Diagnosis and Repair of Your Imperial's Freeze Plugs


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Question from Mark:

Are there 2 different sets of plugs: the (hard?) "drain" plugs and the "soft plugs." Are there 2? Are the soft plugs located behind the hard ones? Do I need a wrench to remove the outer hard plugs, or do they pop out?  How about the inner ones?

Reply from Dick:

The two types of plugs are alongside each other, both just above the pan rail on each side of the block. The drain plugs are screw in plugs which look very much like normal pipe plugs (which can in fact be substituted, they use pipe threads). They have either a hex head or a square head, I've seen both types. Usually they are either 1/4 or 3/8 pipe thread plugs, (which means they are 1/2" to 5/8" OD). They are for the purpose of draining coolant out of the block, and should be opened when ever you are trying to flush all the old gunk out of a car's cooling system. The core plugs, AKA soft plugs, AKA freeze plugs are in the same area, but are much larger, about 1 1/2" diameter, cup shaped, and press fit into holes in the block. There are 3 per side on Mopar blocks of this era. They have to be removed carefully, and there are at least two ways to do this. One way is by cutting them through the center with a very sharp cold chisel (be careful not to drive the plugs through the hole, they can drop down inside and be a bitch to retrieve), and be careful not to smack the plug so hard that you damage a cylinder jacket, which is right behind some of them. When you have cut a large enough hole through the center, grab the remains with a good channel-lock or vise-grip and twist and turn the bugger until you work it out of the hole. They are tough, but once you've done one, you'll get the hang of it. The other way to do it is to drive one edge of the core plug into the block so as to get the plug to tilt out from the opposite edge, then grab that with your channel-locks and twist and pull it out. It helps to know how deep you can drive an edge in, and this is dependent on how close the cylinder bore is to the plug. You can sight down from the spark plug and see what is behind each one. If you do one that is between cylinders, you don't have to worry about hitting anything. If you do one that is right on a cylinder, you may hit the cylinder jacket before you can get it to go in far enough to tilt out the opposite edge. You'll just have to experiment with each one. Stay with it, you can do it! You install new ones by cleaning the holes carefully to scrape off all the old remains and rust, then put a skin coat of RTV sealant on the outer sealing surface of the new plug, and drive it into the block (NOT TOO FAR!) with a socket and breaker bar that fits well so you can tap on the end of the handle to drive it in squarely until it fits like the original. You'll find they are available in either brass or steel. I use brass for all but the center plug on each side (the brass is forever, the steel is to be a sacrificial anode and protect the block if you ever get lax on your anti-corrosion protection.) I put the steel one in the center position because it is the easiest to change.

Question from John (413):

While at the trans shop trying to figure out why my '65 wouldn't shift right, (see Problem 1) the mechanic and I noticed coolant dripping fairly steadily. Instead of a hose or something, it was seeping out of the side of the block, on the left side, roughly 5 or 6 inches above the oil pan, and roughly in a vertical line with the oil filler cap. shit. Of course, the transmission guy (something of an alarmist) immediately said I need a brand new engine block, but my machinist and mechanic both highly recommend Aluma-Seal in the radiator to plug up a leak like that more or less permanently. Failing that, the machinist suggested cleaning the area down to bare metal with a burr grinder and sealing it with epoxy, which he says is a common repair to newer iron blocks that aren't nearly as heavy as a 413 and are prone to cracks & leaks. Now, this has apparently only been actively leaking for a few weeks (I'd noticed coolant on the ground, but thought it was from another car). I live in Tucson, and the coldest it's been since I've owned the car is in the mid-20s. Of course I have the right coolant/water mix, so there is no way it froze! Nor have I overheated the engine, but no telling what happened to it before I owned it. Why would such a leak just start now? If it's a crack, it must have been there for years, I am told. Maybe someone had sealed it before and it opened up again? The trans guy suggested it basically rusted through from the inside, but neither my mechanic or machinist thought so, having never seen such a heavy block just rust out. OK, my question is: what would you do? Financially, I am NOT in a position to pull and rebuild the engine, get a new block, etc. Is it doomed or is this a minor thing?


From Dan:

The first thing you should have checked are the freeze plugs.  In case you didn't know, freeze plugs are found both in the heads and on either side of the block between the oil pan the the deck (where the heads attach).  Their purpose is to pop out in case the water in the block freezes so it won't crack (water expands when freezing and will crack the block if it doesn't have anywhere to go).  It's not uncommon for these to develop leaks, rust through, of just fall out.  I would take the car somewhere and have it put up on a lift where they can be checked. if they need to be replaced, (usually 3 per side) try to find brass plugs rather than steel, they hold up better. This is probably all that is wrong with your car. 

From Jim:

How about the drain valve on the side of the engine? If the leak is coming from behind the exhaust manifold, it may be difficult to see easily. Or how about the head gasket? Is there coolant in the oil? Does the exhaust have a sweet smell? It seems unlikely that the block would be cracked or corroded. I would try a good stop leak (Dike is my favorite, from Conklin) and see what happens. The prospect of removing the engine is not appealing, and I would want to be pretty confident that it is necessary before loosening the first bolt.

From Joe:

Don't forget to check for freeze plugs being rusted through. Many of these are hard to see with accessories and manifolds in place. I have had several of these fail of Chrysler products over the years. There are helpful replacements with a design which is basically two large flat washers with a rubber doughnut between them and a bolt & nut with lock washer. Remove the old freeze plug and slip the replacement in place and tighten the nut. These are available at most car parts stores.

From George:

Personally, I never cared much for the canned block sealers. I was always afraid they would seal up something else, such as the heater core! Plus, the permanence of such repairs is iffy. Of course, the best repair would be to replace the leaking part. First, I would have the cooling system pressure tested to be as certain as I could of where the leak is coming from. It may be just a rusted out freeze plug, which can be replaced without engine removal. There are 2 types of replacement plugs, the original all steel disc type, which have to be driven in to swell them after removing the old one. The other type is a steel disc with a rubber plug, much on the principle of a boat transom drain plug, but instead of a lever to flip over to swell the rubber part and make it tight, there is a nut to tighten to swell the rubber part. This kind is much easier to install in tight places. Simply stick it in the hole, and tighten the nut until the plug is good and snug. If it turns out to be a external crack in the block, the epoxy method is an acceptable repair, depending on location and severity of the crack. The product named J&B Weld, available at all auto parts stores, has a very good reputation for patching cracks in most anything. Gaining good access and following the directions exactly would probably make a permanent, good repair. Cleaning the crack and surrounding area completely is essential! But, if the engine is a little tired, now may be the time to yank it out and give it what it really needs-----

Question from Steve (413):

On a recent short drive, my car overheated. We pulled into the quickie mart to see what was going on and found the radiator empty. When I tried to add water it runs out the back of the block on the driver side. I am assuming that one of the freeze plugs blew out. Now I need some advice. I figure if one went they all need to be replaced. Can this be done with the engine in the car? When the shop did my block a while back they had a hard time finding freeze plugs. He told me they were special and no one carries them. What is special about these and where can I find them? Last question is how in the heck do you get the old ones out and the new ones in?


From Dick:

I have replaced the core plugs in my 64 (should be the same block) from under the car. I won't say it's easy, but it can be done. If you are certain these were replaced recently, I think you have a claim against the garage that did it. If it is the rear one on the driver's side, you are really lucky, because this one is easily accessible after you remove the starter. The two front ones are really tough to get out, but usually its the rear ones that corrode away first. If you find that the core plugs have not been replaced, you probably need to pop them all out and clean out the gunk in the block. The story about them being hard to find is pure BS. They are in stock at any NAPA store. They have both brass and steel ones. Brass lasts forever, steel will serve as a "fuse" to tell you when you have some corrosion going on in there (never happens if you take good care of the car). I always put one steel one in the center position (easy to get at) and brass in the other locations, so that when I want to check on things years down stream, I can pop the steel one out and look at the back side. Removing them is tricky. Take a small, sharp cold chisel, and make a small cut in the cup near the outer ring (don't scar the block, though) and be very careful not to drive the chisel too far in (more than 1/4 inch) because the cylinder walls are right behind the plug on some cylinders. Once you have the cut made, dig at it with a small screwdriver until you get the hole large enough to grab the ragged edge with your channel-lock pliers. Then you can "lever" it out by rolling the pliers against the edge of the hole. You may have to fight for a while to get it out, but it can be done. When you have it out, get a good light and look around in there. If you see guck, muck, creepy crawly creatures, mud or crud, start digging it out, and remove the other 5 plugs too, as you have a mess in there. It is usually worst at the rear of the block, so if the rear plug area looks OK, you can be confident someone has already done this. Re-installing the cups requires that the block sealing surface be spiffy-spotless clean and undamaged. Place the cup in position and drive it in by tapping on a socket which just barely fits inside the cup. Be very careful to drive it in square to the surface of the block, and only drive it flush with the outer surface, don't bury it in there. If you like to waste money, you can buy an installing tool at any auto parts place. If you think there may be some defects in the sealing surface, use a very, very thin coat of "Ultra-Copper" gasket goop on the plug to fill the defects. DON'T get carried away with this stuff! Loose particles of RTV circulating in your cooling system will cause all kinds of trouble! If you do use the goop, wait 24 hours before filling the cooling system. Don't even think about going without a thermostat. Engines are designed to run best at the temperature provided when the cooling system is in good shape and the thermostat is the right rating. Removing or using a lower temp thermostat may give the illusion of a cooler running engine in cases of troubles elsewhere, but that is a band-aid, not a cure, and will cause much more deterioration in years to come.

From Loren:

Freeze plugs at the bottom of the block on a 413 are easy to see from underneath the car. They can also be removed and replaced with some difficulty. When I did mine, I had to borrow a mechanic's freeze plug installer tool to get the ones above the front frame to drive into the block. I also believe there are freeze plugs on the fire wall end of the cylinder heads that I doubt can be replaced without removing the heads from the block. I replaced the freeze plugs in the bottom of my block but am crossing my fingers and hoping the the freeze plugs in the fire wall end of the cylinder heads last until I am ready to rebuild the engine. If your engine runs well and has good compression you may just have to pull the heads and replace the blown freeze plug. I can't think of any way to drive on in, let alone pull out the old ones at the back of the engine heads.

From 4-toes:

It should not be necessary to remove the heads to replace the freeze (casting) plugs in the heads, near the firewall. You can buy replacement plugs that fit loosely but have an expansion bolt in the middle of the plug. It is tight, but you should be able to get them in and then use an end wrench to tighten. Think of the old type of plug for a thermos jug. I have done this on a '69 383 T&C. I was 8,000 feet up in Big Cottonwood Canyon near Salt Lake City, when it blew. I made a temporary plug by sharpening the end of a piece of oak branch and wedging it into the hole with the other end against the firewall. I loosened the radiator cap to relieve pressure and drove several months before getting around to repairing it with an expansion plug.

From Don:

I had to do this job on my '62 and was advised that the plugs installed easily if you froze them first. They did.

From Joe:

Some of the disc type plugs are very hard to drive in after cleaning and flushing. I have completely eliminated any further pop out problems that do occur from the disc type plugs. I use the rubber with a nut expansion plugs. But then I always take the engines out of my cars no matter how many miles, low or many, and start all over. I put my blocks in an engine stand, rotate them as I flush them out. Heads off. No engine shop is going to spend the extra time to do this. Even the factory did not make sure all was clean inside after removing some of the sand casting materials. After I flush with chemicals to clean and pressure rinse, I scrape as much as I can get to and remove all the scale out I can possibly find. I run mini wire brushes, small scrapers I have bent for the job. Anyway, it is very easy to replace a soft plug with the rubber type and they have never ever popped out like I have had the disc type do in the past 17 years I have driven Imperials. If you are after original looking restorations, then you are on your own. The disc do and will pop out. They did as new cars, and surely will as older cars. 

From PEN:

I would like to discourage rubber core plugs. The 413 came with both cups and disks in the freeze plug holes, depending on the particular year and engine and heads. I can think of four reasons that a cup or disk would push out of its hole: a burned head gasket, a cracked head, or a severely rusted block with blocked cooling passages, or the engine was frozen and the plugs did their job. Improper installation would probably represent a fifth, but that would be unlikely and readily known to the installer, unless the guy is wholly unskilled, which may actually not be far off these days. The one rubber plug I have in mine is the one where my block heater goes in. I want to further impress that we should, where possible, keep these cars as original as possible, only making modifications where absolutely necessary, and if necessary, where those modifications will not be readily seen. The exception to that is adding modern, customary safety features, such as seat belts.

Follow-up from Steve:

Today I was able to get under the car and get all the freeze plugs out. I found the easiest way to get them out was to drill a hole in the middle then I could just stick the screwdriver in popped them right out. Although NAPA insisted they were the cup type (and that is what I bought) sure enough they were discs. The discs themselves were all in good condition but there was evidence of weeping around all of them. The one that had come out was beside the starter and looked like it had been replaced because there were hammer marks on it from being driven in. I don't think the factory would have done that. Now I am considering replacing them with the rubber ones like Joe Machado suggested. Anyone else know any positives or negatives to doing this? I spent about two hours under there with a water hose flushing all the gunk out. Amazing how much stuff collects down there. I don't know if I got it all but it has to be better than it was.

Question from Joe ( 413):

I'm at work and I get a call from my son, the radiator fluid is just pouring out of the car. He adds hot water and drives home, where I meet him. After adding more hot water I see fluid just coming out like crazy from the drivers side of the engine. Fearing a blown gasket or worse, I check around and find that the water is coming from between the 1st and 2nd spark plugs (from the front), near the oil check stick, and there appears to be a freeze plug or something missing, maybe an inch in diameter or so. When I tried to see if the hole were threaded or not, well the engine was very hot and burned my poor little finger so I stopped. Anyhow, this doesn't seem to be a big deal if I can find out how to plug it. Is there a standard sized threaded plug to fit it, or is it a rubber plug or what?


From Steve:

I just went through this on one of ours... If a middle freeze plug went then chances are they are all getting weak. I would pull the starter and all six freeze plugs, clean out the gooey mess that is hiding in the water passages down there and replace the freeze plugs. You can check the archives and see the stir I caused on this but my end result was this: The freeze plugs in my block were flat disks not the newer cupped plugs. I could find no way to get to get cups in and the flat disks were impossible to get in from under the car. On the advice of a few people on the list (and against the advice of a few others) I installed rubber plugs in place of the disks. You can get these rubber plugs at most any auto parts store. They are super easy to put in and so far are working like a charm. You insert the plug then tighten a nut on the front of it. I put loc-tite on the nuts so they wouldn't come loose and I see no way these things are ever coming out. Is this a long term fix? I don't know but I am very happy with the end result. If you want to have the disks replaced I would take it to someone and have it done because access is awkward at best under there

If you are going to temporarily use a rubber-type expansion plug, leave the radiator cap loose. Pressure from the cooling system is great for popping those temporary rubber plugs out.

From Arran:

If it is a freeze plug, which it sounds like, then it is not threaded in but press fitted in. The freeze plugs are basically round bottle cap shaped pieces of steel that is pressed into the holes in the water jacket of at the factory. Contrary to popular belief the freeze plugs are not there to stop the engine block from cracking when the water freezes but are there to fill in the holes left over from the casting process. The holes are there in order for the foundry to get the sand out of the engine block after casting. Your freeze plug probably rusted through with age and the others will be soon to follow so it would probably be a good idea to replace any that haven't already been replaced. Since your car has a 413 wedge it shouldn't be a problem getting some new plugs if you check with your local auto parts store. I am sure that they would be the same as the ones in a 383 or probably any other Chrysler V8 engine from that era. If you have any car manuals around every one of them will tell you how to remove and replace freeze plugs.

From Dick:

Measure the hole, go to your nearest auto parts store, and buy the rubber type expansion plug that is recommended for this size hole. Put it in and tighten the center bolt, and you'll be OK for a while.

Follow-up from Joe:

I couldn't get a micrometer on the freeze plug, so I shoved various sockets into it and mic'ed them. The hole is slightly larger than 1 ", smaller than 1 1/32". Eventually got a rubber expansion thingy that goes from 1" to 1 1/8".

Question from Richard (440):

While poking around under my 75 the other day I noticed one of the freeze plugs in the engine block had developed a leak. From doing a bit of clean-up and some more poking it looks like a small rust hole near the center of the plug. I've run across this sort of thing once before, but that was about 5 years ago, and I can't remember what I used to patch the thing up. So far I've tried a couple of different epoxy metal patches, but they don't want to stick to the plug. I haven't been too gung-ho about getting the surface real clean for fear of making a big hole out of a little hole. So my question is: can anybody recommend a good chemical metal patch, and how feasible is it to try replacing a freeze plug w/ the engine still in the car?? Getting at the plug is more than a little tough, but I think if I drop some of the exhaust I should be able to rip the old plug out a piece at a time, but I'm fairly sure that trying to swing a hammer to seat a new plug is going to be tricky.


From Michael:

Real feasible, if you can get to it. I'd say if you can get to it to patch it, it should be as easy to replace. If you can drillit and pull it, you don't have to risk leaving it in the block. If not, usually they will remain in place enough when you drive it in, to allow you to pull it out. Use brass for the replacement plug.

From Greg:

I try to replace them with new metal freeze plug where I can get access. On the non accessible locations I use the rubber type that are adjustable. The rubber plugs loosen in time, I have had them come out before and ruin my day. If you use them, leave a card board sheet under the engine, because they will seep before they fall out. That way you can avoid a major "Oh Shit" before it happens by tightening the plug nut. The best thing to do is put the engine and replace all of them, because if one is seeping the rest of the freeze plugs are going to need attention too, it is just a matter of time. Once you have new plugs installed, change the antifreeze and back flush every year to avoid doing this nasty job again.

I use an aircraft rivet gun with a round block of steel as a driver to install new freeze plugs. I usually can't get at them with a hammer, or at least get a good swing at them.

From Dick:

It would certainly be a longer term cure to replace the core plug. Usually, if one is leaking, the others are not far behind, and it would be less work in the long run to just remove whatever obstacles are involved and replace them all. This probably means dropping the starter and the exhaust system, but usually, with that out of the way, you can get enough access to work on them. I have found that a socket of the right OD on the end of a 1/2 breaker bar can usually be used to drive in the new plugs, or you can buy or rent a tool which is designed for the task. Often, you can remove them easily by driving a screwdriver through the center (but don't drive it in more than 1/4 inch or so, you can damage a cylinder wall), then prying with the screwdriver until the whole deal pops out. Clean up the hole and apply a very thin skin coat of Ultra Copper on the bore of each hole before you drive in the new plug. It will make it easier to drive it in, and will seal if there are any pits in the surface of the block from rust. You should clean out the water jackets while you have access to them, there is sure to be a buildup of glop in the block, especially to the rear of each side. To prove this to yourself, remove the block drain plugs, I bet you nothing runs out until you poke in there with a small rod. This is a messy dirty job, and you will probably not see any difference in operation of then engine, but your block will be much happier with cooling water circulating all the way around and up and down all 8 cylinders. You will have more reserve cooling ability for severe conditions, also.

Question from Keith (440):

I'm about three miles from my house when I smell antifreeze.  I look at the temp. and it's rising FAST.  By the way I was cruising at 85 on the highway not an exit until mine and my house is in the corner of the highway.  Anyway, I pull into the drive a nervous wreck and RUN to the hose in the yard and begin to gently rain water on the motor, while it's running of course, to cool it. By this point the temp. was pinned over H. When the temp. went down to 3/4 after several minutes I tried to turn the car off and it kept running, the internals were still so hot the fuel kept detonating. It sounded like a machine gun. So I turned the key back ON so the detonation would stop, being a two-stroke motorcycle road racer I KNOW from 1st hand experience what detonation can do to pistons.  I continued to cool the motor VERY carefully so as not to cool too fast and crack anything. when the gauge read COOL I finally turned off the ignition. The next day when I looked at it I could finally see the problem. The drivers side 1-5/8 in. freeze plug had popped out. So I put a new rubber one in and she runs w/o a knock. Same as she ever was.  What a piece of machinery.

Reply from Larry:

If your bung-hole isn't cracked or distorted, I think you may want to replace the rubber with brass. Keep an eye on the water pump and  head gaskets - heads may have warped a smidgen- please keep us posted as to how she runs after a couple of k.  If anything can survive this kind of mishap - it is a 440.

Question from Roger (440):

I want to replace all core plugs on my engine.  How complicated of a job is it?


From Gregg:

Freeze plugs are cheap, get a brass one. Any parts store should carry them in various sizes. One was leaking on my other-leading-brand car. I had to jack up the car and take off the starter, dig out what was left of the steel freeze plug, then tap in a new brass one. Cost me $.50. The trick is to get a socket the same size as the plug and tap it in evenly. If you warp it, throw it away and start with another one. It doesn't mean you have to replace all of them. But if you get the motor rebuilt, get them all replaced with brass ones.

From Rodger:

When a freeze plug starts to leak it shows what is happening within the engine. As one goes through the ordeal of removing the plug, it only makes sense to replace the others while you are wearing your best clothes. This is a case of preventive maintenance when you are under there doing the corrective maintenance  on the one leaking plug.

Reply from John:

On the other hand, none of the other freeze plugs may ever fail. In 34 years, I've only had one fail. If you have the engine out, it makes sense to change all but otherwise these are a bear to deal with. I'd leave the others alone.

Reply from Mark:

I would say it depends on what you find when you replace the one bad plug. If there's a lot of black gunk behind it (like there was on my old '63), then it's time to pull all the freeze plugs, and flush it out, using a bent coat hanger, if necessary, to get all that black gunk out, which is what I had to do. In doing this, I found that two other plugs, located towards the back of the engine, were paper thin. When I was done, I had all the confidence in the world that I wouldn't be on the side of I-80 in October because a freeze plug went bad, and so drove my '63 from Denver to Montreal in October of '97 without incident. (Except for the tire that blew out near Nappanee, but that wasn't a freeze plug problem....) Yes, they are a bear to deal with, and it's a messy, messy job. But, better to deal with a messy bear in your driveway, than on a cold (or hot) roadway, miles from home....

From Steve:

Changing the core (freeze-out) plugs on your 440 is a royal pain in the neck. There are 6 of them total. 3 on each side of the engine...and to make matters worse...1 or them is under each motor mount. Another is behind the starter. I don't remember what size they are off the top of my head 1 48/64 th's or 1 21/64 th's. (???) Factory ones are made of steel. Replacement ones are made of either brass or steel. There is a special tool made for installing them which makes things a little easier. Workable space is limited and much patience is needed. HELPFUL HINTS: 1)Prime and paint the new plugs. Especially the coolant side since they usually rust from the inside out (I don't think anyone ever does this). 2) This is a great time to check the motor mount condition. 3) The most thorough cooling system flush can be done with all of the core plugs out. All the "gunk" will be accessible. Loosen it with a coat hanger and wash it out.

Question from Mark (440):

How do you flush a block?  Stick a hose in there? Or just let it drain? I would probably have to have the car up on blocks, yes? I can't reach the drain plugs from anywhere but crawling underneath, I suppose?

Reply from Dick:

Yes, you have to get under the car. The drain plugs are not large enough to get much crud out with a flush. The only way I know is to knock out the soft plugs and just dig out the mud with spoons, hacksaw blades, or whatever you find that works. The drain plugs are useful in telling you if you have a problem, though, since opening them either drains the block (good) or doesn't (bad!). They'll all drain if you poke in the hole, but if you have to, you know the mud is at least that high in the block, and since the block is tilted in the car, look how high it is toward the back of the engine. #'s 7 and 8 aren't getting much cooling circulation at all!

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