caused the damage?

BTW, isn't that a solid metal area, with no coolant?

[Plainsman30]

Thanks all for the input.

My builder does not think the pistons and rings were too tight or they would have been scored all around the pistons instead of just on one side. The wrist pins seem OK.

When the pistons scored, it was loud and sounded like a semi truck applied the engine brakes right beside me. There was a lot of smoke, but the engine didn't seize up and I did get home with it. The middle ring only on both pistons is melted into the pistons on just the one side of the piston. There is signs of stress on the rod bearing inserts, and they will be replaced. Most of the scratches on the cylinder walls came out when honed. We will again check for water jacket blockage.

[Mike V. Florida]

What did you use for connecting rods? Were they trued up before installation?

[Flathead]

From what I have seen, the original Ford Model A pistons were a split skirt design. The replacement pistons now available are a solid skirt design. A split skirt can be fit with less clearance than a solid skirt which may explain why Ford called for a tighter fit with their pistons. Use the specs that come with the pistons you have, not specs for a different piston. Food for thought.

[Tom Wesenberg]

Can you post pictures of the cylinder damage, piston damage, and rod bearings that show stress.

I'm wondering if the rods were bent, or the cylinders bored at an angle?

[George Miller]

Jims clearance of .oo38 min is right on. The ring welding to the piston sounds like not enough ring gap also. The piston may not score all the way around, it runs hotter on the thrust side. The piston has a lot of load on the thrust side, when the cylinder fires. When the cylinder fires it pushes the rod down.The rod is turning the crank which is going more and more at a angle. This forces the piston hard against the cylinder wall.

[Dave in MN]

[QUOTE=tbirdtbird;1188653]"My builder does not think the pistons and rings were too tight or they would have been scored all around the pistons instead of just on one side. "

a very common misconception indeed

has it been noted what clearance was actually used?

To answer the very important question tbirdtbird poses, please measure the cylinder bore and then both of the un-scored Teflon coated pistons. Then take the two remaining middle rings and place them in the cylinders and measure the ring gap.
All this may be for nothing as the cylinders have now been honed to remove surface damage but it may give you and the rest of us a clue as to what happened.
Most importantly as the guys above have said, make sure you have at least .003" clearance. For hardworking touring engines, I set them up at .0035". FWIW: The engine I built for the Great Race last summer was set up with .004" clearance.
Hope all goes well upon reassembly.
Good Day!

[Tom Wesenberg]

Yes, to what George just said, and that's why I'm surprised to hear the damage was in line with the wrist pin, and not on the thrust side. Pictures would help to determine what caused the damage.

[George Miller]

You really need to find out for sure what happened. If not it will happen all over again. Piston clearance is very important on The A engine.

Also a lean mixture is bad news, and so is slow timing or to far advanced
You need to check piston clearance and ring gap. Plus make sure you have a clean water jacket around the cylinders.

[Plainsman30]

We used new rods rather than inserting Model A rods. The rods and bores were both checked at machine shop after they were removed and were OK. I will post more information and pictures soon as I don't have them right now. Thanks,

[racer32]

Originally Posted by Plainsman30

My builder does not think the pistons and rings were too tight or they would have been scored all around the pistons instead of just on one side.

That sentence right there says a lot about your engine builder. As a former 2-stroke motorcycle racer who has seen a few seized cylinders, I can tell you that a piston seizure will not score the cylinder all around. The piston "welds" itself to the spot where the oil film is rubbed off (or washed off in water-cooled engine). That usually happens at the hottest spot in said cylinder. Sounds to me like that "hot spot" turned out to be between cylinders 3 and 4 if I read right.

[H. L. Chauvin]

FWIW:

Just something you may want to consider:

If an "experienced" dentist cannot figure out why one's expensive set of false teeth does not fit ........and/or .......... if an "experienced" optometrist cannot figure why one cannot see better with one's new prescription glasses ........ and/or when your garage roof caved in with a 10 mph breeze, and the "experienced" carpenter cannot figure out why ........ and/or when an "experienced" engine rebuilder cannot figure out why one's simple Model A engine failed so quickly .......

After reading your comments: "but would like the engine to hold together next time" and, "Any other ideas would be appreciated."

Maybe just relax and think about it tomorrow with a cup of coffee?

Hope this helps.

[Plainsman30]

Yes the failure was in the area between 3 and 4. I don't think there are stillany water passages in this area.

[James Rogers]

What you don't understand is the concept of siamesed cylinders. When you have 2 cylinders that are as close as to not have any water passage between them they are considered siamese and connected. In this case the cylinders will heat up excessively in the siamese area and cause galling of the pistons because of the expansion of the iron in that area. The remedy is to have steam holes in the area which re-leaves the heat so the cylinders remain at a relatively constant temp. The area will gain some excessive temp but it is controlled somewhat and will keep the gaskets from blowing and the cylinders as close to round as possible. The 400 chevy small block engine has this configuration and is controlled the same way. The suggestion at this point is, DRILL THE HOLES, Ford knew what he was doing.

[Plainsman30]

I did talk to the builder. The piston clearance was .030 on all 4 cylinders when it was assembled the first time. Cylinders have now all been honed again, so clearance will be a little greater than .030. The builder will measure all 4 cylinders before final assembly to check clearance. The builder said rod bearings didn't show too much stress, but he polished the journals and will put in all new bearing inserts.