After driving 50 miles then sitting at an idle for 20 min.the 48 began to over heat and steam...I have no problems at normal speeds...Thermostat is good 160 deg. and fluid is clean.Any solutions?

Chef, I don't have the answer for you, just a question.

Mine gets up to 180-190 driving around town. I haven't put many miles on it yet. Was wondering if I should get a 160 or 170 and put in it.

The motor is still under 5000 miles and normally runs between 160-170 deg except for steep hills...I was arriving at a show of over 900 cars and sat at idle to 1 MPH for a good 45 min. it was reading 220 the whole time and then began to steam...I made it by keeping it in neutral with a little gas to keep the fan moving

Normal - You can't let an engine idle for any length of time after highway driving. I would suggest pulling out the hand throttle and running it at a very fast idle so the fan can pull some air through the core. The 1948's came with a 4 pound pressure cap. This will raise the boling point a little as will 50/50 antifreeze. If the cores are in good shape you could run a 7 pound cap. This will raise the boiling point a few more degrees.....and an accessory overflow tank will catch the coolant and return it to the system if it does decide to puke.
An eletric booster fan in front of the core will also help.

My 52 has always had the same problem, mostly on real hot days in parades never boils but pegs the temp gage on the high end. I found a 5 blace fan from a truck engine, it works well for the slow stuff but at 50 it will drive you nuts. Back to the 4 blade. There are 6Volt fans available for about $75.00 and I have contemplated purchasing one and installing as a pusher to add to the stock fan in parades.

Chef, you say that the engine has 5K on it. I assume the block and head were properly cleaned by a dip process. Is this right? If so, then maybe the problem is elsewhere. I suggest making the following checks:

Radiator
a. Drain the radiator. (catch the antifreeze mix, of course.)
b. Take off the top and bottom hoses.
c. Hold your hand over the bottom outlet and the top one as well.
d. Have a helper fill the radiator to overflowing.
e. Discontinue filling.
f. Release hand from bottom outlet, observing height of flow and listening for a sucking sound in radiator's flues. Good, immediate evacuation of fluid (water) and almost no relatively long sucking sound will tell you the radiator is probably OK. A relatively slow evacuation of water and a relative long sucking sound indicates that some of the flues are either stopped up or partly stopped up.
g. If not good evacuation and/or prolonged sucking sound, reverse flush the radiator by using you air chuck (wrap a towel around it) and blow the radiator out from the bottom through the top. Use a hose or something to route the water over board, if you care to. Otherwise it'll wet everything. Do the reverse flush process until you have a better flow.
h. If no good results, either get the radiator rodded out, a new radiator or new core.

Head gasket
a. Take the thermostat out.
b. Replace the thermostat housing (leave hose off).
c. Fill the radiator until the water is level with the top of the thermostat housing opening.
d. loosen the fan belt and trip it so that the water pump won't turn.
e. Have someone crank the engine over. Water should just sit there.
f. Look for compression bubbles coming out the thermostat outlet.
g. If none at "f" above, then start the engine for a short to see if any form then. Again, water should just sit there as if minding its own business.
h. If none, then the head gasket is OK.

Radiator fins
a. Check to see if the radiator fins are stopped up with bugs (or them there 7 or 14 year zakadas (sp) or what ever year or proper name for those critters happens to be. (I drove my 41 through MS/LA one summer back in the early seventies. I was a cruising along, heading west on I-20 at about 55MPH when all of a sudden there was this whap! whap, whap. I didn't know what was a-going on. Soon discovered in the headlights these alien UFOs a-coming at me and splattering themselves against the car and my windshield. Just turrble! They wuz bug guts everywhere. You bayou folk know what I mean.)
b. If stopped up, take the radiator out.
c. Use an air blower chuck on your compressor, water spray or whatever and blow the bugs out. Back to front will work best (after all that's the opposite way they wuz going when they got stuck in there in the first place.

Best,
Charlie

BTW: I think Chevgene's advice that the engine will heat up if sitting at slow idle for any extended period. Unless, it is worn slap out of course.

BTW2: If the block was cleaned and none of the above defects are discovered, then go into the house, get your Colt 45 and shoot the thing. (Grin, of course)

Thanks for your input guys....I found a new discovery... 1 Missing nut and other loose nut on header pipe to manifold flange..I also think the gasket(donut)is missing,I have not taken it apart yet..By looking at the flange I think it is the wrong one it does not torque up tightly leaving an exhaust leek...Would this problem have anything to do with exceesive heat problems??

I can't imagine how. Noise for sure but acerbating the overheating? I doubt it.

Yep, Charlie has it right. There is no way an exhaust leak would cause your engine to over heat. It might be noisey or burn off some wiring or paint but it would not make the engine run hot.

I have been driving my dads '40 Special Deluxe every weekend for the past month. I've noticed that the car will start to run warm when driving slow in town, not bad but closer to 190 - 200. If I get in a line of cars going into a show or real heavy rush hour traffic it will get close to the red (210). Once on the road it starts to cool down from about 35 up to 55. I have been using the throttle to bring the idle up when it gets close to being really hot and that seems to help maintain it. Its never boiled over / puked coolant out. It doesn't seem to matter if the weather is 90 or 70, the car still acts the same. I've just learned to not get over excited and now feel comfortable how the car is going to act. Gene has noted exactly what I'm finding with our car and it seems to be normal to run a tad bit warm without air going through the radiator. Don

After replacing the header to manifold gasket,brass nuts and tightening it up good I lost another nut after about 100 miles...I had a real good exhaust leak .I was on a tour at the NW meet and when we arived at a rest stop I pulled a 3/8"nut from the bumper guard and mended the leak temperarly...after returning to the hotel a member just happened to have a new gasket and 2 brass nuts..When I dug into it I found the gasket had totally unravelled like a cork screw and was inside the pipe??I have made it home but I have to keep re-tightening the nuts often
The only advice so far is to replace the studs...what do you think???

Use steel nuts.

Chev Nut,I've had both types drop off..Brass and Steel..If the alignment is a bit off creating a poor seal and the exhaust is blowing on the nuts could this be the cause for them to loosen and eventuly fall off??

Alignment a bit off?....Are ther three alingment rings is place and not distorted? This is a must. I would stat by replacing the studs and using steel nuts like the factory used. If they continue to loosen I would find thin check or lock nuts and double nut it,

Are you talking about the exhaust pipe flange to intake manifold bras nuts? I usually try to use a "real stamped metal flange" rather than the flat piece of steel plasma cut one with the slotted bolt holes. But if you have only the repop flat piece for a flange double nut the brass with two steel nuts and jam lock them together.

Three alignment rings??There are no additional hardware on my 48...gasket to manifold..pipe to gasket and flange around the pipe...I do have an old gasket in my drawer that has a metal bushing inside the gasket.Would this be the correct one other then the one thats just a donut??The flange on the car presently is the cast type but I did purchess the flat one thinking the casted flange was not correct..The threads on the studs are good and I am a bit concerned about having to remove them in fear of them breaking
Why do they sell the brass nuts as replacements if steel is correct?I thought the brass did not expand as much?

There are also three alignment sleeves that install in the intake ports of the cylinder head to align the manifold with the cylinder head when it is installed. Do you have those as well?

There is an illustrated view of the manifold sleeves in your parts book if you don't think you have them installed.

OK - I think he is talking about the exhaust pipe packing. The original for a 1948 was a thick abestose ring, thats all. It slipped over the pipe extending above the flange. There is also a heavy duty version that is metal but the same size/shape. I would suggest starting with a new packing, use regular steel nuts and retighten after a few miles of driving (so the ring seats in) and then install a seconed sets of nuts below the first set. They will lock the nuts in place and prevent their loosening. Try this experiment and no one will notice double nuts under the flange,

Some years used steel nuts and some brass. In the dealership we used to use brass for replacement thinking was they would not rust tight. They can tend to loosen easier due to ledd friction on the threads.

I guess we are not all talking about the same thing???header pipe to exhaust manifold

That is the one I was talking about (double nut the flange to the intake)

NOT:
The 3 alignment rings that go between the manifold and the cylinder head.

Yeh MrMac I got yah.With the casted flange I do not have room for a double nut..I will try the gasket will the bushing in the middle,it is flat at the top and may create more room on the stud