3.8 core plugs?

Relative to a post by “Ed”. I wonder how common it is for a 3.8 style core plug to blow out. I’m not talking corrosion - say your block is good with no corrosion. I know it is common practice for race motors to have the plugs strapped. Is this a solution looking for a problem or does it happen often?
Lots of 3.8’s running around with no straps (including mine…). I used some JB weld around the perimeter on a very clean block. Now there isn’t much purchase on that perimeter, as these aren’t like the 4.2 and most other style plugs, so inquiring minds would like to know how much of a danger these plugs present.

Common! After both rebuilds on my 3.8, I had several plugs remove themselves spontaneously within a few thousand miles. My solution was to remove ALL of them, and replace them with Dorman copper expandable plugs. I’ve never had one of those come out. The old ones can be removed using a long slide hammer, and the new Dormans installed, with the engine in-situ.

Ray L.

Ouch. When I did the engine (possible memory problem) I thought I had trouble finding plugs that would fit in the odd recess in the block. I had never seen a seating area like on the 3.8. The fact that I used some epoxy is evidence of my suspicion. Should have acted on it.

The 3.8 uses “flat” plugs, not the “cup” plugs used on the 4.2. Most of them are 1-3/4", a very common size. You need to THROUGHLY clean the seat, put a bead of sealant on the seat, push in the plug, the flatten it with a suitable drift to expand it and lock it into the hole. My personal belief is a large diameter drift, or large socket, should be used to completely flatten the plug. But the most common way to install them is to give the plug a whack with a ball-peen hammer. I do NOT believe this is adequate, as evidenced by the several plugs, all them installed that way, in my engine which simply fell out while driving.

Problem is, the seats are often corroded, which will lead to leaks. I always put a can of stop leak in the radiator after changing the plugs, to take care of any small leaks.

Ray L.

Just taking rough estimates (like Enrico Fermi) they are about 2 sq. inches. With a 7 lb cap that means they need to hold a minimum of ~ 14 lbs.

I’d say if a bunch of plugs popped out in short order, they were probably not installed correctly. And I’m not sure that a ball-peen hammer is the best idea for expanding a 3.8 type domed core plug. I think it would be more likely to dent the center rather than actually expanding the plug enough.

When I rebuilt my 3.8 engine, I removed all the core plugs and cleaned a ton of crud out of the water passages, then I cleaned the core hole surfaces and installed new plugs, probably with a bit of Permatex non-hardening form-a-gaskket. I gave each one a smart smack with the flat side of a 16 oz. hammer. I’d never done this before but I never had any leaks or seepage from them in all the years I had my car on the road.

I’ll just add my 2c worth. Many years ago my ‘S’ was used in a TV series here, on the last shoot of the day I was driving up a moderately steep hill, car going great, ‘clink’ tinkle tinkle.
Hmm what was all that white smoke out the back?
Reaching the top of the hill I did my usual scan of the gauges, what no heat???
Stopped and deduced that the rear core plug had popped out (clink) and the smoke was all the water hitting the exhaust pipe in <> 5sec
Called a tow truck and drove the car on, then off at the shoot location and then back onto the flat bed.
The actors probably thought ‘tosser’
Got home fairly late that night, I hate to think what it cost the production company, I agreed to a 1/2 payment :slight_smile:

We’ve probably rebuilt 20 or 30 3.8 and 4.2 motors for customers over the last 5 or so years and have not had any problems with core plugs. None of our motors are for racing.

We have the blocks chemically cleaned and sandblasted and on occasion have had to have minor repair done to the plug holes but nothing more usually that a bit of die grinding to tidy them up. I don’t recall ever seeing one need brazing.

We use the “watchglass” shaped plugs from the usuals and put a bead of Devcon epoxy sealant under the plug before inserting them. Then they are peened using a 1/2" piece of brass bar. As Ray says you need to get them flattish to get a good tight fit. Then more Devcon is applied around the edge of the plug. Seems to work for us.

My bad experience with the core plugs was wholly due to poor workmanship by the rebuilder 15 years ago. I didn’t make the first mile before one popped out entirely. I never had a problem with the factory installation. What really pissed me off was evidence that the rebuilder knew he had a problem, but just shipped the car anyway. I bought every Dorman expandable plug in Chattanooga before I realized it was beyond my ability to fix a simple leaking core plug. And I’ve made a good living fixing leaks in nuclear power plants.
After messing with it for a year, I took the car back to Texas insisted they fix it. That’s when they put the straps on instead of really fixing the problem. Some still leaked, but at least none popped out.
My current old school automotive machine shop had never seen straps used, but put them back on. They didn’t use any sealant on the threads and we didn’t catch it until the engine was in and filled with antifreeze.
Like many on these forums I don’t like having problems, but do get satisfaction solving them.
I’m happy now, done with my core plug issues, and optimistic. :slight_smile:

What was the problem? I’ve replaced all the large plugs in-situ on several E-Types, and found it quite easy, and none ever came out again. The only slightly tricky one is the one on the back of the head, and even those are not all that hard to do.

Ray L.

Hi Ray,
Personal shortcomings in mechanical ability and dexterity. :slight_smile:
Good news is I’m getting better and have some great help, including here.

MGB’s also used the ‘disc’ style core plugs, like a 3.8 Jag, right up until the end of the production run.

They also have a reputation for popping out, but as Ray mentioned, popular wisdom is that a healthy dimple in the center should hold them in, so that’s the approach most people take.

It doesn’t take a whole lot of smarts to realize that getting the disc as flat as possible gives it the largest interference with the block recess. For that reason, I’ve always used an aluminum drift that is the same diameter as the plug, and hammer them as flat as I can get them…I’ve never had one of mine come out.

Guys who vintage race, at engine speeds of 7,000 rpm+, with coolant temps over 200F, and 18-20 psi radiator caps have a legitimate need to add safety-straps to their plugs, but on a street driven car, they shouldn’t be needed.

It’s also worth noting that there are different brands of plug out there, and some fit better than others. It probably goes without saying that the disc should JUST BARELY fit in the block recess before deforming it. If it rattles around in the pocket, it’s too small, and you’re betting against yourself.

I installed the plugs using Permatex and whacked the snot out of them (not flat, though). At about 450 miles on the rebuilt engine, I see antifreeze drips on the floor below one of the plugs. I’m thinking I’ll try Dorman plugs if I can get the old plugs out without stripping the whole left side.

I don’t think the exact number, size and type of plug has been called out for the 3.8. When you get that info it would be nice to log it somewhere where we can find it. It may already exist, but sometimes a search needs to have a magic code word, or even incorrect info…

Here’s the part number

Some in the MG world have resorted to these plugs. They look cleaner than the Dormans plugs, and they apparently work well.

Unfortunately, the cast pockets and apertures in the MG block aren’t all that consistent, and sometimes require some customizing to make these work…I’m guessing the same would hold true for the Jag blocks.


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I often thought about making something similar, but could not convince myself there was enough room for them, without removing all of mine, which I am loathe to do. I also think I would use copper washers instead of O-rings.

That said, I’ve been running the Dorman’s since ~2003, with zero problems.

Anyone here have a spare block sitting around that could verify how much space is available behind all the plugs? Especially the one on the back of the head. I think getting this style plug in the hole on the back of the head in-situ could be a challenge, but I’m sure do-able.

Ray L.

Ben, Do you know if those stainless plugs are 1 3/4" ?

I’m not sure, but I can measure some MGB plugs this evening.

Thanks. The do look like a great solution.